Houdini looping particles

Looping fluid sim

For a while I’d been planning to look into making looping particle systems in Houdini, but hadn’t found a good excuse to jump in. I don’t really do much VFX related work at the best of times, something I need to do more of in the future πŸ™‚

Anyway, I was recently chatting with Martin Kepplinger, who is working on Clans Of Reign, and he was looking to do a similar thing!

So begins the looping particle journey…

Technique overview

I won’t go into the fluid sim setup, it doesn’t really matter too much what it is.

There are a few conditions that make my approach work:

  • Particles must have a fixed lifetime
  • The first chosen frame of the simulation must have a lead up number of frames >= the particle lifetime
  • The last frame of the loop must be >= the first frame number + the particle lifetime

I have some ideas about how to get rid of these requirements, but not sure if I’ll get back to that any time soon.

For the example in this post, I am keeping particle lifetime pretty low (0.8 to 1.0 seconds, using a @life attribute on the source particles, so a 24 frame loop).

The fluid sim I’m using is some lumpy fluid going into a bowl:

Full fluid sim

The simulation is 400 frames long (not all shown here), but that ended up being overkill, I could have got away with a much shorter sim.

Going back to my rules, with particles that live 24 frames I must choose a first frame >= 24 (for this example, I’ll choose 44).
The last frame needs to be after frame 68, so I’m choosing 90.
This makes a loop that is 46 frames long, here it is with no blending:

Looping particle system with no blending

The technique I’m going to use to improve the looping is somewhat like a crossfade.

For this loop from 44 –> 90, I’m modifying the particles in two ways:

  1. Deleting any particles that are spawning after frame 66 (i.e, making sure all particles have died before frame 90)
  2. From frames 66 to 90, copying in all the particles that spawn between frame 20 –> 44.

This guarantees that all the particles that are alive on frame 89 match exactly with frame 44.

To illustrate, this gif shows the unedited loop on the left, and next to it on the right is the loop with no new particles spawned after frame 66 (particles go red on 66):

Particles stopped spawning after frame 66

Next up is the loop unedited on the left, and on the right are the new particles from frames 20 – 44 that I’m merging in from frame 66 onward:

Pre loop particles added to end of the unlooped sim

And now, the unedited loop next to the previous red, green and blue particles combined:

Pre spawn and end spawn particles combined

And finally, just the result of the looping by itself without the point colours:

Final looping particles

One thing that might be slightly confusing about the 2nd gif with the green pre-loop particles is that I’m always spawning particles in the bowl itself to keep the fluid level up, in case you were wondering what that was about πŸ™‚

Setting up the loop

This is the SOPs network that takes the points of a particle sim, and makes it loop:

Full SOPs network for looping particles

The first part of setting up the looping simulation is picking the start and end frames (first and last) of the loop.

I created a subnetwork (FrameFinder, near the top of the above screenshot) that has parameters for two frame numbers, and displays a preview of the two frames you are selecting, so you can find good candidates for looping:

FrameFinder subnetwork preview

The loop setup I chose for the Unity test at the top of the blog was actually a bit different to the range I chose for the breakdowns in the last section.
For Unity, I wanted the shortest looping segment I could, because I didn’t want it to be super expensive (memory wise), so I chose start and end frames 25 frames apart.

You can see that the frames don’t need to match exactly. The main thing I wanted to avoid was having a huge splash in the bowl at the bottom, or over the edge, because that would be hard to make look good in a short loop.

Node parameters

In the screenshot above, you can see that I have First Frame and Last Frame parameters on my FrameFinder network.

I don’t tend to make my blog posts very tutorial-y, but I thought I’d just take the time to mention that you can put parameters on any node in Houdini.

Example:

  • Drop a subnetwork node
  • Right click and select “Parameters and Channels –> Edit Parameter Interface…”:Parameter editing
  • Select a parameter type from the left panel, click the arrow to add the parameter, and set defaults and the parameter name on the right:Edit parameter interface dialog
  • Voila! Happy new parameter:
    floaty

You can then right click on the parameter and copy a reference to it, then use the reference in nodes in the subnetwork, etc.
In the edit parameter interface window, you can also create parameters “From Nodes”, which lets you pick an existing parameter of any node in the sub network to “bubble up” to the top, and it will hook up the references.

If this is new to you, I’d recommend looking up tutorials on Digital Assets (called “HDAs”, used to be called “OTLs”).

I do this all the time on subnetworks like on the FrameFinder, but also to add new fields to a node that contain parts of an expression that would get too big, or that I want to reference from other nodes, etc.

On Wrangles (for example), I find myself adding colour ramp parameters a lot for use within the wrangle code.

FrameFinder

This subnetwork has two outputs: the particles with some Detail Attributes set up, and another output that is a preview of the two frames which I showed before, but here’s what that looks like again:

FrameFinder subnetwork preview

It’s the first time I’ve created multiple outputs from a subnetwork, usually I just dump a “Preview” checkbox as a parameter on the network, but I like this more I think, particularly if I end up turning the whole thing into an HDA.

Here is what the FrameFinder network looks like:

Contents of framefinder subnetwork

In this project, I’m using Detail attributes to pass around a lot of data, and that starts in with the attribcreate_firstAndLastFrame node.

This node creates Detail parameters for each of the frames I chose in the subnet parameters (firstFrame and lastFrame):

Create details attributes for first and last loop frame

Right under the attribCreate node, I’m using two timeshift nodes: one that shifts the simulation to the first chosen frame, and one to the last frame, and then I merge them together (for the preview output). I’ve grouped the lastFrame particles so that I can transform them off to the right to show them side by side, and also giving all the particles in both frames random colours, so it’s a little easier to see their shape.

Time ranges and ages

Back in the main network, straight after the frameFinder subnetwork I have another subnetwork called timeRangesAndAges, which is where I set up all the other Detail attributes I need, here is what is in that subnetwork:

Time Ranges and Ages subnetwork

The nodes in the network box on the right side are used to get the maximum age of any particle in the simulation.
In hindsight, this is rather redundant since I set the max age on the sim myself (you could replace all those nodes with an Attribute Create that makes a maxAge attribute with a value of 24), but I had planned to use it on simulations where particles are killed through interactions, etc πŸ™‚

The first part of that is a solver that works out the max age of any particle in the simulation:

Solver that calculates maximum particle life

For the current frame of particles, it promotes Age from Point to Detail, using Maximum as the promotion method, giving me the maximum age for the current frame.

The solver merges in the previous frame, and then uses an attribute wrangle to get the maximum of the previous frame and current frame value:

@maxAge = max(@maxAge, @opinput1_maxAge);

Right after the solver, I’m timeshifting to the last frame, forcing the solver to run through the entire simulation so that the MaxAge attribute now contains the maximum age of any particle in the simulation (spolier: it’s 24 :P).

I then delete all the points, since all I care about is the detail attribute, and use a Stash node to cache that into the scene. With the points deleted, the node data is < 12 Kb, so the stash is just a convenient way to stop the maxAge recalculating all the time.
If I turn this whole thing into an HDA, I’ll have to rethink that (put “calculate max particle age” behind a button or something).

There are two more wrangle nodes in time ranges and ages.

One of them is a Point wrangle that converts the particle age from seconds into number of frames:

@frameAge = floor(@age/@TimeInc);

And the next is a Detail wrangle that sets up the rest of the detail attributes I use:

// Copy max age from input 1
int maxAge = i@opinput1_maxAge;

// A bunch of helper detail variables referenced later
int loopLength = i@lastFrame - i@firstFrame;
int loopedFrame = (int)(@Frame-1) % (loopLength+1);

i@remappedFrame = i@firstFrame + loopedFrame;

int distanceFromSwap = loopedFrame - loopLength;

i@blendToFrame = max(1, i@remappedFrame - loopLength);
i@numberOfFramesIntoBlend = max(0, maxAge + distanceFromSwap);

When I hit play in the viewport, I want to see just the looping segment of the simulation over and over, so that complicates all this a little.

With that in mind,Β there are 3 important attributes set up here.

@remappedFrame

If my start frame is 20, for example, and the end frame is 60, I want to see the 20-60 frame segment over and over, hence the wrapping loopedFrame temporary variable.

So if my viewport time slider is on 15, I really want to see frame 34, the value of remappedFrame will be 34. It will always be a number between firstFrame and lastFrame.

@blendToFrame

This takes the remappedFrame, and shifts it back to before the start of the loop.
I only use this value when we hit the last 24 frames of the loop, but I’m clamping it at one just so the timeshift I use later doesn’t freak out.

This attribute will be used in the 2nd part of the technique: combining in pre-loop particles.

@numberOfFramesIntoBlend

When we are getting into the last 24 frames of the loop, this value increases from 0 to 24.
It’s used in the 1st part of the technique to stop spawning particles that have an age less than this value.

Timeshifts and recombining particles

Back to the main network:

Full SOPs network for looping particles

After the timeRangesAndAges node, the network splits: on the left side, I’m timeshifting the simulation using the remappedFrame detail attribute as the frame using this expression:

detail("../timeRangesAndAges/", "remappedFrame", 0)

On the right side I’m time shifting the simulation using the blendToFrame attribute as the frame using this expression:

detail("../timeRangesAndAges/attribwrangle_calcTimeRanges", "blendToFrame", 0)

I’ve colour coded the nodes in the network with the same colours I’ve shown in the gifs in the technique section.

Since I’ve timeshifted the simulation, the detail attributes get time-shifted too.
But I don’t really want that, so I’m attribute transferring the two detail attributes I care about (remappedFrame and numberOfFramesIntoBlend) back onto the remapped sims using Attribute Transfer.

After the attribute transfers, on both sides I’m creating a new point group called inBlendingFrames.

Group expression node for particles in blending range

detail(0, "numberOfFramesIntoBlend", 0) > 0

I probably didn’t need a point group for this, considering every particle is either in or out of this group on a given frame, it just made life easier with the Split node I use on the left.

On the left side, I do a split using inBlendingFrames.
When we’re not in the blending range, we don’t have to do anything to the particles, so that’s the blue colour node.

For both the red and green nodes section, I start with deleting anything not in the inBlendingFrames group.

For the green particles (the pre-loop particles that we’re merging in), we’ve already got the right frame, due to the timeshift up the top.
If we’re on blending frame 2 of the blend (for example), we will still have particles that were spawned 24 frames ago, but we really only want particles that should spawn after the blend starts.
I use an attribute wrangle to clean the older particles up, since I have the frameAge attribute I can use:

if ((@frameAge > i@numberOfFramesIntoBlend))
{
	removepoint(0, @ptnum);
}

Here’s what that looks like for a frame about halfway through the blend.

Pre loop particles with older particles removed

For the red nodes section (where we take the original loop, and delete any particles that start spawning after the blend), I use an attribute wrangle to clean the new particles up:

if (@frameAge < i@numberOfFramesIntoBlend)
{
	removepoint(0, @ptnum);
}

Particle loop end with new particles deleted

So, I merge the red, blue and green particles all together, and we end up with the result I showed in the technique section!

Pre spawn and end spawn particles combined

Here again uncolourised:

Final looping particles

Unity, Alembic and all that jazz

This post is already crazy long, so I’m just going to gloss over the Houdini –> Unity stuff.
If anyone is really interested in those details, I might do another post.

So now that I have a looping particle system, I can use a regular Particle Fluid Surface with default settings, and a polyreduce node to keep the complexity down:

A frame of the looped fluid sim remeshed

I exported the range of frames as an Alembic file, and imported it into Unity with the Alembic plugin.

I threw together a really quick monoBehaviour to play the Alembic stream:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UTJ.Alembic;

[RequireComponent(typeof(AlembicStreamPlayer))]
public class PlayAlembic : MonoBehaviour
{
	public float playSpeed = 0.02f;

	AlembicStreamPlayer sPlayer;

	// Use this for initialization
	void Start ()
	{
		sPlayer = GetComponent();
	}

	// Update is called once per frame
	void Update () 
	{
		sPlayer.currentTime += playSpeed;
		sPlayer.currentTime = sPlayer.currentTime % 1.0f;
	}
}

As a last little thing, I packaged the network up a lot neater, and dumped it in a loop that ran the process on 16 different 46 frame range segments of the original simulation.
The idea being, why try to find good first and last frames when you can just go for a coffee, and come back and have 16 to choose from!

The loops with big splashes definitely don’t work very well (they look like they aren’t looping because lots of particles dying on the same frame), but there are some fun examples in here:

16 different looping segments

Faking Catmull-Clark creased sub-d in UE4

Vector displaced sub-d wheel

I’ve been hoping for edge creased Catmull-Clark subdivision in game engines ever since I started using Modo about 10 years ago.
I previously made a tool to build LODs from sub-d surfaces in Unity, just to have some way of getting a sub-d like mesh in engine. Super expensive LOD meshes…
This was not a very useful thing to do.

There are a few games out there using real-time sub-d, including the folks at Activision who have demonstrated CC sub-d with creases in a game engine:

Efficient GPU Rendering of Subdivision Surfaces using Adaptive Quadtrees

It is unclear if they shipped edge-creasing in any of their released games, but they definitely use CC subdivision surfaces.

And sure, technically there has been real-time subdivision in games going back to TruForm on ATI cards in the early 2000s, and probably before then for all I know, but I’m specifically interested in Catmull-Clark sub-d, and specifically edge creasing πŸ™‚

Why creases?

Creasing gives you control over the sharpness of edges, without having to manually bevel all of your edge loops.
This is nice for keeping a low poly mesh, but also allows you a little more flexibility.
For example, if you come back and change the model, you don’t have to un bevel and re-bevel edges.
If you bake out a normal map, and decide the bevels aren’t quite wide enough, you can just change the crease value and re-bake.

Here are some loops on my wheel model that are heavily creased:

Wire frame sub-d

If I were to bevel those edges instead, my base model would go from 3924 vertices to 4392.

If I removed creases across the whole model, and beveled all the edges to get the same end result I’d need a base mesh around 6000 vertices (2000 vertices more than the creased version).

For the sake of showing how much work the creasing is doing for me, here is the base model vs Sub-d vs Creased Sub-d:

Comparison between sub-d and creased sub-d in Modo

Vector Displacement approach

I’m not likely to be able to implement the Call Of Duty approach myself, so I’ve done something far more hacky, but slightly less gross than my previous Unity attempt πŸ™‚

My new method is:

  • In Houdini, tessellate the model completely flat
  • Also tessellate it using Catmull Clark creased sub-d
  • Bake the difference in positions of the vertices between these two meshes into a vector displacement map and normal map
  • In UE4 (or engine of choice) flat tessellate the model
  • Apply the vector displacement map to push the vertices into their sub-d positions

It’s very expensive from a memory point of view (and probably performance for that matter), so this is not something you’d want to do for a game, but it does show off how nice creased sub-d would be in UE4 πŸ™‚

Houdini Vector displacement map generation

First up, here’s the un-subdivided model in Modo:

Low poly wheel in Modo

And this is the edge weighting view mode in Modo, so you can see which edges are being creased:

Modo edge weight visualisation

There are two things I want to bake out of Houdini: A vector displacement map and a normal map.
I’m not baking this data by projecting a high poly model onto a low poly, I don’t need to because the high poly model is generated from the low poly, so it has valid UVs, I can just bake textures straight out from the high poly.

Here’s the main network:

Houdini network for generating vector displacement

The right side of the graph, there are two Subdivide nodes.
The Subdivide on the left uses “OpenSubdiv Blinear”, and on the right is “OpenSubdiv Catmull-Clark”, and they are both subdivided to a level of 5, so that I have roughly more vertices in the meshes than pixels that will get baked out:

Bilinear vs Catmull-Clark sub-d

The “bilinear” subdivision is pretty close to what you get in UE4 when you use “flat tessellation”. So what we want to do is work out how to push the vertices from the left model to match the right model.
This is very easily done in a Point Wrangle, since the point numbers match in both models πŸ™‚

v@vDisp = @P - @opinput1_P;
@N = @opinput1_N;
f@maxDimen = max(abs(v@vDisp.x), abs(v@vDisp.y), abs(v@vDisp.z));

Or if you’d prefer, as a Point VOP:

Vector displacement wrangle as VOP

Vector displacement (vDisp) is the flat model point position minus the creased model point position.
I am also setting the normals of the flat model to match the creased model.

When I save out the vector displacement, I want it in the 0-1 value range, just to make my life easier.
So in the above Wrangle/VOP I’m also working out for each Point what the largest dimension is (maxDimen).
After the Wrangle, I promote that to a Detail attribute (@globalMaxDimen) using the Max setting in the Attribute Promote SOP, so that I know the maximum displacement value across the model, then use another Wrangle to bring all displacement values into the 0-1 range:

v@vDisp = ((v@vDisp / f@globalMaxDimen) + 1) / 2;
@Cd = v@vDisp;

The displacement is now stored in Point colour, in the 0-1 range, and looks like this:

Vector displacement displayed on model

Bake it to the limit!.. surface

You might have noticed that the Normals and Displacement are in World space (since that’s the default for those attributes in Houdini).

I could have baked them out in Tangent space, but I decided for the sake of this test I’d rather not deal with tangent space in Houdini, but it’s worth mentioning since it’s something I need to handle later in UE4.

To bake the textures out, I’m using two Bake Texture nodes in a ROP network in Houdini.

Bake texture nodes in ROP

I’ve only changed a few settings on the Bake Texture nodes:

  • Using “UV Object”, and no cage or High Res objects for baking
  • Turned on “Surface Unlit Base Color” as an output
  • Set the output format for Vector Displacement as EXR
  • Set the output format for Normal map as PNG
  • Unwrap method to “UV Match” (since I’m not tracing from one surface to another)
  • UDIM Post Process to Border Expansion

And what I end up with is these two textures:

Baked vector displacement map

Baked normal map

I bake them out as 4k, but lodbias them down to 2k in UE4 because 4k is a bit silly.
Well, 2k is also silly, but the unwrap on my model is terrible so 2k it is!

Testing in Houdini

If you look back at the main network, there is a section on the left for testing:

Houdini network for generating vector displacement

I created this test part of the network before I jumped into UE4, so I could at least validate that the vector displacement map might give me the precision and resolution of data that I would need.
And also because it’s easier to debug dumb things I’ve done in a Houdini network vs a material in UE4 (I can see the values of every attribute on a vertex, for example) πŸ™‚

I’m taking the flat tessellated model, loading the texture using attribvop_loadTexture, copying the globalMaxDimens onto the model, and then the attribwrangle_expectedResult does the vector displacement.

The attribvop_loadTexture is a Vertex VOP that looks like this:

Vertex VOP used for loading the vector displacement texture

This uses the Vertex UVs to look up the vector displacement map texture, and stores the displacement in vertex colour (@Cd). It also loads the object space normal map, and moves it from 0-1 to -1 to 1, and binds it to a temporary loadedNormals attribute (copied into @N later).

Then at the end of the network, the expectedResult wrangle displaces the position by the displacement vector in colour, using the globalMaxDimen:

@P -= ((@Cd * 2) - 1) * f@globalMaxDimen;

If you’re wondering why I’m doing the (0 –> 1) to (-1 –> 1) in this Wrangle, instead of in the VOP (where I did the same to the normal), it’s because it made me easier to put the reimportUvsTest switch in.
This (badly named) switch allows me to quickly swap between the tessellated model with the displacement values in vertex colour (before bake), and the tessellated model that has had that data reloaded from texture (after bake), so I can see where the texture related errors are:

Animated difference between texture loaded displacement and pre bake

There are some errors, and they are mostly around UV seams and very stretched polygons.
The differences are not severe enough to bother me, so I haven’t spent much time looking into what is causing it (bake errors, not enough precision, the sampling I’m using for the texture, etc).

That’s enough proof in Houdini that I should be able to get something working in engine, so onwards to UE4!

UE4 setup

In UE4, I import the textures, setting the Compression on the Vector Displacement Map to be VectorDisplacementmap(RGBA8), and turn off sRGB.
Yay, 21 Mb texture!

I can almost get away with this map being 1024*1024, but there is some seam splitting going on:

Low res vector displacement broken seams

That might be also be solved through more aggressive texture Border Expansion when baking, though.

Here is what the material setup looks like (apologies for the rather crappy Photoshop stitching job on the screenshots, but you can click on the image to see the details larger):

Tessellation material in UE4

The value for the DisplaceHeight parameter is the @globalMaxDimen that I worked out in Houdini.

Since both textures are Local (Object) space, I need to shift them into the right range (from 0-1 to -1 to 1), then transform them into World space (i.e, take into account the objects rotation and scale in the scene, etc).

The Transform node works fine for converting local to world for the Normal map.
I also needed to set the material to expect world space normals by unchecking Tangent Space Normal:

Checkbox for disabling tangent space normals in UE4

The Transform node works fine for normal maps, but does not work for things that are plugged into World Displacement.
Tessellation takes place in a hull / domain shader and the Local -> world transformation matrix is not a thing it has access to.
To solve this properly in code, I think you’d probably need to add the LocalToWorld matrix into the FMaterialTessellationParameters struct in MaterialTemplate.usf, and I imagine you’d need to make other changes for it to work in the material editor, or you could use a custom node to access the matrix.

If you look back at my material, you can see I didn’t do that: I’m constructing the LocalToWorld matrix from vectors passed in as material parameters.
Those parameters are set in the construction script for blueprint for the object:

Wheel contruction script

I’m creating a dynamic material instance of the material that is on the object, applying this new instance to the object, and setting the Up, Right and Forward vector parameters from the Actor. These vectors are used in the material to build the local to world space matrix.

If I wanted the object to be animated, I’d either need to do the proper engine fix, or do something nasty like update those parameters in blueprint tick πŸ™‚

Results in UE4

Please ignore the albedo texture stretching, I painted it on a medium divided high poly mesh in Substance Painter, probably should have used the low poly (something for me to play with more at a later date).

Close shot of the wheel

Toggle between sub-d and not in UE4

Close up toggle of sub-d toggle on wheel in UE4

This is with a directional light and a point light without shadows.
As a side note, Point Light shadows don’t seem to work at all with tessellated objects in UE4.

Spotlight and directional light shadows work ok, with a bit of a caveat.
They use the same tessellated mesh that is used for the other render passes, so if the object is off screen, the shadows will look blocky (i.e, it seems like tessellation is not run again in the shadow pass from the view of the light, which probably makes sense from an optimization point of view):

Spotlight shadow issues with tessellated meshes

And that’s about it!

Seems like a lot of work for something that is not super useful, but it’s better than my last attempt, and being built in Houdini it would be very easy to turn this into a pipeline tool.

For lots of reasons, I’m skeptical if I’ll ever work on a project that has Catmull-Clark creased sub-d, but at least I have a slightly better way of playing around with it now πŸ™‚

 

 

 

 

 

Chopped Squabs – Pt 4

 

Last post was about smoke, particles and weird looking ball things.
This post is about the audio for the video.

I wanted to make a sort of windy / underwater muffled sound, and managed to get somewhat close to what I wanted, just using Houdini CHOPs!

Pulse Noise

I decided not to create the audio in the same hip file, since it was already getting a bit busy, and because the data I want to use is already cached out to disk (geometry, pyro sim, etc).

The new hip file just has a small geometry network and a chops network.
Here’s the geometry network:

Pulse amount network

I wanted the audio to peak at times where the pulse was peaking on the tuber bulbs, so the first step was to import the bulbs geometry:

Tuber ball geometry

Next I’m promoting the pulse amount from points to detail, using “sum” as the promotion method (this adds up all the pulse amounts for all points in the bulbs every frame).
I don’t care about the geometry any more, because the sum is a detail attribute, so I delete everything except for a single point.

I had a bit of a hard time working out how to bring the values of a detail attribute into CHOPs as a channel. I think it should be simple to do with a Channel CHOP, but I didn’t succeed at the magic guessing game of syntax for making that work.

Anyway, since importing point positions is easy, I just used an attribute wrangle to copy the pulse sum into the position of my single point:

@P = detail(@opinput1, "pulse");

Audio synthesis fun

I had some idea of what I wanted, and how to make it work, from experiments in other software (Supercollider, PureData, etc).

I found that creating an interesting wind sound could be achieved through feeding noise into a lowpass filter.

I also tried this out in Unity, grabbing the first C# code I could find for filters:
https://stackoverflow.com/questions/8079526/lowpass-and-high-pass-filter-in-c-sharp

Here is a Unity MonoBehaviour I built from that code above:

using System;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

[RequireComponent(typeof(AudioSource))]
public class PlayerAudio : MonoBehaviour
{
    AudioSource _requiredAudioSource;
    public int _samplerate = 44100;

    private const float _resonance      = 1.0f;
    private float _phase                = 0.0f;
    private float _rampInAmount         = 0.0f;
    private float _frequency            = 800.0f;
    private System.Random _rand1        = new System.Random();
    private float[] _inputHistory       = new float[2];
    private float[] _outputHistory      = new float[3];

    void Start ()
    {
        _requiredAudioSource = GetComponent<AudioSource>();
        AudioClip testClip = AudioClip.Create("Wind", _samplerate * 2, 1, _samplerate, true, OnAudioRead);
        _requiredAudioSource.clip = testClip;
        _requiredAudioSource.loop = true;
        _requiredAudioSource.Play();
    }

    void Update ()
    {
        _rampInAmount = Mathf.Min(_rampInAmount + (Time.deltaTime/2.0f), 1.0f);
    }

    void OnAudioRead(float[] data)
    {
        float c, a1, a2, a3, b1, b2;

        for (int i = 0; i < data.Length; i++)
        {
            // Create a random amplitude value
            double currentRand  = _rand1.NextDouble();
            float amplitudeRand = (float)(currentRand * 2.0f - 1.0f) * _rampInAmount;
            amplitudeRand /= 2.0f;

            // Phase over a few seconds, phase goes from 0 - 2PI, so wrap the value
            float randRange = Mathf.Lerp(-1.0f, 1.0f, (float)currentRand);
            _phase += 1.0f / _samplerate;
            _phase += randRange * 200.0f / _samplerate;
            _phase %= (Mathf.PI * 2.0f);

            float interpolator = (Mathf.Sin(_phase) + 1.0f) / 2.0f;
            _frequency = Mathf.Lerp(100, 200, interpolator);

            // Low pass filter
            c = 1.0f / (float)Math.Tan(Math.PI * _frequency / _samplerate);
            a1 = 1.0f / (1.0f + _resonance * c + c * c);
            a2 = 2f * a1;
            a3 = a1;
            b1 = 2.0f * (1.0f - c * c) * a1;
            b2 = (1.0f - _resonance * c + c * c) * a1;

            float newOutput = a1 * amplitudeRand + a2 * this._inputHistory[0] + a3 * this._inputHistory[1] - b1 * this._outputHistory[0] - b2 * this._outputHistory[1];

            this._inputHistory[1] = this._inputHistory[0];
            this._inputHistory[0] = amplitudeRand;

            this._outputHistory[2] = this._outputHistory[1];
            this._outputHistory[1] = this._outputHistory[0];
            this._outputHistory[0] = newOutput;

            data[i] = newOutput;
        }
    }
}

It’s pretty gross, doesn’t work in webgl, and is probably very expensive.
But if you’re a Unity user it might be fun to throw in a project to check it out πŸ™‚
I take no responsibility for exploded speakers / headphones…

With that running, you can mess with the stereo pan and pitch on the Audio Source, for countless hours of entertainment.

Back to Chops

When testing out Audio in a hip file, I could only get it to play if I use the “Scrub” tab in the audio panel.
You need to point it to a chopnet, and make sure that the chopnet is exporting at least one channel:

Audio panel settings

You should also make sure that the values you output are clamped between -1 and 1, otherwise you’ll get audio popping nastiness.

The CHOP network I’m using to generate the audio looks like this:

Audio generation chopnet

What I’d set up in Unity was a channel of noise that is run through a High Pass filter with a varying Cutoff amount.
I tried to do exactly the same thing in Houdini: Created a noise chop, a Pass Filter chop, and in the Cutoff field sample the value of the noise chop using an expression:

chop("chan1")

I was hoping that the whole waveform would be modified so I could visualize the results in Motion FX View, but what I found instead is that the waveform was modified by the current frames noise value in Motion FX.
With my limited knowledge of CHOPs, it’s hard to describe what I mean by that, so here’s gif of scrubbing frames:

Scrubbing through frames with high pass

It’s likely that it would still have given me the result I wanted, but having the wave form jump around like that, and not being able to properly visualize it was pretty annoying.

So, instead of creating noise to vary the high pass cutoff, I instead created a bunch of noise channels, and gave each of them their own high pass cutoff, then I blend those together (more on that part later).

In my Pass Filter, I created two new parameters (using Edit Parameter Interface) that I refence from a few different nodes:

Custom params on high pass filter

Through trial and error, I found that low cutoff values from 0.3 to 0.6 gave me what I want, so I use an expression to filter each of the channels with a cutoff in that range, based on the channel ID:

ch("baseCutoff") + (fit($C/(ch("passBands")-1), 0, 1, 0.2, 0.5))

The baseCutoff is sort of redundant, it could just be built into the “fit” range, but I was previously using it in other nodes too, and never got around to removing it πŸ™‚

I’m using the “passBands” parameter in that expression, but I also use it in the Channel creation in the noise CHOP:

Noise channel creation from pulse bands param

It probably would have been more sensible just to hard code the number of channels here, and then just count how many channels I have further downstream, but this works fine πŸ™‚

In the Transform tab of the noise, I’m using $C in the X component of the Translate, to offset the noise patterns, so each channel is a bit different. In hindsight, using $C in the “seed” gives a better result.

So now I have some channels of filtered noise!
I’ll keep passBands to 5 for the rest of this post, to keep it easy to visualize:

Filter noise, 5 bands

Combining noise and pulse

The top right part of the CHOP network is importing the pulse value that I set up earlier.

Geometry import to chops

I’m importing a single point, and the pulse value was copied into Position, as mentioned earlier.
I’m deleting the Y and Z channels, since they are the same as X, normalizing the data using Limit, moving the data into the 0-1 range, and then lagging and filtering to soften out the data a bit.

Here is what the pulse looks like before and after the smoothing:

Pulse attribute channel

So this pulse channel is tx0, and I merge that in with all the noise channels (chan1-x).

I want to use the value of this pulse channel to blend in the noise channels I previously created.
So, for example, if I have 4 bands: between 0 and 0.25 pulse value I want to use band 1 noise, between 0.25 and 0.5 I’ll use band 2, 0.5 and 0.75 use band 3, etc.

I didn’t want a hard step between bands, so I’m overlapping them, and in the end I found that having quite a wide blend worked well (I’m always getting a little of all noise channels, with one channel dominant).

This all happens in the vopchop_combineNoiseAndPulse:

vop: Combine Noise And Pulse

The left side of the network is working out the band midpoint ((1 / number of channels) * 0.5 * current channel number).

The middle section gets the current pulse value, and finds how far away that value is from the band midpoint for each channel.

If I just output the value of that fit node, and reduce the overlap, hopefully you can see what this is doing a little more obviously:

Motion view of channel blending

As the red line increases, it goes through the midpoints of each noise band, and the corresponding noise channel increases in intensity.

Since I’ve lowered the overlap for this image, there are actually gaps between bands, so there are certain values of the pulse that will result in no noise at all, but that’s just to make the image clearer.

The rest of the network is multiplying the pulse value with the noise channel value, and I end up with this:

Multiplied noise and pulse value

With that done, I delete the pulse channel, and I add all the noise channels together.

The last thing I’m doing is outputting the noise in stereo, and for that I’m just arbitrarily panning the sound between two channels using noise.

I create two new stereo noise channels together, and then use a vop to flip the second channel:

Noise channel flip

So I end up with this:

Stereo balance waveform

I multiply this back on the original noise:

StereoNoise

There’s a few other little adjustments in there, like making sure the amplitude is between -1 and 1, etc.

Also, I ramp in and out the audio over the first and last 20 frames:

RampInAndOut

And that’s it!

Hopefully you’ve enjoyed this series breaking down the squab video and my adventures in CHOPs. I can’t imagine I’ll use it for audio again, but it was definitely a fun experiment.

Today is Houdini 17 release day, so I can’t wait to dig in, and play around with all the new interesting features, perhaps there will be some vellum sims for future blog posts πŸ™‚

Chopped Squabs – Pt 3

 

Last post, I talked about the tubers, their animation and creation of the pulse attributes.
This post will be about the bulbs on the end of the tubers, the pyro and particles setup.

Great balls of pyro

Since you don’t really get a close look at the tuber bulbs, considering the lighting and pyro cover them quite a bit, all of the bulbs have the same geometry.

Here is what the network looks like:

Bulb geometry network

This is a rather silly thing to do procedurally, I should have just sculpted something for it. Because problem solving in Houdini is fun, I tend to do everything in my home projects procedurally, but there are definitely times where that is a) overkill and b) ends up with a worse result.

Anyway, I’m starting out with a sphere:

  • Jittering the points a bit
  • Extruding the faces (to inset them)
  • Shrinking the extruded faces using the Primitive SOP on the group created by the extrude
  • Extruding again
  • Subdividing

That gives me this:

Animation of bulb geometry from primitives

I’m then boolean-ing a torus on one side, which will be where the tuber connects to the bulb.
I didn’t really need to Boolean that, since just merging the two objects and then converting to VDB and back gives me pretty much the same result, which is this:

Bulb vdb geo

Then I remesh it, and apply some noise in an Attribute VOP using the Displace Along Normal.

Before the Attribute VOP, I’ve set up a “pores” attribute, which is set to 1 on the extruded inside faces, and 0 everywhere else.
The VOP adds some breakup to the pores attribute, which is eventually used for emissive glow:

Attribute VOP for displacement and emissive

There was a very scientific process here of “add noise nodes until you get sick of looking at it”.

Here is the result:

Bulb with displacement

Looks ok if you don’t put much light on it, and you cover it with smoke πŸ™‚

Spores / smoke

I wanted the smoke to look like a cloud of spores, sort of like a fungus sort of thing, with some larger spores created as particles that glow a bit.

The source of the smoke is geometry that has a non zero “pulse” attribute (that I set up in earlier posts), but I also just add a little bit of density from the pores on the tuber bulbs all the time.

So on any given frame I’m converting the following geometry to a smoke source:

Pores and tuber geo for smoke source

The resulting smoke source looks like this:

Smoke source animation

I also copy some metaballs onto points scattered on this geometry, which I’m using in a to push the smoke around:

Magnet force metaballs

I’m feeding that metaball geometry into the magnet force in a SOP Geometry node:

DOP network for particles and pyro

I’m not going to break down the dynamics network much, it is mostly setup with shelf tools and minimal changes.

With the forces and collisions with tuber geometry, the smoke simulation looks like this:

Smoke viewport flipbook

Particles

The geometry for the tubers and bulbs is also used to create spawn points for emissive particles:

Particle spawn network

Kind of messy, but on the right I’m capping the tuber sections that I showed before, remeshing them by converting to vdb, smoothing then converting back to polygons, scattering some points, adding velocity and some identifying attributes.

On the left side, I’m scattering points on the bulbs when the density (pulse value) is high.

In both cases, I’m setting particle velocity to a constant value * @N, so they move away from the surface, and faster from the bulbs than the tubers.

There are quite a lot of them, but in the final render the emissive falls off pretty quickly, so you don’t see them all that much:

Animation of generated particles

That’s it for this post!

Still to come: Breaking down creating audio in CHOPs.

 

 

Chopped Squabs – Pt 2

 

Last post, I talked about the motion of the squab, and the points created for the tubers sprouting from its back.

This post will be about the creating the tuber geometry and motion.

A series of tubes

Here’s the tuber network:

Houdini network for tuber creation
(Click to expand the image)

The Object Merge at the top is bringing in the tuber start points, which I talked a bit about last post.

I’m pushing those points along their normals by some random amount, to find the end points.
I could have done this with a Peak SOP, but wrangles are far more fun!

float length = fit(rand(@ptnum), 0, 1, 1.3, 2.2);

@P += @N*length;

Sometimes I just find it a lot quicker to do things in VEX, especially when I want to do value randomization within a range like this.

The points chopnet, near the top, is straight out of the “Introduction To CHOPs” by Ari Danesh. I highly recommend watching that series for more detailed information on CHOPs.

The chopnet takes the end point positions, adds noise, uses spring and lag CHOPs to delay the motion of the end points so they feel like they are dragging behind a bit:

The CHOPnet for tuber end points

After that, I create a polygon between each end and start point using an Add sop:

Add sop to create tuber lines

I’m then using Refine to add a bunch of in-between points.

In the middle right of the network, there are two network boxes that are used to add more lag to the centre of each wire, and also to create a pulse that animates down the wires.

When the pulse gets to the bulb at the end of the tuber, it emits a burst of particles and smoke, but more on that in a later post.

Here is the chopnet that handles the pulse, the lag on the centre of the wires, and a slightly modified version of the pulse value that is transferred onto the bulbs:

Chopnet that creates wire centre movement and pulse attributes

The wire lag is pretty simple, I’m just adding some more lag to the movement (which I latter mask out towards both ends of the wire so it just affects the centre).

The pulse source is a little more interesting.
Before this network, I’ve already created the pulse attribute, initialized to 0. I’ve also created a “class” attribute using connectivity, giving each wire primitive it’s own id.

When I import the geometry, I’m using the class attribute in the field “organize by Attribute”:

Class used to create a channel per wire

This creates a channel for each wire.

I also have a Wave CHOP with the type set to “ramp”, and in this CHOP I’m referencing the number of channels created by the geometry import, and then using that channel id to vary the period and phase.

Here are the wave settings:

Pulse source chop Wave waveform settings

And here is where I set up the Channels to reference the Geometry CHOP:

Channel settings for Wave Chop

To visualize how this is creating periodic pulses, it’s easier if I add a 0-1 clamp on the channels and remove a bunch of channels:

Wave CHOP clamped 0-1, most channels removed

So hopefully this shows that each wire gets a pulse value that is usually 0, but at some point in the animation might slowly ramp up to 1, and then immediately drop to 0.

To demonstrate what we have so far, I’ve put a polywire on the wires to thicken them out, and coloured the pulse red so it’s easier to see:

NoodleFight

It’s also sped up because I’m dropping most of the frames out of the gif, but you get the idea πŸ™‚

The “Pulse Source Bulbs” section of the chopnet is a copy of the pulse, but with some lag applied (so the pulse lasts longer), and multiplied up a bit.

Tuber geometry

The remaining part of the network is for creating the tuber geometry, here is that section of the network zoomed in from the screenshot early in this post:

Tuber geometry creation network section

I’m creating the geometry around a time shifted version of the wires (back to the first frame), and then using a lattice to deform the geometry each frame to the animated wires.

Tuber cross-section

By triangulating a bunch of scattered points, dividing them with “Compute Dual” enabled, convert them to nurbs and closing them, I get a bunch of circle-ish shapes packed together.
There are probably better ways to do this, but it created a cross section I liked once I’d deleted the outside shapes that were distorted:

Splines created for gemetry of tuber cross section

To kill those exterior shapes, I used a Delete SOP with an expression that removes any primitive with a centre that was a certain distance from the centre of the original circle:

length($CEX, $CEY, $CEZ) > (ch(“../circle4/radx”) * 0.6)

This cross-section is then run up the wire with a Sweep:

Geometry for a single tuber

The Sweep SOP is in a foreach, and there are a few different attributes that I’m varying for each of the wires, such as the number of twists, the scale of the cross-section.

The twisting took a little bit of working out, but the Sweep sop will orient the cross section towards the Up attribute of each point.
I already have an Up attribute, created using “Add Edge Force” with the old Point SOP, it points along the length of the wire.

The normals are pointing out from the wire already, so I rotate the normals around the up vector along the length of the wire:

Rotated wire normals

The Sweep SOP expects the Up vector to be pointing out from the wire, so I swap the Normal and Up in a wrangle before the Sweep.

So, now I have the resting tuber geometry:

Tubers geometry undeformed

To skin these straight tubers to the bent wires, I use a Lattice SOP:

Tuber lattice geo

I Lattice each wire separately, because sometimes they get quite close to each other.
Last thing I do with the tubers is push the normals out a bit as the pulse runs through.

I already have an attribute that increases from 0 – 1 along each tuber, called “rootToTip”.
On the wrangle, I added a ramp parameter that describes the width of the tuber along the length (so the tuber starts and ends flared out a bit).

The ramp value for the tuber shape is fit into a range I like through trial and error, and I add the pulse amount to it, then use that to push the points out along their normals

This is the wrangle with the ramp parameter for the shape:

Tuber width wrangle

@tuberProfile = chramp("profile", @rootToTip, 0);

float tuberWidth = fit(@tuberProfile, 0, 1, 0.0, .04);
float pulseBulge = ((@pulse)*0.1*(1-@rootToTip));

@P = @P + (@N * (tuberWidth + pulseBulge));

This gives me the pulse bulge for the tubers:

Tuber pulse bulge

That does it for the tubers!

In future posts, I’ll cover the stalk bulbs, particles, pyro, rendering and audio.

 

Gears of Washroom – Pt 6

Last post was all about materials, this time around I’ll be talking rendering settings and lighting.

Rendering choices

Being one of my first renders in Houdini, I made lots of mistakes and probably made lots of poor decisions in the final render.

I experimented a bit with Renderman in Houdini, but after taking quite some time to get it set up properly, enable all the not so obvious settings for subdivision, etc, I decided this probably wasn’t the project for it.

I ended up using Mantra Physically Based Rendering, and chose to render at 1080p, 48 fps. Well… I actually rendered at 60 fps, but realized that I didn’t like the timing very much when I’d finished, and 48 fps looked better.
This is something I should have caught a lot earlier πŸ™‚

Scene Lighting

I wanted two light sources: an area light in the roof, and the explosion itself.
Both of these I just dumped in from the shelf tools.

The explosion lighting is generated from a Volume Light, which I pointed at my Pyro sim.

I was having quite a lot of flickering from the volume light, though.
I suspected that when the volume was too close to the walls, and probably a bit too rough.

To solve this, I messed around with the volume a bit before using it at a light source:

VolumeLight

So I import the sim, drop the resolution, blur it, then fade it out near the wall with a wrangle:

VolumeTrim

For the sake of the gif, I split the wall fading and density drop into separate steps, but I’m doing both those things at once in the Wrangle:

@density -= 0.12;
float scale = fit(@P.x, -75, -40, 0, .2);
@density *= scale;

So between a X value of -75 (just in front of the wall) to -40, I’m fading the volume density up from 0 to 0.2.

After that, I had no issues with flickering, and the volume lighting looked the way I wanted it!

VolumeLightFrame.png

Render time!

I think that’s it all covered!

Some stats, in case you’re interested:

  • Explosion fluid and pyro took 3 hours to sim
  • Close up bubbling fluid took about 1 hour to sim
  • Miscellaneous other RBD sims, caches, etc, about 2 hours
  • 184gb of simulation and cache data for the scene
  • Frame render times between 10 – 25 minutes each.
  • Full animation took about 154 hours to render.
    Plus probably another 40-50 hours of mistakes.
  • 12gb of rendered frames

My PC is a i7-5930k with a NVidia GeForce 970.

Hopefully I’ve covered everything that people might be interested in, but if there’s anything I’ve glossed over, feel free to ask questions in the comments πŸ™‚

Gears of Washroom – Pt 5

Last post I went through all the setup for the bubble sim, now for lighting, rendering, materials, fun stuff!

Scene materials

I talked about the texture creation in the first post, but there are also quite a lot of materials in the scene that are just procedural Houdini PBR materials.

Materials.png

Most of these are not very exciting, they are either straight out of the material palette, or they are only modified a little from those samples.

The top four are a little more interesting, though (purplePaint, whiteWalls, wood and floorTiles), because they have some material effects that are driven from the simulation data in the scene.

If you squint, you might notice that the walls and wood shelf get wet after the grenades explode, and there are scorch marks left on the walls as well.

Here is a shot with the smoke turned off, to make these effects obvious:

WetAndScorched.png

Scorch setup

To create the scorch marks in a material, I first needed some volume data to feed it.
I could read the current temperature of the simulation, but that dissipates over a few frames, so the scorch marks would also disappear.

The solution I came up with was to generate a new low resolution volume that keeps track of the maximum value of temperature per voxel, over the life of the simulation.

PyroMaxTemp

To start out with, I import the temperature field from the full Pyro sim, here is a visualization of that from about 2/3rds the way through the sim:

FullSimSmoke

I only need the back half of that, and I’m happy for it to be low resolution, so I resample and blur it:

SimplifiedSmoke

Great! That’s one frame of temperature data, but I want the maximum temperature that we’ve had in each voxel so far.

The easiest way I could think of doing this was using a solver, and merging the current frame volume with the volume from the previous frame, using a volume merge set to “Maximum”:

VolumeMaxSolver

And the result I get from this:

SimplifiedSmokeMax

So that’s the accumulated max temperature of the volume from the current frames, and all the frames before it!

Scorch in material

Back in the whiteWalls material, I need to read in this volume data, and use it to create the scorch mark.

Here is an overview of the white walls material:

whiteWallsMaterial.png

Both the wetness and scorch effects are only modifying two parameters: Roughness and Base Colour. Both effects darken the base colour of the material, but the scorch makes the material more rough and the wetness less rough.

For example, the material has a roughness of 0.55 when not modified, 0.92 when scorched and 0.043 when fully wet.

The burnScorch subnet over on the left exposes a few different outputs, these are all just different types of noises that get blended together. I probably could have just output one value, and kept the Scorch network box in the above screenshot a lot simpler.

Anyway, diving in to the burnScorch subnet:

BurnScorchSubnet.png
(Click for larger image)

One thing I should mention straight up: You’ll notice that the filename for the volume sample is exposed as a subnet input. I was getting errors if I didn’t do that, not entirely sure why!

The position attribute in the Material context is not in world space, so you’ll notice I’m doing a Transform on it, which transforms from “Current” to “World”.
If you don’t do that, and just use the volume sample straight up, you’ll have noise that crawls across the scene as the camera moves.
I found that out the hard way, 10 hours of rendering later.

Anyway, I’m sampling the maximum temperature volume that I saved out previous, and fitting it into a few different value ranges, then feeding those values into the Position and in one case Frequency of some turbulence noise nodes.

The frequency one is interesting, because it was totally a mistake, but it gave me a cool swirly pattern:

SwirlyNoise.png

When combined with all the other noise, I really liked the swirls, so it was a happy accident πŸ™‚

That’s really it for the scorch marks! Just messing about with different noise combinations until I liked the look.

I made it work for the white walls first, then copied it in to the purple walls and wood materials.

Wetness setup

Similar concept to what I did for the temperature, I wanted to work out which surfaces had come in contact with water, and save out that information for use in the material.

WetnessSetup

On the left side, I import the scene geometry, and scatter points on it (density didn’t matter to me too much, because I’m breaking up the data with noise in the material anyway):

WetnessPoints

The points are coloured black.

On the right side, I import the fluid, and colour the points white:

WetnessPointsSim

Then I transfer the colour from the fluid points onto the scatter points, and that gives me the points in the current frame that are wet!

As before, I’m using a solver to get the wetness from the previous frame, and max it with the current frame.

WrangleWetness

In this case, I’m doing it just on the red channel, because it means wetness from the current frame is white, and from the previous accumulated frames is red.
It just makes it nice to visualize:

WetnessSolver

I delete all the points that are black, and then cache out the remaining points, ready to use in the material!

Wetness in material

I showed the high level material with the wetness before, here is the internals of the subnet_wetness:

subnet_wetness.png
(Click for larger image)

So I’m opening the wetness point file, finding all points around the current shading point (which has been transformed into world space, like before).
For all wetness points that are within a radius of 7 centimetres, I get the distance between the wetness point and the current shading point, and use that to weight the red channel of the colour of that point.
I average this for all the points that were in the search radius.

In the loop, you’ll notice I’m adding up a count variable, but I worked out later that I could have used Point Cloud Num Found instead of doing my own count. Oh well πŸ™‚

I take the sampled wetness, and feed it into a noise node, and then I’m basically done!

If you want an idea of what the point sampled wetness looks like before feeding it through noise, here is what it looks like if I bypass the noise and feed it straight into baseColour for the white walls (white is wet, black is dry):

WetnessPointSample.png

Next up, Mantra rendering setup and lighting, should be a rather short post to wrap up with πŸ™‚

Gears of Washroom – Pt 4

Last post was all about Pyro and FLIP preparation for the explosion.
I’m going to go off on a bit of a tangent, and talk about the bubbling fluid sim at the start of the animation!

16.5 to the rescue

Houdini 16.5 came out just as I was starting to build this scene, and they addedΒ air incompressibility to FLIP.

I’d been doing some experiments trying to make bubbling mud in a separate scene:

FlippedOut

As you can probably tell, I didn’t have a great deal of luck πŸ˜‰

Fluid spawning setup

BubbleFluid

The network starts with importing the glass sections of the grenades.

There are three outputs: The geometry to spawn fluid from, a volume used for collision and volumes for fluid sinks.

The fluid spawn and collision is pretty straightforward, I’m using the Peak SOP to shrink the glass down, then the Fluid Source SOP to output an SDF for the collision:

CollisionVolume.png

^ The collision volume is pretty πŸ™‚

The sink volumes are a little more interesting.
The Copy SOP is copying onto some points I’ve spawned.
The points are scattered on the bottom part of the glass:

CopyTemplate

The geometry that is getting copied onto those points is a sphere that scales up and down over a certain frame range.

In the Copy SOP I’m stamping the copy id for each point, and the sphere is scaled using an attribute wrangle:

float lifeSpan = 220;
float maxScale = 0.9;
float minScale = 0.4;
float randParticle = @copyID+(rand(@copyID)*.3);
float bubbleLife = 0.06;

float zeroToOneTime = (@Frame%lifeSpan)/lifeSpan;
float distanceFromTimePoint = abs(zeroToOneTime - randParticle);
float dftpZeroOne = fit(distanceFromTimePoint, 0, bubbleLife, 0, 1);

float scale = cos(dftpZeroOne * ($PI/2));
if (scale > 0) scale += minScale;
@P *= scale * maxScale;

This ended up being far more complicated than it needed to be, I was randomly adding to it until I got something I liked πŸ™‚

I could strip most of the lifespan stuff out entirely: I was originally using that so that each point could spawn spheres multiple times, but that ended up being too aggressive.

Anyway, for each point, there is a range of frames within the lifespan where the sphere is scaled up and down.

With a low lifespan, this is what I’m getting:

Bubbles

The spheres get converted to a sink volume, and is used to allow fluid to escape the sim.
Where the fluid escapes, bubbles are created!

This is another case where I used the shelf tools for sink volumes in another scene, had a look at what it produced, then recreated it here.
I really recommend doing that sort of thing, it can really help with iteration time when your scenes get complicated!

Fluid sim and trim

BubbleFlip

The flip solver is pretty straightforward!

The collision volumes are imported as source volumes, and passed in to velocity and post solve sourcing (again, I worked this setup out with shelf tools).

Air incompressibility is just a checkbox option:

BubbleFlipAir

That’s it for the sim, on to the surfacing network:

fluidCloseSurface

I had some issues when solving on lower settings, where I’d get a lot of fluid leaking out of the container.
To solve this, I’m importing the glass into this network, then converting to a VDB:

TrimVDB

Right under that, I use an attribute wrangle to copy “surface” from the VDB to the points, and I use that surface value to delete any outside points.

Here’s a mockup example (since my final sim only had very minor leaking), where I’ve manually moved a bunch of points outside the glass, and I’m colouring them with the surface value:

VDBCull

Now time to surface the fluid.

Usually I would just use the Particle Fluid Surface SOP to do this, but I tried a number of approaches with it, and I was always either losing the internal bubbles, or not getting the desired results, so I built the surface with some of the nodes that the Particle Fluid Surface SOP uses internally.

First of all, VDB from Particles, here is a cutaway of that:

VDBFromParticles

The surface is pretty rough, though!
I smooth it out with a VDB Smooth SDF:

VDBSmooth1

I didn’t want to smooth out the interior any further, but the exterior is still too rough.
Similar to how I was deleting points before, I use the glass to create another VDB, and I use that as the mask for a second VDB smooth:

VDBSmooth2

And that result, I was happy with!

You might have noticed by now that I’ve only been simulating one grenade in the fluid bubble sim.
I figured that since I was using heavy depth of field, I could get away with using the same fluid sim for all of the grenades, and just copy it onto each grenade:

CopyStampSim.png

To make the duplication less obvious, I simulated the fluid sim a bit over 60 frames more than I needed, and I used a time shift on the 2nd and 3rd grenade to offset the simulation, so the same bubbles aren’t coming up at exactly the same time.

The copy node stamps the GrenadeID, which the timeshift node uses:

TimeShiftStamp

And now we have bubbling grenade water sims!

RemeshedOffsetSims.png

(Re-meshed in this shot, because the render mesh is 1 million polygons :))

Still to come in this series of posts: Mantra rendering setup, lighting, materials, fun with scorching and wetting walls!

Gears of Washroom – Pt 3

Last post I talked about some of the RBD prep for the grenade explosions, now time to talk about Pyro and Flip prep!

Every Sim in its right place

All of the simulations (RBD, Pyro and FLIP) live in the same DOP network:

AutoDop_fluid
(Click on image to expand)

The network itself isn’t terribly exciting, it was mostly built using by testing out shelf tools in other files, then I’d reconstruct them to try and understand them a bit better. I might dive into it a little later in this blog series.

Now, on to the networks that feed this beast!

Explosion fuel and force

FuelAndForce
(Click on image to expand)

In this network, I’m importing the grenade geometry and using it as a basis for generating fuel for the Pyro simulation, and also forces that affect the FLIP and RBD sims.
Sorry about the sloppy naming and layout of this one, it’s not as readable as it should be.

Splodey time!

Early in the grenades network, I created a primitive attribute called “splodeyTime”, set about 10 frames apart for each grenade.
This attribute drives the timing of all of the explosion related effects.

For example, I use it to set the active time for the RBD pieces in an attribute wrangle:

if (@Frame > (@splodeyTime-2)) i@active = 1;

You can also see it in the first loop in my Explosion Fuel and Force network:

FuelAndForce_foreach1

Here, I’m doing some similar promotion tricks like in the last post:

  • Iterating over pieces (in this case each piece is a grenade base)
  • Promoting splodeyTime up to a detail attribute.
  • Creating a single new point in the centre of the current grenade, using the Add SOP:
    AddPoint
  • Deleting all geometry except for that point.
  • Also delete the point, if:
    • Current frame number < (splodeyTime – 2)
    • Current frame number > (splodeyTime + 2)

So at the end of the foreach, each grenade will be represented by a single point, that will only exist for 4 frames around the splodeyTime of that grenade.

Great metaballs of fire!

The first thing I do with those points, is generate a cigar of metaballs:

MetaballCigar
I’m using this rather dubious looking thing to break constraints on the RBD (in the Remove Broken SOP solver in the DOP network diagram at the top of the post).

There are two more branches that come off the metaball cigar.

MetaballAndPoints

The left branch just inflates each of the metaballs, it is fed into a Magnet Force, and is only used on the RBD objects, to blow the grenade fragments apart.

The right branch is very similar, except after a slightly more aggressive inflation, I’m generating points from the volume of the metaballs, and adding a velocity attribute to them.

The points, that you can see in the image, are used to drive force in the water that explodes out of the grenades, in the FLIP fluid.

One thing I wanted to do here was to have fluid that is near the wall shoot out pretty straight, but then add a bit more randomness when the fluid is near the chain end of the grenade.

Not the best way to visualize it, but these are the force vectors for that effect:

FluidForceVectors

To start out, I set the velocity attribute to {10,0,0} in an attribute wrangle.

Then, the randomness is achieved in the creatively named attribvop1 in the right branch a few images back, and here is what that looks like:

ForceVectorVOP.png

Stepping through this network a bit…
I’m creating a new noisey velocity, and mixing that with the existing straight down X velocity.

I always want the noisey velocity to be positive in X, but to be much stronger in Y and Z, hence the splitting out of the X component and the abs after the bottom noise.

The mix node uses the X position of the point to work out how much of the straight velocity to use versus the noisey velocity.
So between 0 – 5 in X, I’m blending the two. When the position is greater than 5, we have 100% of the noisey vector.

I messed about with lots of different vector / noise patterns, but I liked the movement this gave my fluid, so I stuck with this πŸ™‚

Fuel

Backing up a bit, there’s one more branch of this network, and it creates the fuel for my Pyro simulation:

FuelNetwork

I’m copying a sphere onto the explosion points, and then usingΒ a fluid source SOP to create a volume for fuel, and using the built in noise settings to get some variation.

The sphere itself is getting scaled by splodeyTime, using copy stamping in uniform scale:

fit((stamp(“../copy2/”, “splodeyTime”, 1) – $F), -2, 2, 4, 10)

I also move it forward each frame using an attribute wrangle:

@P.x += ((@Frame+4) – @splodeyTime)*2;

This is what the fuel looks like over the few frames it is generated:

Fuel

Flipping glass

One last thing for simulation prep, I’m using the glass sections of the grenade as the fluid source of the exploding Flip sim.

Again, this uses the splodeyTime to delete the glass when it is outside of the frame range, so I’m creating the surface volume and particles just for a few frames:

FlipParticlesAndSurface

That’s pretty much each for simulation prep, at least for the main explosion!

Still to come, the bubbling water at the start, and all things materials and rendering.

Gears of Washroom – Pt 2

Last post I talked about the static scene setup for the above video, this post focuses on the RBD preparation and setup for the explosion.

It’s pretty much Houdini from this post onwards, here is the top level layout of my scene:

SceneView

Complete with a “TODO” section on the left that I forgot to clear out πŸ˜‰

The grenades nodes in the middle are where most of the prep work for the RBD is done, so the internals of those will be the focus for this post.

Import once, fracture twice

Although I probably should have just manually rotated and placed the chains, I decided to do an RBD sim to hang the chains in place, and over the wooden shelf.

GrenadesSimmed

I couldn’t get Concave collision to work with the chains.
They would sim ok for a handful of frames, and then generally start intersecting, or explode apart, or do any number of annoying things.

I had a chat to one of our VFX Artists at work, Mandy Morland, who does all sorts of cool things in Houdini.
She suggested I could fracture the chains, and then simulate the chain links as clusters of Convex Hulls, which was a super awesome idea.

As long as each new fracture piece in a single chain link has the same value in the name attribute, it can be assembled and passed to DOPs as a single object.

PresimFracturing_Chain

So, for example, all of the highlighted sections above would have the same name attribute value “piece2”.

However, the name attribute is created before the fracture, and the fracture creates new internal faces which will not have the correct name attribute!

ChainNewFaces

The above image visualises this problem with colour as a primitive attribute.
The pieces have a green colour applied to them before fracture, and after fracture the internal pieces can’t know what colour to have, so they get initialized to default (I’ve then coloured them blue to make it more obvious).

To avoid this issue, I’m looping over chain pieces, promoting the piece attribute to Detail, then fracturing the chain, then promoting the piece attribute back to Primitive:

PresimFracturing
(The piece attribute is just temp storage for Name).

This is a Houdini trick I do a lot of, in general form:

  • Loop over geometry pieces that I want to share an attribute value
  • At the start of the loop, promote an attribute to Detail.
    Only the current piece of geometry will be affected by this, Detail is essentially per piece at this point.
  • Do the things I want to do in the loop that generate new geometry, or invalidate the attribute in some way I don’t want
  • Promote the attribute back from Detail to whatever context it was before (primitive, point, etc)

Side note on fracturing in a loop…

Fracturing inside a loop isn’t the best idea, it’s going to be slower than running fracture once on a single geometry object.

When fracturing geometry that represents combined objects, and that has per-object attributes you want to keep (eg: a string attribute called “chainID”), a better way might be:

  • Fracture the geometry (new faces will have a blank chainID).
  • Use Connectivity on Primitives to get a attribute (let’s call it Bob)
  • Do a foreach loop on Bob:
    • Promote chainID to Detail (using “maximum”)
    • Promote chainID back to Primitive.

Very similar, but at least it gets the fracture out of the loop.

In the end, node network performance wasn’t too big an issue for me: I go crazy with cache nodes (184 GB for this project), so I only had to wait for it to finish once, and it was still pretty quick for such simple geometry.

Hanging out

With that done, the chains were ready for the hanging sim, I just needed to make sure the bullet sim was creating convex hulls for connected prims:

ChainsConvex

ChainSettle

Back in my Grenades_fractured network, I’m importing the base un-fractured geometry, and then importing the hanging simulation with a dopimport node, with an Import Style set to “Transform Input Geometry”, and using timeshift to pick a frame I like.

So now I have the base un-fractured geometry in a good starting pose:

PosedGrenades

With the grenades now in the right pose, I fracture them again, ready to be blown up!
They don’t get made active straight away, but more on that, explosions and fluids in the next blog post πŸ™‚