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.

 

Chopped Squabs – Pt 1

 

The above video is a thing that wasn’t really supposed to be a project, but turned into one πŸ™‚

This series of blog posts will break down how I made it!

This first post will be some background on the project, and how I set up the movement of the squab and spawned the initial points for the tubers that are attached to it.

But why

This whole thing was a feature exploration of CHOPs that went off the rails a bit.

While working on another Houdini rendering project at home, I needed to dynamically re-parent an object half way through an animation.

Luckily, there is a shelf tool for that!

Parent blend shelf tool in Houdini

When I ran it, Houdini created a constraints network, and it seemed like nothing else had changed.
The object was suddenly dynamically parented at that frame, but there were no attributes, or connected nodes, or anything that I would expect to see generated.

Output constraints network from parent blend

Just a new network, and a reference to the constraints on the geometry node at the obj level.
So, magic basically?

Get to the chopper

I’ve never really used CHOP networks before, and it seemed like a good time to see how they work. Much like the other contexts in Houdini, there is an endless amount of learning and exploration that can be done here, but I started out watching the great “Introduction To CHOPs” videos by Ari Danesh (@particlekabob).

https://www.sidefx.com/tutorials/lesson-1-intro-to-chops/

My squab video was the result of me getting distracted and going off on a tangent while watching the training!

If you’re interested in CHOPs, I’d really recommend watching those videos. I’m not going to go very in depth over things that Ari already covered.

Setting up the squab

In case you’re not familiar with it, the “squab” is a test asset that ships with Houdini.

Houdini squab test asset

If you happen to know who made the squab, please let me know and I’ll edit this post with that info πŸ™‚

I decided I wanted to create tuber like growths off the side of it, with bulbous end parts that would emit spores, which travel up the tuber in a pulse.

The first network to dive into sets up the points on the squab that will spawn tubers, and also sets up the FEM (soft body) data for the squab.

Network for setting up squab and tubers and bulbs

I won’t go into details on the two bottom right network branches: These are selecting some points that are fixed for the FEM solver, and mostly standard nodes created by the FEM Organic Mass shelf tool.

The left “Tuber points” branch scatters some points on the squab body, taking the normals from the surface.
I didn’t want many tubers coming out towards the front of the squab, so I deleted points based off the magnitude of the Z component of the normal (a Z value of 1 means the point is facing straight forward in “squab space”, since the squab is aligned to the world Z).

Delete points by z component magnitude

The next issue is that some of the generated tubers would crash through the squab geometry.
I didn’t need to entirely solve this, since it’s a dark scene with enough going on to mask some intersections, but there were some pretty obvious bad ones:

Tuber intersection with squab geometry

The first thing I tried worked out well enough, which was to ray trace out from the tuber points and see if I hit any squab geometry. If I hit geometry, I delete the point.

It’s a little ugly, but here’s the VOP that collects the hits:

Ray trace VOP to exclude points that would intersect squab

There are two non-deterministic random nodes, and to force those into the loop body I’ve just exposed the “type” as a constant, and fed it into the loop. Makes no sense, might not be necessary, but it gets the random nodes into the loop body πŸ™‚
That’s part of what makes it messy.

Each iteration of the loop is a sample, the number of samples is a parameter exposed to the network.
I use a SampleSphere node to create sample points on a cone around the normal, with an angle of 45 degrees. I use the vector to those points as the raydir of an intersect node, and trace out from the point and see if I hit anything. I store the max of the hitprim into an attribute, and then I just delete any points where this “hitval” is greater than 0 (the default hitprim is -1).

Tuber geo with reduced points from ray trace intersection

You can see that running this pass removes quite a lot of invalid tubers, I didn’t mind it being too aggressive.
A smarter person would have done this non procedurally and just manually placed points, I probably need to start doing that more often πŸ™‚
Proceduralism for the sake of it is a great way to waste a lot of time…

Chops transforms

On the main network graph, there are three Transform nodes that I’ve coloured aqua (the ones toward the bottom of each of the three network branches), these are fed data from the “motionfx” CHOPs network.

I’m using random noise to move the body of the squab (and the attached tuber points) around.
The “Body Motion” is almost straight out of the tutorial by Ari Danesh, here’s what the “motionfx” network looks like:

Chops network for body and tuber points motion

First thing to note, the tuber points and chops body get animated in exactly the same way.
The top left of the graph I use channel nodes to create transforms for the tuber points and the squab body:

Channel creation for tuber points

Then I merge them together, and work with 6 channels for the rest of the network.
However, this is unnecessary!

I could have worked with a single set of data, and then made the export node write out to the multiple transforms. This is something I learnt from the tutorials after I’d implemented things this way πŸ™‚

Move yo body

The Body Motion group of nodes was largely trial and error: I added a sine wave to some noise, and added some filters and lags until the motion looked nice.

Not very scientific, but here’s what the waveform results of each of those nodes are:

Body motion waves and noises

That’s over 600 frames, btw.

And that gives me something like this:

Squab body movement without rotation gif

I wanted to get a sort of floating in water currents feel, and I was fairly happy with the result.

I also wanted the body to rotate a little with the movement.
I tried a few things here, such as a CHOPs lookat constraint, various VOP things.
In the end, I did a very hacky thing:

  • Took the position of the original waveform, and moved it one unit down in Y
  • Applied a lag to that, then subtracted it from the original position waveform
  • Multiply it by some constants
  • Rename it to be the rotation channels

If you imagine the squab body as a single animated point, this is like attaching another point a little below it, and attaching this new point to the first point using a spring.
Then, measuring the distance between the points, and using that distance as a rotation.

A bit of a weird way to do it, but it gets the job done! πŸ™‚

I’ve overexaggerated the rotation and lag in this gif just to make it obvious:

Squab rotate gif

In future posts, I will break down the creation of the tubers, some of the FX work, and also creating the sound in CHOPs!

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™‚