Frame Blending with Motion Vectors

Introduction

This is my variation of the frame blending technique first developed by Guerrilla Games for Killzone 2, you can find a link to their publication and other resources at the bottom of the post. The aim is to extend the utility of animated textures by distorting them with motion vectors to procedurally generate the inbetween frames. This comes at a cost of shader complexity and texture memory but the results are worth it, especially for special cinematic moments.

Here’s an example of what you can expect to achieve with the technique described in this tutorial:

Those of you familiar with video editing and compositing might liken this technique to what a plugin called Twixtor from RevisionFX does, it’s essentially the same thing but in our case the motion vectors aren’t generated on the fly, we’ll have to create them manually.

For this breakdown I’ll use Unreal 4, FumeFx for 3dsMax and After Effects with a RevisionFX plugin called “Motion Vectors: Create” that comes with the pro edition of Twixtor. You should be able to achieve the same with any other software that can output normalized motion vector data but your results may vary. The end result of this technique depends entirely on the motion vector data so this can be the most arduous step to get right and it might require some experimentation.
 

Motion vector data

To render motion vectors out of FumeFX you’ll need to make sure you’re outputting the velocity channel before you simulate. Additionally you’ll need to go to Rendering – Environment, select “FusionWorks Renderer” and under parameters check “Create Channels” and “Image Motion Blur” or the velocity vectors won’t render. Afterwards you can render them as a separate render pass called “FusionWorks Velocity”. You can play with the “max velocity” parameter to make sure you’re not hitting 0 or 1 if you check the pixel values, you want to stay midrange otherwise you’re not getting a full range of motion data.

When you bring the velocity pass to After Effects (or other compositing software) you might have to modify the gamma. If you’re not sure, one way to check is to remove the blue channel and make sure the empty area has the following values:

 
 
 
 
 
It’s worth pointing out you might have to invert x or y components of your motion vector to make sure Unreal interprets it properly.

Alongside motion vectors you’ll also want to render the base texture with an alpha channel along with any other texture passes you might have. This will be the final output and we’ll use the motion vector texture to drive it.

Use the alpha channel to stencil the motion vector texture, see image below. You can also blur it sligtly, this seems to help with frame blending later in Unreal.

Next I use the “Motion Vectors:Create” effect that comes with Twixtor pro to generate additional screen based motion vectors from the base texture. I found this step to be a requirement if I wanted to use a single motion vector texture. The main idea behind this is to generate some extra motion data beyond the range of our simulation, otherwise the pixels won’t distort beyond that edge. I also needed to invert the red channel to match the output of FumeFX.

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
You can see the result of the “Motion Vectors:Create” vectors I generated from the base texture on the left and FumeFX motion vector layer added on top of that on the right. Because I’ve stenciled and blured the FumeFX layer it blends nicely with the one on the left.

Below is an example of final textures you might end up with. I’m going with a 8×8 SubUV, so 64 frames, roughly 2 seconds at 30fps but we’ll be able to extend that more than 10 times while keeping the same perceived frame rate. In production you’ll probably go with a smaller amount of SubUVs to keep the texture size smaller. The motion vector texture can be scaled down significantly, I used a 512 for a 4096 base texture but it must be uncompressed in the engine, otherwise you’ll get a lot of artifacts. Also make sure to untick sRGB when importing to Unreal.

 

Shader

The shader is pretty straight forward. The main idea is to build custom SubUV cross-blending, similarly to what Particle SubUV expression does automatically but we’ll need to do it manually to control the next step. In addition to interpolating from one subimage to the next we’ll need to distort the current subimage pixels towards the position of the next and similarly distort the next one back towards the current one so they kind of meet in the middle. We’ll achieve this distortion using the motion vector texture we’ve made.

Here’s the completed shader network we’ll be creating followed by a breakdown:

SubUV Function:

As you can see we’re using a function in our shader, to get the SubUV funcionality. We’ll modify the existing one that comes with Unreal 4 so it outputs texture coordinates instead of an RGB channel, see image below.

SubUVCoordinates_Function:

Frame control:

We’ll use two sets of SubUV functions we created, one for “Current” and one for “Next” frame. For testing purposes we can drive the animation with a time expression as seen below, later we’ll want to use a particle color channel or a dynamic parameter to drive it in Cascade. It’s important to always add 1 to the “Next” frame value to offset the SubUVs by one frame.

Move to float range:

We’ll plug the SubUV function output into our motion vector texture. Following up we’ll first use a component mask to mask out the z component of the vector, we’re not using it. Next we multiply the map value by 2 and then subtract 1 to move it into the -1.0 to +1.0 float range, this will keep the distortion centered. We’ll do this for both branches of the shader, current and next.

Current Frame:

After getting the motion vector in float range we’ll use the frac to figure out the percentage of where in the frame we currently are and multiply the distortion amount based on that value, there will be no distortion at the start of the frame and full distortion by the end. Afterwards we’ll subtract the output from the SubUV function to add the distortion to the base texture.

Next Frame:

Very similarly, after getting the motion vector in float range we’ll use the inverted frac to figure out the percentage of where in the frame we currently are but this time backwards so there will be full distortion at the start of the frame and no distortion by the end. Afterwards we’ll add the output to the SubUV function to subtrack the distortion from the base texture, reversing the process.

The scalar parameter “Distortion_Strength” will control the amount of distortion as described above. This is an ambiguous value you’ll have to eyeball, it has a direct correlation with the SubUV size and speed of your texture, if you have fewer frames, you’ll usually want this value to be higher, UVs will need to get distorted further to catch up to the next frame. I generally found myself using very low values with this setup.

Cross Blend:

Finally we’ll use a linear interpolate expression of both the Current and Next frame with a frac of the time expression for the alpha to cross-blend them together as we flow through the frame. You’ll need to do this for every texture channel you’re using.

It can be difficult to grasp the logic but to put it simply what happens is, the current frame starts as it is, then it gets more and more distorted towards the position of the next frame, similarly but inverted, the next frame starts fully distorted back towards the current frame and then goes forward to its original position. When we finally lerp the two they cross-fade to each other to produce a seamless fluid motion. This is happening continuously frame to frame.
 

Summary

I see this being especially useful for large scale effects, slowmos and sequences where you need the effects to drive the shot and evolve naturally. Most of the time however,  visual effects in games tend to have a short lived screen presence for gameplay reasons so regular animation ranges and cross-fading work just fine, this definitely doesn’t replace the existing workflow but it’s a very nice addition to any artist’s arsenal.

The main difference between my approach and the technique Guerrilla Games described is in the motion vectors. They apparently use 2 motion vector textures, one is made specifically for blending to the next frame, and the other for blending back to the current one. As far as I can tell I’ve been able to reach the same visual fidelity with the benefit of using less texture memory, because I use a single motion vector texture. However, their technique might be more accurate when dealing with smaller SubUV textures and I believe they generate their vectors procedurally with Houdini, taking some of the manual labour out of the process. Ultimately I’m not familiar with their approach enough to make any final calls so don’t take this at face value.
 
Resources:
– http://www.guerrilla-games.com/publications.html
– http://www.revisionfx.com/products/twixtor/

Credits:
– Ryan DowlingSoka of Black Tusk for helping me figure this stuff out.
– Guerilla Games for inspiration.