This plugin provides the means to extract optical flow data from Unreal Engine. It is based on ProfFan's optical flow demo.
Currently this plugin requires a patched custom-built Unreal Engine 4. Tested for versions 4.20 and 4.21.
You would need to get UE4 source code, apply OptiFlowUE4.patch and then compile the engine. To apply the patch, simply put OptiFlowUE4.patch into the root directory of the engine and run patch -p1 UE4.patch.
After compiling UE4, use it to create a new project or open an existing one, and then copy OptiFlow directory into Plugins directory (you may need to create it in the root of the project).
After that, you can use OptiFlow/Content/PostProcess/VelocityMaterial.uasset as a post process material. It has two parameters: VelocityBufferCoefficient and StaticObjectCoefficient, which control how quickly movement saturates colors for stationary/movable and static objects respectively.
Currently this plugin correctly displays optical flow data for movable and stationary UE4 actors, based on velocity frame buffer (also used by engine for motion blur). Static actors' optical flow may be estimated incorrectly.
Images below are taken using VelocityMaterial as a post processing material with VelocityBufferCoefficient set to 100, and StaticObjectCoefficient set to 20 (this is more than default to emphasize the effect). They are taken using UE4's default "Rolling Blueprint" example, which features a controllable rolling ball with camera attached to it. In our example, a camera is closer to the ball than default.
This image is taken by capturing a ball rolling to the right:
This image is taken by capturing a ball rolling forward along the wall to the left:
