warp

How the previous frame is bent on its way back, per pixel.

Null keeps the plain behaviour: the frame returns scaled and turned, which is all a whole picture can be put through in one go. A WarpSpec lets every pixel work out where it came from separately, which is what turns the feedback loop from an echo into a shape.

Declare it as a property rather than a getter. The shader behind it is compiled once and kept, and a getter would build a new one sixty times a second.