Feedback Loops
Like a SOP Solver in Houdini!
The setup is a little different but the same logic applies. A TOP feedback loop is regularly used to blur, fade or transform pixels like you usually do with attributes in Houdini.
The most common setup is built using a feedback and a comp node like this:
In the feedback node I pointed to the comp9 node which overlays the feedback with the current frame. Everything in between the comp9 node and the feedback node can be seen as the inside of a SOP solver.
In this example I used a level node to multiply the brightness with 0.95 every frame and blur it a little which leads to this fading effect.
sources / further reading: