On how to use the Feedback TOP in Touchdesigner to do nice stuffs.
Imagine that we are stacking tracing paper, with each a black dot with a different position. As the tracing paper is quite transparent, we see the dot on the top paper full black, then the dot on the second paper a little bit less, and the one underneath a even less, and so forth.
Now in 12 fps, as if we were adding new pieces of paper.
The basic feedback loop network : one texture TOP, going to Composite TOP and to a Feedback TOP looking at the Composite TOP, going to a modifier TOP (level, blur, etc), and also in the Composite.
The texture TOP goes into the Feedback TOP only to get the resolution, as the Feedback get the image from the Composite TOP.
In the parameters of the Feedback TOP, we drag and drop the Composite TOP in the Target TOP slot, to have the grey arrow link between the TOPs closing the loop.
In the parameters of the Level TOP, in the Post tab, we set the opacity to a lower value like 0.9, to decrease the opacity of the layers as they go through the feedback loop.
The Level TOP creates the slowly-disappearing effet : the opacity of the first frame post-Feedback is at 0.9 for example, so the frame underneath is at 0.9 * 0.9 = 0.81, then the one underneath is at 0.9 * 0.9 * 0.9 = 0.72, and so forth.
In the parameters of the Composite TOP, we chose a operation mode like Over, Add, or Atop.
The feedback loop only consist of a Circle TOP moving randomly with two noises CHOP for x and y, the Feedback TOP and the Composite TOP, with a white background added after the loop.
The moving circle is therefore continuously drawing a line of its position as it goes through the feedback loop.
To clear the background, we only need to pulse the Reset button in the Feedback parameters.
If we add a Level after the Feedback with an opacity of .99, we can see the line very slowly fading.
If we add a Blur, and a Transform with -0.01 in Translate Y, we get the effect of a fading, falling, blurring line.
If we add a Displace with 0.01 as Displace Weight, and with a slowly animated, large and not really constrasted noise, the fading line keeps slowly moving.
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Matthew Ragan's explaination of the Feedback TOP
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Video "What is feedback? feedback in Touchdesigner" by Noto The Talking Ball
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Video "feedback TOP TouchDesigner 088" by Grady Sain
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Explaination What is video feedback synthesis