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When you use code on a 2D canvas to draw in an updating loop you have to draw over the previous frame or wipe it or else you get these trails of moving objects. In blender you don't have to wipe the previous frame because each frame is rendered from scratch, so I was wondering if there was a way to achieve the same effect in blender because you would be able use it to paint by moving objects around in 3D space.

So imagine you rendered each new frame on top of the result of the previous render as your background, that's basically what I want to do.

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  • $\begingroup$ since output files are predictably named, you should be able to import the last rendered frame into the compositor (current frame - 1) and Mix -> Overlay the current render on top. a script may help, but I'm not familiar enough $\endgroup$ Apr 1 at 19:21
  • $\begingroup$ Very similar to blender.stackexchange.com/a/213637/29586 but with rendered image instead of video frames. $\endgroup$ Apr 1 at 19:22
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The answer posted by Rich Sedman is the answer: motion trail effect on video in VSE

Rather than doing this in the VSE you can create this effect using a feedback technique similar to How to do this type of trailing effects where the previous frame is held over to include in the current frame. The only real difference is in masking out the current frame so as to overwrite all but the background each time.

The setup of the scene is the same (ie, a camera pointing at the plane) with the following material on the plane :

enter image description here

material

Here the feedback is handled by the bottom branch - with the Image Texture pointing at the location of the render result, offset by 1 frame to pickup the previous frame. The top branch injects the video (filed infront of a black background) with the Subtract used to darken the image (this was used as a crude way to remove background artifacts since my video background wasn't perfect) with the Less Than used to detect the 'foreground' so that the feedback can be cut out wherever the new image is above a certain threshold. The images are combined and the Emission with a value less than 1 effects the fade out as the trail ages.

This produces the following result :

enter image description here

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