Well, look… if you can make a post process to color up the bounds of geometry by checking 8 pixels around the current pixel and doing stuff with the results… (and you most definitely can, it’s one of the most used shaders in games I think). I see no reason why you couldn’t apply some similar logic to the post process to create the streak like aftereffect.
Things that will help here are custom stencils, exactly like the outlines.
the motion and length on screen are the hardest parts to figure out. Though technically you shouldn’t care about motion and just lenght on screen / draw interval.
For example, if you can draw an outline every frame, and the object moves every frame but the outline remains for 10 frames overlaying to each other you already sort of got the base effect ready to go.
No idea how one would go about preventing the re-render or leaving the outline in place. Or redrawing the old outline in the same spot every frame… and that would be what needs to be figured out really.
and yes. Buffers make sense, but buffers are 2d.
So maybe you can cook up a few render textures with a scene capture 2d, isolate the character via stancil, and make the postproces fade in the texture of the capture that matches the screen.
Obviously it’s not very performant. But in a pinch it would probably give the result you are looking for with somewhat minimal setup.
Best tutorials to look up for that are the reactive water ones. The wiki has a few and they aren’t very well explained.
However the process they have in the content examples loops and updates at least 3 different render targets over time to generate the geometry on the water plane.
This is by nature already very similar to what you need. Maybe you can adapt the whole concept.
and no, its not only a post process effect anymore…
If you can access the buffer ofc, the process would be similar to the logic of the content example as well…