sub-frame temporal stretch/blend for video?

Greetings,
I’m working on a light accumulation patch and have run in to a problem. I’m trying to eliminate the non-image data coming through each frame of video.

I believe one way is to adjust the vertical blank interrupt upon capture… which involves lower level programming.

A workaround with Max is with some kind of temporal stretch for each frame. I found a frame blending abstract vade did in 2008,
https://cycling74.com/forums/sharing-is-fun-gpu-powered-frame-blending-working-now
but it doesn’t seem to help, nor does any type of xfade. Blur just smears the nuances of the light.  I think what I’m looking for is some sub-frame solution.

Below is a side by side example comparing traditional time-lapse photography with my failed attempt.

The patch has the main idea of the accumulation loop. I’ve run gl shapes through the loop so I think the logic of the patch is fine… maybe not. Another thought was to ping-pong or blend 2 textures with a .5 frame offset… but there is no such thing… is there?

Oh… and this is all live…  camera to Max to projector.

Any ideas?