Is it possible to write from a post-process material to a buffer?

Hey all, I’m working on kind of a weird problem. I’ve generated some shapes inside of a post-process material, this is the intended effect-

As it’s just a regular material, it’s per-pixel, which is fine except that I want to do more advanced effects, starting with edge detection. Technically this is possible to do per-pixel but, I’d need to sample the surrounding pixels and that would get very expensive, very quickly. Instead, I tried writing the output of the mask to a RenderTarget and then using that as a base for further effects, like so-

Unfortunately, there are some serious drawbacks with this method- it’s slow, low res, and has a fixed aspect ratio. Intuitively I feel like the move here is to write the output of the mask to a buffer but, it’s very difficult to find information on how to do that. I did find this thread but, I’m hoping to avoid modifying engine code. Is this possible? Am I barking up the wrong tree? Any insight is appreciated here