Post effect with temporary texture

Hi,
I'd like to do a post effect where it's rather hard to calculate pixel color for particular destination position (inside frag function), but it's really easy to calculate position/color for each source pixel.

Is that possible at all to go through all source pixels, calculate the position/color for each and write to some temporary texture and finally return this texture from a shader?

I'm not sure how to write to this temporary texture inside a shader.

I've try to google, but I'm not sure what to search exactly...
What possible approaches can be applied here?

Thank you, Roman