LÖVE

You are right, only one would be sent to it, but you could define an extern in your shader, that will hold a (dest) canvas, and you can use that, and the supplied drawable (source) to draw with a shader implementing a blending mode... though you will probably need two canvases in total to do this, since iirc you shouldn't read from and draw to the same canvas... though that's usually with passing the canvas into love.graphics.draw, not with shader:send... didn't test this yet, to be honest.

As for how much this will tank performance... the biggest issue would probably be sending that canvas through shader:send, but i believe that doing this once per frame probably won't cause too much slowdown... probably.

Anyway, here's a site i googled that have some relevant formulas: http://renderingpipeline.com/2012/06/ph ... modi-glsl/

That looks awesome