[muyyatin2]

Ahh yes, for exact filtering it does need to be constant colour. I'm looking into seeing whether it can be done for gradients. However in practice, it works quite well visually to compute the "average color of the polygon" for each piecewise section, and blend those together.

[pixelpoet]

If you look at old papers by James Arvo, he has done analytic illumination from linearly varying lights, maybe this is helpful. Here for example his thesis: https://www.cs.cornell.edu/courses/cs667/2005sp/readings/Arv...

There's also this work on analytic antialiasing by Michael Mccool: https://www.researchgate.net/publication/2524514_Analytic_An...

[dahart]

Arvo’s work is also using Green’s theorem, in much the same way this article is. Integrating the phong-exponent light reflections is a little bit insane though (and btw I’ve seen Arvo’s code for it.)

The problem you run into with non-constant polygon colors is that you’d have to integrate the product of two different functions here - the polygon color and the filter function. For anything real-world, this is almost certainly going to result in an expression that is not analytically integrable.