pouët.net

So this one stores polynomial coefficients defining three discs. For each disc, each point inside the disc is plotted in black and then that point is reflected through the screen center and plotted in white. This has the useful property that when a disc is congruent with its reflection and points are plotted in raster order the result will be two differently-coloured semicircles. I used a column-major plotting order to get a vertical split, which can be easily seen because it renders so slowly.

Sometimes performing pixel-processing serially can be advantageous.