I wrote a tri-filler a while back, and it ran pretty fast (did a textured version too, but it''s slower). The general idea is to render the triangle as 2 tris, one flat bottomed and the other flat topped. Of course you just bypass one half if your tri has a flat top or bottom (sort the vertices by y value, and compare y1 to y2, y2 to y3).
To render the top half, you can either compute the slope for the whole v1-v3 edge (might be slightly faster, since you can reuse it on the second half, but it loses a little accuracy), or find the x value on that edge at y2 (here''s how I do it: v1.x + (v3.x - v1.x) / (v3.y - v1.y) * height (where height is v2.y - v1.y)) and use that to find your slope. Also check wether that x value is greater than v2.x, and if so, swap them (store them in xLeft and xRight or whatever you want), then compute your slopes like (xLeft - v1.x) / height and the same for xRight. Then simply set xStart and xEnd to v1.x, and do a for y = v1.y to v2.y, adding the slopes to xStart and xEnd each time through, fill between xStart and xEnd and you have a half-triangle. The rest is just the same, except going from xLeft and xRight to v3.x, instead of v1.x to xLeft and xRight.

If you want, I can explain how I do clipping too (probably not the best method, but it seems to work good), but I don''t really have time right now, so I hope this helps as it is^_^


-Deku-chan

DK Art (my site, which has little programming-related stuff on it, but you should go anyway^_^)