Right-triangle would be simple. Any kind of triangle would be slightly trickier, but shouldn't be too hard.
Given a triangle defined by the points ABC, you can figure out 3 line segments.
A
B C
In line AB, start at point A and traverse towards B. Plot a pixel on that line and move horizontally. Move onto the next line when you've reached line segment AC. Once you reach point B, do the same for BC again moving towards AC. This will fill any missing spots if the triangle happens to be rotated.
Hopefully that made sense.
"Only the educated are free" ~Epictetus
"Imagination is more important than knowledge..." ~Einstein
