Surprisingly tricky being something around two or three dozen lines of C for filling a triangle (with some constraints), after it's been transformed appropriate

Surprisingly tricky being something around two or three dozen lines of C for filling a triangle (with some constraints), after it's been transformed appropriately. But that is not the only way. You can iterate over a plane and test whether the coordinates are within a triangle. It ends up being very similar to the code you'd have for ray tracing a triangle.