Assuming you have an already transformed and projected triangle (aka 2D)

I believe that this is correct...
TriangleArea = 0.5f * ((x1 - x2) * (y3 - y1) - (y1 - y2) * (x3 - x1));

Edit:
This site should be able to explain it:
http://softsurfer.com/Archive/algorithm_0101/algorithm_0101.htm

Quote:Original post by meisawesome
Assuming you have an already transformed and projected triangle (aka 2D)

I believe that this is correct...
TriangleArea = 0.5f * ((x1 - x2) * (y3 - y1) - (y1 - y2) * (x3 - x1));

Edit:
This site should be able to explain it:
http://softsurfer.com/Archive/algorithm_0101/algorithm_0101.htm

This isn't remotely related to what the OP's question was!
He didn't want to know the area of a triangle!?

Back on track:
I guess there might be some performance tricks, but the naive way of doing it is to clip the triangle against the four pixel borders, then calculate the area of the resulting polygon.

I assume you use this for some sort of AA?
If you accept some quality loss you might get away by subdividing the pixel (2x2, 4x4 etc) and then calculate how many of the sub pixels centers are within the triangle.