C# SharpDX no option to check if triangle is front facing or back facing

Question

I want to check if a single triangle is front facing or back facing to the camera. I tried to find out if there is a simple option for directx11. However, I could not find one.

Something like: CheckFacing((Vector3)TrianglePoints, Camera(vector3)){
Formula... //The formula to check it but I do not know if I should manually check or if directx11 has an option
return CameraFacing; //(front or back)
}

Answer 1

Basically you're looking for the cross product of the triangle. That's the vector which is parallel to the triangle's normal.

Vector3 cross = Vector3.Cross(TrianglePoints[1] - TrianglePoints[0], TrianglePoints[2] - TrianglePoints[0]);
bool facescam = cross.Z < 0;

If the triangle faces the cam the cross product's Z value will be negative. Note that the output depends on the ordering of the triangle points.

If the cam is arbitrarily located, you need to calculate the dot product between the cam and the cross vector:

bool facescam = Vector3.Dot(cross, Camera) < 0;

Note that for simplicity I assume that Camera already holds the direction the viewer is facing to, that is Camera = viewPoint - camLocation. And regarding performance I'd go with the code here instead of calling an interface, it's quite fast.