3D Solid Cube

Backface culling is really quite easy. What you want to check is wether the triangle's normal is facing the camera or not. You do this like so:

d = DotProduct(TriangleVertex - CameraPosition, Normal);
if( d >= 0 )
// The camera is behind the triangle
else
// The camera is in front of the triangle

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The dot product of two vectors, U and V, is calculated as

DotProduct = U.x*V.x + U.y*V.y + U.z*V.z;

The mathematical formula for the dot product is

DotProduct = LengthOf(U)*LengthOf(V)*cos(SmallestAngle(U,V))

This will have the effect that if the vectors are perpendicular to each other the dotproduct is 0, if they are facing in the same general direction (angle < 90 degrees) the dotproduct is positive and negative if facing away from each other (angle > 90 degrees).

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You can also do the test after 3D to 2D projection by computing the signed area of the triangle:

Area = (U.x*V.y - U.y*V.x)/2;
if( Area < 0 )
// The triangle is invisible
else
// The triangle is visible

The formula for the signed area is derived from the cross product (But that is not relevant to your question).

WitchLord

Edited by - WitchLord on 4/18/00 4:01:38 PM

hehe, thank you very much WitchLord