determinantAffine

Computes and returns the determinant of this matrix assuming it is affine, saving some computations.

Only the upper-left 3x3 block participates; the bottom row is assumed to be [0, 0, 0, 1]. For affine matrices (an object's world matrix, say) the result equals determinant but is cheaper. The two values generally differ for projective matrices, such as a perspective projection, whose bottom row is not [0, 0, 0, 1].