/**
* Recover the z component from a 2d transformed point by finding the intersection
* of a line through the point in the z direction and the transformed plane.
*
* We want to solve:
*
* point = normal . (p0 - l0) / normal . l
*/
static gfxFloat RecoverZDepth(const Matrix4x4& aTransform, const gfxPoint& aPoint)
{
const Point3D l(0, 0, 1);
Point3D l0 = Point3D(aPoint.x, aPoint.y, 0);
Point3D p0 = aTransform * Point3D(0, 0, 0);
Point3D normal = aTransform.GetNormalVector();
gfxFloat n = normal.DotProduct(p0 - l0);
gfxFloat d = normal.DotProduct(l);
if (!d) {
return 0;
}
return n/d;
}
