// Inverse transformation of x and y. It works slow because
// it explicitly inverts the matrix on every call. For massive
// operations it's better to invert() the matrix and then use
// direct transformations.
public void inverse_transform(ref double x, ref double y)
{
Perspective t = new Perspective(this);
if (t.invert()) t.Transform(ref x, ref y);
}
// Multiply inverse of "m" by "this" and assign the result to "this"
public Perspective premultiply_inv(Affine m)
{
Perspective t = new Perspective(m);
t.invert();
Set(t.multiply(this));
return this;
}