[cairo] Extracting The Scaling Component Of A Matrix

[Lawrence D'Oliveiro]

On Sat, 12 Dec 2015 12:15:17 +0100, Behdad Esfahbod wrote:

> On 15-12-11 01:51 AM, Lawrence D'Oliveiro wrote:
>
> > I construct a matrix that implements just the scaling component of
> > the CTM in the Cairo context “ctx”:
> > 
> >     scaling = Matrix.scale(ctx.user_to_device_distance(Vector(1,
> > 1)))  
> 
> This wouldn't do what you think it does for general matrices.  Or
> maybe I don't understand what you are trying to do.  Are you only
> interested in non-skewed matrices?

    import sys
    from qahirah import \
        Matrix, \
        Vector

    for \
        mat \
    in \
        (
            Matrix.scale(3),
            Matrix.skew((0, 1)) * Matrix.scale(3),
            Matrix.scale(3) * Matrix.skew((1, 0)),
            Matrix.skew((1, 1)),
            Matrix.skew((1, -1)) * Matrix.scale(3),
            Matrix.scale(3) * Matrix.skew((-1, 1)),
        ) \
    :
        sys.stdout.write \
          (
            "scaling part of {} = {}\n"
            .format
              (
                mat,
                mat.mapdelta(Vector(1, 1))
              )
          )
    #end for

    ...

    scaling part of Matrix(3, 0, 0, 3, 0, 0) = Vector(3, 3)
    scaling part of Matrix(3, 3, 0, 3, 0, 0) = Vector(3, 6)
    scaling part of Matrix(3, 0, 3, 3, 0, 0) = Vector(6, 3)
    scaling part of Matrix(1, 1, 1, 1, 0, 0) = Vector(2, 2)
    scaling part of Matrix(3, -3, 3, 3, 0, 0) = Vector(6, 0)
    scaling part of Matrix(3, 3, -3, 3, 0, 0) = Vector(0, 6)

Hmmm ...