[Pixman] [PATCH 01/15] demos/scale: Compute filter size using boundary of xformed ellipse, not rectangle

[Oded Gabbay]

On Sat, Dec 12, 2015 at 8:06 PM,  <spitzak at gmail.com> wrote:
> From: Bill Spitzak <spitzak at gmail.com>
>
> This is much more accurate and less blurry. In particular the filtering does
> not change as the image is rotated.
> ---
>  demos/scale.c | 43 ++-----------------------------------------
>  1 file changed, 2 insertions(+), 41 deletions(-)
>
> diff --git a/demos/scale.c b/demos/scale.c
> index d00307e..71c7791 100644
> --- a/demos/scale.c
> +++ b/demos/scale.c
> @@ -55,50 +55,11 @@ get_widget (app_t *app, const char *name)
>      return widget;
>  }
>
> -static double
> -min4 (double a, double b, double c, double d)
> -{
> -    double m1, m2;
> -
> -    m1 = MIN (a, b);
> -    m2 = MIN (c, d);
> -    return MIN (m1, m2);
> -}
> -
> -static double
> -max4 (double a, double b, double c, double d)
> -{
> -    double m1, m2;
> -
> -    m1 = MAX (a, b);
> -    m2 = MAX (c, d);
> -    return MAX (m1, m2);
> -}
> -
>  static void
>  compute_extents (pixman_f_transform_t *trans, double *sx, double *sy)
>  {
> -    double min_x, max_x, min_y, max_y;
> -    pixman_f_vector_t v[4] =
> -    {
> -       { { 1, 1, 1 } },
> -       { { -1, 1, 1 } },
> -       { { -1, -1, 1 } },
> -       { { 1, -1, 1 } },
> -    };
> -
> -    pixman_f_transform_point (trans, &v[0]);
> -    pixman_f_transform_point (trans, &v[1]);
> -    pixman_f_transform_point (trans, &v[2]);
> -    pixman_f_transform_point (trans, &v[3]);
> -
> -    min_x = min4 (v[0].v[0], v[1].v[0], v[2].v[0], v[3].v[0]);
> -    max_x = max4 (v[0].v[0], v[1].v[0], v[2].v[0], v[3].v[0]);
> -    min_y = min4 (v[0].v[1], v[1].v[1], v[2].v[1], v[3].v[1]);
> -    max_y = max4 (v[0].v[1], v[1].v[1], v[2].v[1], v[3].v[1]);
> -
> -    *sx = (max_x - min_x) / 2.0;
> -    *sy = (max_y - min_y) / 2.0;
> +    *sx = hypot (trans->m[0][0], trans->m[0][1]) / trans->m[2][2];
> +    *sy = hypot (trans->m[1][0], trans->m[1][1]) / trans->m[2][2];
>  }
>
>  typedef struct
> --
> 1.9.1
>
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Could you please add some comment in the code about where this
calculation comes from (reference to some mathematical
equation/proof), and detail the mapping of the variables in the code
to the arguments of the mathematical equation ?

Otherwise, this patch is:

Reviewed-by: Oded Gabbay <oded.gabbay at gmail.com>