[Pixman] [PATCH v9 14/15] pixman-filter: Added big comment to filter

[Oded Gabbay]

On Fri, Jan 22, 2016 at 11:42 AM,  <spitzak at gmail.com> wrote:
> From: Bill Spitzak <spitzak at gmail.com>
>
The title of the patch confused me a bit.
Maybe rewrite it as:
pixman-filter: Add description to pixman_filter_create_separable_convolution()

> v9: Described arguments and more filter combinations, fixed some errors.
> v8: first version

No need of v8:first version
>
> Signed-off-by: Bill Spitzak <spitzak at gmail.com>
> ---
>  pixman/pixman-filter.c | 46 +++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 45 insertions(+), 1 deletion(-)
>
> diff --git a/pixman/pixman-filter.c b/pixman/pixman-filter.c
> index 639d5e6..74819fe 100644
> --- a/pixman/pixman-filter.c
> +++ b/pixman/pixman-filter.c
> @@ -378,7 +378,51 @@ filter_width(pixman_kernel_t reconstruct, pixman_kernel_t sample,
>  }
>
>  /* Create the parameter list for a SEPARABLE_CONVOLUTION filter
> - * with the given kernels and scale parameters
> + *
> + * Returns pointer to the block of numbers
> + *
> + * n_values: pointer to integer to store size of the block
> + *
> + * scale_x/y: amount to stretch the sample filter. Note that scale
> + * is the inverse of how the image is being scaled, if the image is
> + * being made smaller, the filter gets larger.
> + *
> + * reconstruct_x/y: filter that is not scaled.
> + *
> + * sample_x/y: filter that is scaled. Resoult is convolved with reconstruct filter.
> + *
> + * subsample_bits_x/y: If positive there are 2^n subpixel positions computed for
> + * the filter. If negative then -n (rounded up to the next power of 2) positions
> + * are calculated when scale==1, but fewer are computed for larger scales and
> + * more for smaller scales.
> + *
> + * Some interesting reconstruct.sample combinations:
> + *
> + *  IMPULSE.x - Uses the sample function only. This is what many other pieces of
> + *              software do. Does not work for scale < 1.0.
> + *
> + *  BOX.x - For scale < 1.0 this produces square pixels. For scale > 1.0 this
> + *          approaches IMPULSE.x as scale gets larger.
> + *
> + *  x.IMPULSE - Uses the reconstruct function only, scale is ignored.
> + *
> + *  BOX.BOX, LINEAR.IMPULSE, IMPULSE.LINEAR - at scale==1 these all match and
> + *              also produce same result as bilinear filtering.
> + *
> + *  BOX.BOX - Produces a trapazoid-shape. Narrowest possible filter with antialiasing.
> + *            Matches a lot of other software, some call it "box", others call it "linear"
> + *
> + *  BOX.LINEAR - At scale==1 this is what some software calls a Quadratic filter
> + *
> + *  IMPULSE.LINEAR - Some software calls this "linear" or "triangle". Not a good filter.
> + *
> + *  LINEAR.LINEAR - non-negative cubic. What some software calls "cubic interpolation".
> + *
> + *  IMPULSE.LANCZOS2 - Close to what a lot of other software calls "cubic interpolation"
> + *
> + *  IMPULSE.CUBIC - Called "Mitchell" in other software
> + *
> + *  IMPULSE.GAUSSIAN - Best removal of aliasing but usually considered too blurry
>   */
>  PIXMAN_EXPORT pixman_fixed_t *
>  pixman_filter_create_separable_convolution (int             *n_values,
> --
> 1.9.1
>
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With those changes, this patch is:

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