[Pixman] [PATCH v14 16/22] pixman-filter: distribute normalization error over filter

[Søren Sandmann]

On Sun, Mar 6, 2016 at 8:06 PM, <spitzak at gmail.com> wrote:

> From: Bill Spitzak <spitzak at gmail.com>
>
> This removes a high-frequency spike in the middle of some filters that is
> caused by math errors all being in the same direction.
>
> Signed-off-by: Bill Spitzak <spitzak at gmail.com>
> ---
>  pixman/pixman-filter.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/pixman/pixman-filter.c b/pixman/pixman-filter.c
> index 36dd811..ab62e0a 100644
> --- a/pixman/pixman-filter.c
> +++ b/pixman/pixman-filter.c
> @@ -282,8 +282,18 @@ create_1d_filter (int              width,
>             p[x] = t;
>         }
>
> +       /* Distribute any remaining error over all samples */
>         if (new_total != pixman_fixed_1)
> -           p[width / 2] += (pixman_fixed_1 - new_total);
> +       {
> +           pixman_fixed_t delta = new_total - pixman_fixed_1;
> +           pixman_fixed_t t = 0;
> +           for (x = 0; x < width; ++x)
> +           {
> +               pixman_fixed_t new_t = delta * (x + 1) / width;
> +               p[x] += new_t - t;
> +               t = new_t;
> +           }
> +       }
>

I think there is a sign error in this code: delta is new_total - 1, which
is positive when new_total is bigger than 1. But this positive delta is
then added to the samples, making the total even bigger.

Also, I would write the code like this:

        pixman_fixed_t error = pixman_fixed_1 - new_total;
        for (x = 0; x < width; ++x)
        {
            pixman_fixed_t d = error * (x + 1) / width;
            p[x] += d;
            error -= d;
        }

to get rid of the temporary and to make it more obvious that there is an
error that is being distributed.

Another possibility is to do error diffusion in the /* Normalize */ loop.


Søren
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