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

[Bill Spitzak]

On Fri, Mar 18, 2016 at 6:54 PM, Søren Sandmann <soren.sandmann at gmail.com>
wrote:

> 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.
>

Yes I believe you are right. Looks like my mistake there, and you can see
the other line that this replaces is in the correct direction so I'm not
sure how I managed to do this. This incorrect version still hid the
artifact in the filter (a small dip/spike in the middle of large ones for
sync filters), so I guess I concluded I got it right.

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.
>

That will not distribute the error evenly. I could just add
error*(x+1)/width-error*x/width and avoid the temporary entirely, though I
guess there has to be a warning comment that doing the obvious-looking
optimization will make it not work.

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

I don't think that will work because it needs to take into account the
rounding to pixman_fixed_t after the division.

>
>
> Søren
>
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