[Pixman] [PATCH v14 12/22] pixman-filter: fix subsample_bits == 0

[Bill Spitzak]

That's why this only changes the behavior when the number of samples is odd.


On Sun, Apr 3, 2016 at 11:17 AM, Søren Sandmann <soren.sandmann at gmail.com>
wrote:

> I don't believe this patch is correct.
>
> As an example, consider an image with an identity transformation and a
> cubic filter (which has width 4) with subsample_bits = 0. The current code
> will compute a matrix
>
>     [ cubic(-2), cubic(-1), cubic(0), cubic(1) ]
>
> Now suppose we are filtering a pixel located at x = 17.5. The code in
> bits_image_fetch_pixel_separable_convolution() will eventually end up
> convolving with the pixels at locations 15.5, 16.5, 17.5 and 18.5, which is
> the correct result since cubic(-2) = 0 [1].
>
> With your code, the matrix computed would be
>
>     [ cubic(-1.5), cubic(-0.5), cubic(0.5), cubic(1) ]
>
> which would not be correct no matter what: With an identity
> transformation, the pixel at 17.5 should be multiplied with cubic(0).
>
> There is a detailed explanation of these issues in the file
> pixman/rounding.txt.
>
>
> Søren
>
> [1] You might consider optimizing this case away and generate a matrix
> with width 3, but since this would only work with subsample_bits=0, it's
> not worth the special-casing.
>
>
> On Sun, Mar 6, 2016 at 8:06 PM, <spitzak at gmail.com> wrote:
>
>> From: Bill Spitzak <spitzak at gmail.com>
>>
>> The position of only one subsample was wrong as ceil() was done on an
>> integer.
>> Use a different function for all odd numbers of subsamples that gets this
>> right.
>>
>> Signed-off-by: Bill Spitzak <spitzak at gmail.com>
>> ---
>>  pixman/pixman-filter.c | 5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/pixman/pixman-filter.c b/pixman/pixman-filter.c
>> index ac89dda..f9ad45f 100644
>> --- a/pixman/pixman-filter.c
>> +++ b/pixman/pixman-filter.c
>> @@ -252,7 +252,10 @@ create_1d_filter (int              width,
>>          * and sample positions.
>>          */
>>
>> -       pos = ceil (frac - width / 2.0 - 0.5) + 0.5 - frac;
>> +       if (n_phases & 1)
>> +           pos = frac - width / 2.0;
>> +       else
>> +           pos = ceil (frac - width / 2.0 - 0.5) + 0.5 - frac;
>>
>>         total = 0;
>>         for (x = 0; x < width; ++x, ++pos)
>> --
>> 1.9.1
>>
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>>
>
>
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