[Pixman] [PATCH v10 13/15] pixman-filter: Negative subsample values produces scaled result

[Bill Spitzak]

On 02/04/2016 02:47 AM, Oded Gabbay wrote:
> On Tue, Feb 2, 2016 at 8:28 AM,  <spitzak at gmail.com> wrote:
>> From: Bill Spitzak <spitzak at gmail.com>
>>
>> If a negative value is used for the subsampling, then -n subsamples are
>> used at scale==1, and fewer are used at larger scale, more are used at
>> smaller scale, so that the total number of samples is approximately
>> the same. The computed value is rounded up to the next power of 2.
>>
>> The scale demo is modified to allow these negative numbers, and initially
>> uses -12.
>>
>> Signed-off-by: Bill Spitzak <spitzak at gmail.com>
>> ---
>>   demos/scale.ui         |  5 +++--
>>   pixman/pixman-filter.c | 24 ++++++++++++++++++++++++
>>   2 files changed, 27 insertions(+), 2 deletions(-)
>>
>> diff --git a/demos/scale.ui b/demos/scale.ui
>> index b62cbfb..1e77f56 100644
>> --- a/demos/scale.ui
>> +++ b/demos/scale.ui
>> @@ -24,12 +24,12 @@
>>       <property name="page_size">10</property>
>>     </object>
>>     <object class="GtkAdjustment" id="subsample_adjustment">
>> -    <property name="lower">0</property>
>> +    <property name="lower">-256</property>
>>       <property name="upper">12</property>
>>       <property name="step_increment">1</property>
>>       <property name="page_increment">1</property>
>>       <property name="page_size">0</property>
>> -    <property name="value">4</property>
>> +    <property name="value">-12</property>
>>     </object>
>>     <object class="GtkWindow" id="main">
>>       <child>
>> @@ -321,6 +321,7 @@
>>                         <object class="GtkSpinButton" id="subsample_spin_button">
>>                           <property name="visible">True</property>
>>                          <property name="adjustment">subsample_adjustment</property>
>> +                       <property name="value">-12</property>
>>                         </object>
>>                         <packing>
>>                           <property name="left_attach">1</property>
>> diff --git a/pixman/pixman-filter.c b/pixman/pixman-filter.c
>> index f28cc29..6e8024a 100644
>> --- a/pixman/pixman-filter.c
>> +++ b/pixman/pixman-filter.c
>> @@ -342,14 +342,38 @@ filter_width(pixman_kernel_t reconstruct, pixman_kernel_t sample,
>>               double* size, int* subsample_bits)
>>   {
>>       int width;
>> +    /* IMPULSE.x does not work for size < 1.0 */
>>       if (reconstruct == PIXMAN_KERNEL_IMPULSE && *size < 1.0)
>>          *size = 1.0;
>> +    /* Convolution adds the widths of the filters together */
>>       width = ceil (filters[reconstruct].width + *size * filters[sample].width);
>> +    /* If there will only be one sample, it must be 1.0 due to normalization,
>> +       and subsampling is useless. */
>>       if (width <= 1)
>>       {
>>          width = 1;
>>          *subsample_bits = 0;
>>       }
>> +    else if (*subsample_bits < 0)
>> +    {
>> +       /* The intention was to do -n / size rounded up to the next power of 2,
>> +          but this non-linear function seems to work better. For large size
>> +          it is the width of the BOX.BOX filter. For small size it reduces
>> +          samples by 2 at maximum. */
>> +       double desired_samples = -*subsample_bits;
>> +       if (sample == PIXMAN_KERNEL_IMPULSE)
>> +           ; /* For x.IMPULSE no scaling is done */
>> +       else if (*size >= 1.0)
>> +           desired_samples *= 2.0 / (*size + 1.0);
>> +       else
>> +           desired_samples *= 2.0 / ((*size + 1.0) * *size);
>> +       *subsample_bits = (int) ceil (log2(desired_samples) - .01);
>> +       if (*subsample_bits < 0)
>> +           *subsample_bits = 0;
>> +       else if (*subsample_bits > 8)
>> +           /* Assume we cannot see more than 256 different shades and limit subsampling */
>> +           *subsample_bits = 8;
>> +    }
>>       return width;
>>   }
>>
>> --
>> 1.9.1
>>
>> _______________________________________________
>> Pixman mailing list
>> Pixman at lists.freedesktop.org
>> http://lists.freedesktop.org/mailman/listinfo/pixman
>
> I tried applying just the first hunk (the scale.ui change) without the
> second hunk to see If I get any visual errors in scale demo, but
> couldn't see anything wrong. So, I would need to see some form of
> test/example that is affected by this patch, as I don't have the
> knowledge to even ACK it.
>
> Otherwise, you will need to get someone else to review this (Soren ?).
>
> Oded
>
Run it with the graph output and you will see that the new one produces 
far fewer subsamples and changes the number of them as the scale changes.