[Pixman] [PATCH 07/14] pixman-image: Added enable-gnuplot config to view filters in gnuplot

[Bill Spitzak]

On Tue, Apr 12, 2016 at 1:55 PM, Søren Sandmann <soren.sandmann at gmail.com>
wrote:

>
>   1-wide filters - looks triangular, but a 1-wide box would be more
>>>                    accurate
>>>
>>
>> Because you are not plotting the two dummy points at (0,±width/2), a
>> 1-wide filter is actually just a single point.
>>
>> You may be right that leaving the dummy points off the plot may make it
>> easier to figure out what is going on.
>>
>
> Okay, I will update the comment.
>
> I don't like to make up fake data points, but maybe I should add
> [x=-width/2:width/2] to the gnuplot command line.
>

Yes that sounds like a very good idea.


>
> diff --git a/pixman/rounding.txt b/pixman/rounding.txt
>>
>>> index b52b084..1c00019 100644
>>> --- a/pixman/rounding.txt
>>> +++ b/pixman/rounding.txt
>>> @@ -160,6 +160,7 @@ which means the contents of the matrix corresponding
>>> to (frac) should
>>>  contain width samplings of the function, with the first sample at:
>>>
>>>         floor (frac - (width - 1) / 2.0 - e) + 0.5 - frac
>>> +     = ceil (frac - width / 2.0 - 0.5) + 0.5 - frac
>>>
>>
>> Unfortunately this produces numbers that don't agree with the filter
>> generator or filtering code.
>>
>> For a width==4 filter with n_phases==1, the generator seems to produce
>> values at -1, 0, 1, 2, so the first sample is at -1. It also appears the
>> filtering sampler is using the same rule, otherwise these filters would
>> produce an obvious shift in the image.
>>
>
> If you add printf's on top of this series
>
> diff --git a/pixman/pixman-filter.c b/pixman/pixman-filter.c
> index 176dfae..4a8743e 100644
> --- a/pixman/pixman-filter.c
> +++ b/pixman/pixman-filter.c
> @@ -129,12 +129,15 @@ general_cubic (double x, double B, double C)
>  static double
>  cubic_kernel (double x)
>  {
> +  double v = general_cubic (x, 1/3.0, 1/3.0);
> +
> +  printf ("cubic(%f) => %f\n", x, v);
>      /* This is the Mitchell-Netravali filter.
>       *
>       * (0.0, 0.5) would give us the Catmull-Rom spline,
>       * but that one seems to be indistinguishable from Lanczos2.
>       */
> -    return general_cubic (x, 1/3.0, 1/3.0);
> +  return v;
>  }
>
> and run scale with Reconstruction=Cubic, Sample=Impulse, and Subsample=0,
> it prints
>
> cubic(-2.000000) => 0.000000
> cubic(-1.000000) => 0.055556
> cubic(0.000000) => 0.888889
> cubic(1.000000) => 0.055556
>
> so the filter generator *does* generate values at [ -2, -1, 0, 1 ]. And as
> mentioned, the sampling code, if given the value 17.5 will convolve with
> the pixels at 15.5, 16.5, 17.5, 18.5, so I'm pretty sure this matrix is
> correct.
>
> (I suspect your changes to the integral() arguments caused it to generate
> different values, so you should check without them included.
>

Looks like you are correct, toggling between this and nearest shows that
the filtering code is reading the arrays this way.


>
> This series is available as a git repository here:
>
>
> https://cgit.freedesktop.org/~sandmann/pixman/log/?h=spitzak-for-master
> )
>
>
> Søren
>
>
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