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

[Søren Sandmann]

>   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.

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.

This series is available as a git repository here:

    https://cgit.freedesktop.org/~sandmann/pixman/log/?h=spitzak-for-master
)


Søren
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.freedesktop.org/archives/pixman/attachments/20160412/8373769d/attachment.html>