[Pixman] [PATCH v13 13/14] pixman-image: Implement PIXMAN_FILTER_GOOD/BEST as separable convolutions

Bill Spitzak spitzak at gmail.com
Sat Mar 5 03:12:36 UTC 2016

On Fri, Mar 4, 2016 at 4:17 PM, Søren Sandmann <soren.sandmann at gmail.com>

> On Wed, Feb 10, 2016 at 1:25 AM, <spitzak at gmail.com> wrote:
>> From: Bill Spitzak <spitzak at gmail.com>
>> Detects and uses PIXMAN_FILTER_NEAREST for all 8 90-degree rotations and
>> reflections when the scale is 1.0 and integer translation.
>> GOOD uses:
>>  scale < 1/16 : BOX.BOX at size 16
>>  scale < 3/4 : BOX.BOX at size 1/scale
>>  larger : BOX.BOX at size 1
>>  If both directions have a scale >= 3/4 or a scale of 1/2 and an integer
>>  translation, the faster PIXMAN_FILTER_BILINEAR code is used. This is
>>  compatable at these scales with older versions of pixman where bilinear
>>  was always used for GOOD.
>> BEST uses:
>>  scale < 1/24 : BOX.BOX at size 24
>>  scale < 1/16 : BOX.BOX at size 1/scale
>>  scale < 1 : IMPULSE.LANCZOS2 at size 1/scale
>>  scale < 2.333 : IMPULSE.LANCZOS2 at size 1
>>  scale < 128 : BOX.LANCZOS2 at size 1/(scale-1) (antialiased square
>> pixels)
>>  larger : BOX.LANCZOS2 at size 1/127 (antialias blur gets thicker)
>> v8: Cutoff in BEST between IMPULSE.LANCZOS2 and BOX.LANCZOS2 adjusted for
>>     a better match between the filters.
>> v9: Use the new negative subsample controls to scale the subsamples. These
>>     were chosen by finding the lowest number that did not add visible
>>     artifacts to the zone plate image.
>>     Scale demo altered to default to GOOD and locked-together x+y scale
>>     Fixed divide-by-zero from all-zero matrix found by stress-test
>> v11: Whitespace and formatting fixes
>>      Moved demo changes to a later patch
>> v12: Whitespace and formatting fixes
>> Signed-off-by: Bill Spitzak <spitzak at gmail.com>
> The short answer to this patch is NACK - it doesn't make sense to change
> the meaning of GOOD and BEST.

Okay that is going to need a big WTF?

There are at least three reasons:
> 1. Pixman is a low-level API that is in the camp of "do what I say" not
> "do what I mean". When users such as cairo specify GOOD and BEST, they are
> essentially asking Pixman to make a tradeoff that it doesn't have the
> knowledge to do. As such, I think it was a mistake to have these values in
> the first place (and indeed also a mistake to have them in the Render
> extension since X11 is also supposed to be a do-what-I-say API). As such, I
> think they should remain aliases to BILINEAR as they are now.

I have no idea why having three ways to say BILINEAR is necessary to "do
what I say". One seems sufficient.

2. The motivation for doing this in Pixman (and not in cairo) is that
> supposedly the X server will then pick it up automatically and use the
> high-quality filters when PictFilterBest and PictFilterGood is set. But
> that doesn't actually work because the X server doesn't map those Render
> filters to the corresponding Pixman filters. See:
>      https://cgit.freedesktop.org/xorg/xserver/tree/fb/fbpict.c#n420
> So it looks to me like the whole scheme just won't work. But even if it
> did, you would still need to make all the hardware drivers do the same
> thing, so it's not just a matter of adding some constants in that function.

If X is not currently sending GOOD/BEST through, then this is even a better
reason why it is ok to change them, as it cannot possibly change the
behavior of any X programs. All other pixman-using programs are directly
linked with the library. I did figure X would remain broken for long after
this is fixed, and Cairo would use the image backend for such transforms
until X is updated, or it is replaced with Wayland (where the image backend
is used all the time).

The way to go is to add a new SeparateConvolution filter to Render and make
> fb call into Pixman to implement it. Once that is in place, the hardware
> drivers can then implement it.

You seem to think the hardware drivers will implement this version of
SeparateConvolution? Really???

The purpose of this was so that the hardware drivers could implement
something they call "good" and something they call "best" using whatever
hardware resources are available. This is not going to happen if they are
expected to read the filter array.

3. The patch is completely broken as written. I could go into details, but
> there is no point since I don't think the patch should be pushed even if
> fixed. I'll just point out that if the environment variable
> PIXMAN_DISABLE=fast is set, the patch does absolutely nothing (you can
> verify this with the scale program). And setting this environment variable
> is not supposed to change Pixman's behavior, only potentially its
> performance.

Will look into that. I would like the rest of the detail please.

Currently Linux 2D rendering is a joke. Nobody was using the
SeparateConvolution until I patched up Cairo to do it, but I sure figured
that was a very temporary patch until it could be done in a proper
location. Users want "good", not "low level api". So now Linux is 1.5 years
further behind. Really sad.
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