[cairo] [PATCH] pthread-same-source: Refresh reference images

[Bill Spitzak]

On Mon, Apr 2, 2018 at 11:26 AM, Bryce Harrington <bryce at osg.samsung.com>
wrote:

>
> (It'd still be interesting to see further refinements on the downscaling
> code, I'm sure there's still potential there for more optimization.
> Hopefully some day someone can look again at the convolution filter
> approach and see if it can get better integrated without causing
> test failures.)
>

I have some plans for improving the downscaling, but they are three years
old. The main problem is that my proposed changes to pixman were rejected
and I had to give up after that, as I cannot see any possible way to
improve this unless the filtering decisions are simplified and moved to
pixman.

The basic problem is that the current filter api can define a single size
of filter but the size and form of the filter depends on the transform
algorithm. My proposed 2-pass algorithm, for instance, requires the filter
in one direction to be parallel to an axis in the source image, and the
filter in the second direction to be parallel to an axis in the destination
image, and which one is chosen depends on the rotation and the dimensions
of the image. Schemes that do multiple passes through intermediate images
(ie mipmaps, Catmull-Rom 3-pass, closest-integer follwed by filtering, etc)
put limits on the filters that can be used or how they are specified, as
they must decompose into multple filters that are convolved. And for
non-affine transforms the filter will vary for every single location.

Also just as a basic design criteria, I do not believe users of Cairo or
Pixman are interested in anything other than whether filtering is
happening, and perhaps choosing between "good" and "best" filtering to
trade off speed. Limiting the api to simple keywords such as "good" and
"best" will allow much more freedom for replacing and improving the
filtering.

My proposal basically amounts to this:

1. Move filtering to Pixman and make the _GOOD and _BEST settings do
something different and desirable. First proposal is to make them do
exactly what Cairo is doing, running the existing filtering code (which is
not 2-pass as it runs the 2 "passes" on every pixel, not reusing any
previous results). Apparently there is a bug in the X so that pixman inside
X will never receive the _GOOD or _BEST settings, for now I propose that it
fall back to the image backend in these cases, to support this Cairo will
still retain some redundant code to detect if _BILINEAR can be used so it
does not have to do the fallback.

2. Add a new 2-pass algorithm to pixman and make _GOOD and _BEST run that
(and possibly _BILINEAR as it is possible it will be faster than that for
downscaling). The api is designed so that it is possible to replace this
algorithm, in particular any kind of hardware-supported filtering might be
far faster and should be allowed. This necessarily means that test image
outputs can change, they will need to be changed when Cairo is changed to
new algorithms. Add some non-affine tests to pixman.

3. Probably not strongly related, but add some non-affine transforms to
source images to Cairo so it can use them.

Does anybody see any chance that this sort of change could be approved?
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