[cairo] Using cairo/pixman for raw video in GStreamer

[Soeren Sandmann]

Hi Benjamin,

All of this sounds good to me. Below are a few comments on how YUV
formats could be integrated in pixman.

> 1) Add extensive YUV support to pixman

Extensive YUV support would be a very useful addition to pixman. Apart
from the benefits you listed, I think it also makes sense to have YUV
support in XRender as a more powerful way of doing textured video than
Xv.

* Tiles

Writing one pixel in a chroma subsampled format requires access to a
2x2 tile of RGB pixels, but the current general compositing only
provides one scanline.

A solution to that may be to move to a tiled architectured where
general_composite() processes destination tiles instead of
scanlines. This would require changing all the scanline accessors, but
hopefully that is a mostly mechanical process.

Aside from hopefully solving the subsampling problem, tiles would also
have better cache behavior for rotated or filtered sources.

* Format specification

Pixman already has some support for YUV formats:

    PIXMAN_yuy2 =        PIXMAN_FORMAT(16,PIXMAN_TYPE_YUY2,0,0,0,0),
    PIXMAN_yv12 =        PIXMAN_FORMAT(12,PIXMAN_TYPE_YV12,0,0,0,0)

but you can't write to them because of the subsampling problem
mentioned above, and so there is pixman_format_supported_destination()
API. It was probably a mistake to add that API, and future formats
should always be supported for both reading and writing.

Having a pixman_format_type_t like PIXMAN_TYPE_YUY2 and TYPE_YV12 for
each video format is not going to scale, so we'll need some new scheme
to describe video formats. I don't know enough about video formats to
have an opinion on how to do this, but I don't think there is anything
particularly great about the two exisiting format codes, so hopefully
we can get away with deprecating them and respecifiying within the new
scheme.


Thanks,
Soren