[cairo] target XLIB <-> Image

Carl Worth cworth at cworth.org
Fri Apr 29 18:09:52 PDT 2005

On Fri, 29 Apr 2005 22:39:31 +0200 (MET DST),  wrote:
>  - compilation of libpixman & cairo from todays CVS is straight
>    forward

That's good to know.

>  - All test tell me that they fail. The pictures with target Image look
>    awfull,

Could you describe "look awful" in more detail? Perhaps one of those
pictures worth a thousand words would help? Otherwise this is "hard"
to diagnose.

>            but the pictures with target XLIB look nice at first glance. In
>    the latter case only pixels next to the "sharp" edges in the pictures
>    are "wrong"

Does this happen with all of the tests or only some? If some, which?
And what X server are you using (flavor and version)?

>  1) Am I right to conclude that the target XLIB test are OK and that the
>     differences occur only due to the use of a different compiler and a
>     different CPU chip?

We may certainly accept some minor rasterization differences in cairo
output, (eg. slight differences along antialiased edges), since we
haven't made any sort of pixel-perfect specification for
cairo. However, the Render extension, (which cairo uses when drawing
to Xlib), does have more precisely defined rasterization and
compositing. So any differences we see here might indicate we should
look more closely at the relevant Render implementation.

For example when I recently added the Xlib tests I found some minor
differences along the edges of a very few tests, (clip-twice and
coverage I think), which seem to indicate a single-bit error in the
accelerated Render compositing of Xati, (while another kdrive server,
the all-software Xfake, doesn't show the same problems).

>  2) Can you give me some hints to get the "target Image" tests better?
>     i.e. what code should I try to debug first?

See above request for more details. But if the Xlib tests are working
fine, then you'll likely be wanting to look in libpixman, (or the
libpixman-specific portions of cairo, eg. cairo-image-surface.c).

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