[cairo] Transform+Repeat patch for libpixman

[David Reveman]

On Mon, 2004-09-06 at 11:22 -0400, Owen Taylor wrote: 
> Here's a patch for libpixman that makes sources/masks with both 
> a transform and repeat work correctly.

great!

> 
> Notes:
> 
>  - I've extended the pixman_compositeOperand 'transform'
>    union member:
> 
>         op->u.transform.transform = image->transform;
>         op->u.transform.filter = image->filter;
> +       op->u.transform.repeat = image->repeat;
> +       op->u.transform.width = image->pixels->width;
> +       op->u.transform.height = image->pixels->height;
> 
>    All five are likely best replaced with:
>     
>         op->u.transform.image = image;
> 
>    But I didn't want to do that in this patch since it would obscure
>    the logic of the patch.
> 
>  - This is far from brilliantly efficient, but it isn't horrible
>    and actually improves the case of 1x1 source since
>    pixmap_compositeGeneral isn't getting called for every pixel
>    individually. (Though that case could be optimized a *lot* more, the
>    transform can be ignored for a 1x1 source.)
> 
>  - I've interpreted the clip as clipping *after* the repeat,
>    I think this matches what happens without the transform

The patch looks good to me, one little problem though:

I changed
#define mod(a,b)       ((b) == 1 ? 0 : (a) >= 0 ? (a) % (b) : (b) - (-a)
% (b))
to
#define mod(a,b)       ((b) == 1 ? 0 : (a) >= 0 ? (a) % (b) : (a) % (b)
== 0 ? 0: (b) + (a) % (b))

as if 
a == [-b | -2b | -3b... ]
then
(-a) % (b) == 0 
and
(b) - (-a) % (b) == b
which is a coordinate outside the image.

With this fixed, I've now committed your patch along with some byte-
order and clipping fixes that I've been sitting on. I've attached a copy
of the final version of your patch.

I've also added a pixman backend to the rendertest program, glitz and
pixman output now matches for all transform+repeat tests. 

> Assuming that this patch looks good, there should be a fairly
> obvious port to the X server.

yeah, we should probably modify cairo's xlib and xcb backends so that
they fall-back to image rendering when transform+repeat is used until
the X server port is done.
 
-David
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