[cairo] xlib-xcb glitch art

[Chris Wilson]

On Fri, May 22, 2015 at 10:55:41AM +0200, Uli Schlachter wrote:
> Hi,
> 
> Am 22.05.2015 um 08:19 schrieb Behdad Esfahbod:
> > See:
> > http://www.hellocatfood.com/glitch-gimp/
> 
> https://bugs.freedesktop.org/show_bug.cgi?id=67505
> 
> AFAIR Chris made gimp create a shm-backed image surface, properly flush, then
> change, then mark it dirty and use it as a source. Then it modifies another part
> of the surface without calling mark_dirty() again and expects the new stuff to
> be drawn. I understand our API documentation as "always call mark_dirty after
> modifying the image surface outside of cairo" while Chris thinks this should work.

It's a trick that permugates from the kernel in order to avoid
synchronisation - our buffer management is all or nothing, we don't have
subsurface serialisation (and probably not for the forseeable future).
And what this allows is for gimp to render directly into video memory
using unsynchronised wrties (like
glMapBufferRange(GL_MAP_UNSYNCHRONIZED_BIT | GL_MAP_PERSISTENT_BIT)).
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre