Re: cairo x11 surfaces

[Eric Wasylishen]

Hi Fred,

I had a look at implementing it today, and it turned out to be easier than 
expected so I finished & committed it.

If we run in to problems we can switch back to XGCairoXImageSurface before the 
next release, but it looks promising. In particular, I tried X forwarding to 
Apple's X11.app, which only supports 24-bit windows, and the new surface is 
significantly faster than XGCairoXImageSurface, and the alpha channel of images 
is correctly preserved (unlike XGCairoSurface).

Riccardo, this should fix GNUstep on the 16-bit display configuration where it 
wasn't working for you. If you could test it some time and let me know if it 
works, that would be great.

Cheers,
Eric


On 2011-10-13, at 1:08 AM, Fred Kiefer wrote:

> This sounds very promising. Maybe we can get rid of XWindowBuffer that way. 
> Do you think this change should go in before the release or after?
> 
> Fred
> 
> On 12.10.2011 18:54, Eric Wasylishen wrote:
>> Hi, I just wanted to report on a bit of research I did on this. There was a 
>> suggestion on the cairo mailing list [1] to implement double buffering on X 
>> by doing the following:
>> 
>> - create a cairo xlib surface for the window (could be 32-bpp with alpha, 
>> 16-bpp no alpha, etc.)
>> - call cairo_surface_create_similar [2] with CAIRO_CONTENT_COLOR_ALPHA. As 
>> far as I know the resulting surface is guaranteed to have an alpha channel. 
>> I skimmed over the implementation and it looks like it tries to create an 
>> xlib surface with alpha; if that fails, it creates a 32-bpp image surface 
>> and returns that.
>> - when expose events are received, use cairo drawing methods to copy the 
>> desired region from the back buffer to the window. This should boil down to 
>> a efficient X call if both surfaces are stored on the server, or otherwise 
>> cairo should handle using shared memory to transfer the 32bpp image surface 
>> to the X server, and converting it to the needed format.
>> 
>> Now, I haven't tested this, but it looks like it should work and looks like 
>> exactly the behaviour we want, and as a bonus, cairo does all of the 
>> messy/hard work. :-)
>> 
>> 
>> It's amazingly hard to find tutorials on how to set up double buffering. 
>> Here's another comment I found: "A conversation with keithp indicates that 
>> his current thinking is that the right way to do double buffering is via an 
>> explicit copy from a separate pixmap. This is portable to absolutely 
>> everywhere, and requires no magic. Probably there will soon be a convention 
>> for the compositing manager to handle the double buffering on systems where 
>> one is running, since it needs to buffer anyhow. But this would be in the 
>> future." [3]
>> 
>> --Eric
>> 
>> [1] http://lists.freedesktop.org/archives/cairo/2007-February/009586.html
>> [2] 
>> http://www.cairographics.org/manual/cairo-cairo-surface-t.html#cairo-surface-create-similar
>> [3] http://xcb.freedesktop.org/XcbNotes/
> 
> 
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