[cairo] Redoing the xlib surface constructors

[Keith Packard]

Around 10 o'clock on Mar 12, Owen Taylor wrote:

> Now, RENDER really doesn't handle monochrome drawing at all, whether
> indexed or not. It's all tied to three-channel colors. RENDER doesn't
> have any provisions for direct-color visuals. And RENDER's handling
> of pseudocolor is crude, at best.

Render handles indexed visuals (i.e. StaticGray, GrayScale, StaticColor 
and PseudoColor) by treating them all the same.  For GrayScale and Pseudo
Color, it allocates a fixed set of colors within the default colormap, you 
can even query what those colors are.  With that in place, all of these 
visuals become easy enough to manage -- just match the nearest color for 
each pixel generated and lookup each pixel in the colormap.  There are 
caches in both directions making performance acceptable.

The Render spec includes dithering, so applications could choose which 
parts of the output would be dithered.  But, no-one has bothered to 
implement that part of the spec, and I'm not sure it will ever happen.

I briefly considered allowing users to specify which colors should be used 
for rendering, but discarded that notion as it would make sharing colors 
between windows much harder (note that the ICCCM 'standard colormaps' 
notion never took hold) and would require significant changes in the 
color->pixel code which uses the wide spacing of colors in the fixed 
Render map to reduce the precision needed within a precomputed table.

> I thought about mentioning it in my mail. I have the colormap in the
> constructors, but as currently, I don't actually use it. Colormaps
> are basically useful only for gray-scale (*), pseudo-color, and 
> direct-color visuals.

When Render is present, the colormaps are never necessary -- applications 
must use the colormap specified in the PictFormat.

The only question is what should be done for these visuals on X servers 
without Render.  Here's what I suggest:

StaticColor/StaticGray:		When the default visual, just use the default
				colormap.

				When not the default visual, just create a
				colormap, query the colors and free it.

PseudoColor/GrayScale:		When the default visual, develop a protocol
				for allocating a reasonable color cube such
				that it can be shared.

				When not the default visual, create a
				colormap and fill it in with a known color cube.
				This will cause flashing in some cases, but
				multiple cairo applications will all use
				the same colormap contents.

DirectColor:			Don't support.  I've never seen an X server
				advertise support for this without also
				supporting TrueColor.

For all of these cases, there isn't a huge amount of code to write as the 
existing X server render implementation should suffice for everything 
except the Xlib interface.

Note: StaticColor and StaticGray colors are per-visual, not per-colormap, 
so creating another colormap is a fine way to find out what the colors in 
that visual are.

-keith


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