Does RENDER accalerate sub-pixel AA?

Adam Jackson ajax at
Tue Sep 6 17:29:04 PDT 2005

On Tuesday 06 September 2005 11:51, Clemens Eisserer wrote:
> Hello,
> just because of interrest: Does RENDER accalerate sub-pixel font rendering?

Render is not an acceleration architecture.  Render is an X protocol extension 
that exposes subpixel alpha blending, among other things.

Both XAA and EXA give the driver enough information to accelerate subpixel 
alpha blending.  XAA's Render acceleration is sufficiently crippled that it's 
not really worth talking about.  EXA is better in this regard, but afaik none 
of the drivers with EXA support accelerate the subpixel alpha case (yet).

Most hardware would have to implement subpixel blending using a multipass 
algorithm, so I suspect the EXA core will gain the ability to attempt 
multipass blending for that case.  EXA wants some fallback logic for doing 
masked blends anyway, for single-tmu hardware.

Hardware with programmable pixel shaders could probably in principle 
accelerate subpixel alpha directly.  It remains to be seen whether a) that is 
a win, b) anyone is masochistic enough to try it within the EXA framework.  
My guess is that the answer to one or more of the above is "no".

I believe newish nVidia hardware does in fact have the ability to accelerate 
subpixel alpha blending directly through the texture engine (ie, no shaders).  
Which would be "easy" to implement in EXA, if we knew how to multitexture on 
nv cards and then also how to tell the chip that the mask is a 
component-alpha texture.

- ajax
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