[Mesa-dev] [PATCH 5/7] intel: Directly implement blit glBlitFramebuffer instead of awkward reuse.

[Eric Anholt]

Kenneth Graunke <kenneth at whitecape.org> writes:

> On 06/05/2013 10:14 AM, Eric Anholt wrote:
>> This gets us support for blitting to attachment types other than
>> textures.
>> +      /* Blit to all active draw buffers.  We don't do any pre-checking,
>> +       * because we assume that copying to MRTs is rare, and failure midway
>> +       * through copying is even more rare.  Given that feedback loops in
>> +       * glFramebufferBlit() are undefined, we can safely fail out after
>> +       * having partially completed our copies.
>
> The first part of this comment makes sense: yes, it's obviously rare.
> The second part I feel could use a little more explanation:
>
> If we fail midway through a blit, we return without clearing 
> BUFFER_BITS_COLOR from the mask.  The caller will fall back to the next 
> method (i.e. meta_BlitFramebuffer) and use that to blit /all/ RTs - even 
> ones we already successfully blit.  Given that feedback loops in 
> glFramebufferBlit() are undefined, we can safely fail out after having 
> partially completed our copies.
>
> Unless there's some kind of blending, it doesn't even seem 
> undefined...meta should just overwrite the colors again, and everything 
> will work out.

Better?

      /* Blit to all active draw buffers.  We don't do any pre-checking,
       * because we assume that copying to MRTs is rare, and failure midway
       * through copying is even more rare.  Even if it was to occur, it's
       * safe to let meta start the copy over from scratch, because
       * glBlitFramebuffer completely overwrites the destination pixels, and
       * results are undefined if any destination pixels have a dependency on
       * source pixels.
       */
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