Re: [Mesa3d-dev] Mesa (gallium-st-api): st/glx: Sync the back buffer to the front buffer.

[Keith W. <kei...@go...>]

On Sun, Mar 14, 2010 at 10:28 AM, Chia-I Wu <ol...@gm...> wrote:
> On Sun, Mar 14, 2010 at 4:53 PM, Keith Whitwell
> <kei...@go...> wrote:
>> This looks like it introduces an extra full-window copying operation
>> at every swapbuffers...  is that correct?
>> If so, I think we should try to figure out an alternative approach
>> without the copying...  would actually flipping between two textures
>> (thus preserving the old back/new front) contents be workable?
> Buffers swapping is handled in xmesa_swap_st_framebuffer.  It is a zero-copy
> operation.
>
> This commit is to fix a bug when this code path is hit
>
>  /* draw something and ... */
>  glXSwapBuffers();
>  glReadBuffer(GL_FRONT);
>  glReadPixels();
>
> In the above, glReadBuffer will cause BUFFER_FRONT_LEFT renderbuffer to be
> added on demand as can be seen in st_ReadBuffer.  The validation list would
> become { ST_ATTACHMENT_FRONT_LEFT, ST_ATTACHMENT_BACK_LEFT }.  When
> glReadPixels is called, st/mesa will validate with the list and read from the
> front buffer.
>
> On the st/glx side, this use pattern is catched.  When the front buffer is
> allocated, the contents of the back buffer will be copied to the front buffer.
> This happens only once.  When the same code is run again, glXSwapBuffers will
> swap the buffers.
>
> The copying is required because xmesa_swap_st_framebuffer does not always swap
> the buffers.  It is so that only the back buffer is allocated when the
> application never draws/reads the front buffer.
>
> --
> ol...@Lu...
>

OK, sorry for being slow & thanks for explaining...

Keith