[Mesa-dev] [PATCH] mesa/st: Don't modify the context draw/read buffers.

[Chia-I Wu]

On Thu, Dec 8, 2011 at 10:00 PM,  <jfonseca at vmware.com> wrote:
> From: José Fonseca <jfonseca at vmware.com>
>
> It sets the wrong values (GL_XXX_LEFT instead of GL_XXX), and no other
> Mesa driver does this, given that Mesa sets the right draw/read buffers
> provided the Mesa visual has the doublebuffer flag filled correctly
> which is the case.
In EGL, when an EGLSurface is created, users can specify whether the
front or back buffer will be rendered to.  The function is used to
make a double-buffered context work with an EGLSurface whose front
buffer is supposed to be rendered to.  But I admit that the function
is hacky.

Since this is brought up, I did this experiment some time ago:

  1. create a single-buffered drawable
  2. create a context with a GLX_DOUBLEBUFFER visual
  3. make the context and drawable current
  4. query GL_DRAW_BUFFER

Mesa returned GL_BACK and nVidia's proprietary driver returned
GL_FRONT.  This difference, IMHO, comes from that Mesa uses the visual
of the context to determine whether the context is double-buffered or
single-buffered, while nVidia uses the visual of the drawable to make
the decision (and at the time when the context is first made current).

What I want to argue here is that, maybe there should be no
single-buffered or double-buffered contexts, but single-buffered or
double-buffered drawables.  Or more precisely, the type of the context
should be determined by the type of the current drawable.  I checked
GLX spec and it seemed that GLX_DOUBLEBUFFER applies for drawables.
Since GL 3.0, GL_DOUBLEBUFFER is also listed as one of the framebuffer
dependent values.  That implies the state may change when the current
drawable changes.  So it is still a correct behavior for the drawable
to determine the type of the context.

I did not have a chance to look deeper into this due to the lack of
time.  So I may be terribly wrong here...


> ---
>  src/mesa/state_tracker/st_manager.c |   64 -----------------------------------
>  1 files changed, 0 insertions(+), 64 deletions(-)
>
> diff --git a/src/mesa/state_tracker/st_manager.c b/src/mesa/state_tracker/st_manager.c
> index d5228d3..55699e7 100644
> --- a/src/mesa/state_tracker/st_manager.c
> +++ b/src/mesa/state_tracker/st_manager.c
> @@ -404,54 +404,6 @@ st_visual_to_context_mode(const struct st_visual *visual,
>  }
>
>  /**
> - * Determine the default draw or read buffer from a visual.
> - */
> -static void
> -st_visual_to_default_buffer(const struct st_visual *visual,
> -                            GLenum *buffer, GLint *index)
> -{
> -   enum st_attachment_type statt;
> -   GLenum buf;
> -   gl_buffer_index idx;
> -
> -   statt = visual->render_buffer;
> -   /* do nothing if an invalid render buffer is specified */
> -   if (statt == ST_ATTACHMENT_INVALID ||
> -       !st_visual_have_buffers(visual, 1 << statt))
> -      return;
> -
> -   switch (statt) {
> -   case ST_ATTACHMENT_FRONT_LEFT:
> -      buf = GL_FRONT_LEFT;
> -      idx = BUFFER_FRONT_LEFT;
> -      break;
> -   case ST_ATTACHMENT_BACK_LEFT:
> -      buf = GL_BACK_LEFT;
> -      idx = BUFFER_BACK_LEFT;
> -      break;
> -   case ST_ATTACHMENT_FRONT_RIGHT:
> -      buf = GL_FRONT_RIGHT;
> -      idx = BUFFER_FRONT_RIGHT;
> -      break;
> -   case ST_ATTACHMENT_BACK_RIGHT:
> -      buf = GL_BACK_RIGHT;
> -      idx = BUFFER_BACK_RIGHT;
> -      break;
> -   default:
> -      buf = GL_NONE;
> -      idx = BUFFER_COUNT;
> -      break;
> -   }
> -
> -   if (buf != GL_NONE) {
> -      if (buffer)
> -         *buffer = buf;
> -      if (index)
> -         *index = idx;
> -   }
> -}
> -
> -/**
>  * Create a framebuffer from a manager interface.
>  */
>  static struct st_framebuffer *
> @@ -471,12 +423,6 @@ st_framebuffer_create(struct st_framebuffer_iface *stfbi)
>    st_visual_to_context_mode(stfbi->visual, &mode);
>    _mesa_initialize_window_framebuffer(&stfb->Base, &mode);
>
> -   /* modify the draw/read buffers of the fb */
> -   st_visual_to_default_buffer(stfbi->visual, &stfb->Base.ColorDrawBuffer[0],
> -         &stfb->Base._ColorDrawBufferIndexes[0]);
> -   st_visual_to_default_buffer(stfbi->visual, &stfb->Base.ColorReadBuffer,
> -         &stfb->Base._ColorReadBufferIndex);
> -
>    stfb->iface = stfbi;
>    stfb->iface_stamp = p_atomic_read(&stfbi->stamp) - 1;
>
> @@ -776,16 +722,6 @@ st_api_make_current(struct st_api *stapi, struct st_context_iface *stctxi,
>          if (stread != stdraw)
>             st_framebuffer_validate(stread, st);
>
> -         /* modify the draw/read buffers of the context */
> -         if (stdraw->iface) {
> -            st_visual_to_default_buffer(stdraw->iface->visual,
> -                  &st->ctx->Color.DrawBuffer[0], NULL);
> -         }
> -         if (stread->iface) {
> -            st_visual_to_default_buffer(stread->iface->visual,
> -                  &st->ctx->Pixel.ReadBuffer, NULL);
> -         }
> -
>          ret = _mesa_make_current(st->ctx, &stdraw->Base, &stread->Base);
>
>          st->draw_stamp = stdraw->stamp - 1;
> --
> 1.7.7.3
>



-- 
olv at LunarG.com