On Thu, Dec 8, 2011 at 10:00 PM, <jfons...@vmware.com> wrote:
> From: José Fonseca <jfons...@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
>
--
o...@lunarg.com
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