On 08/11/2011 11:09 AM, Ian Romanick wrote:
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On 08/04/2011 06:29 AM, Brian Paul wrote:
On 08/04/2011 06:31 AM, Benjamin Franzke wrote:
Fixes https://bugs.freedesktop.org/show_bug.cgi?id=39588
egl_dri2 creates contexts with a doubleBufferConfig when PIXMAP and
WINDOW bit is request, so _mesa_init_color sets DrawBuffer[0] to
GL_BACK.
If a pixmap surface is created egl_dri2 will use a single buffer config,
so MakeCurrent has to adjust DrawBuffer[0] to the current drawable.
---
src/mesa/drivers/dri/intel/intel_context.c | 6 ++++++
1 files changed, 6 insertions(+), 0 deletions(-)
diff --git a/src/mesa/drivers/dri/intel/intel_context.c
b/src/mesa/drivers/dri/intel/intel_context.c
index fe8be08..0eeffc0 100644
--- a/src/mesa/drivers/dri/intel/intel_context.c
+++ b/src/mesa/drivers/dri/intel/intel_context.c
@@ -970,6 +970,12 @@ intelMakeCurrent(__DRIcontext * driContextPriv,
readFb = driReadPriv->driverPrivate;
driContextPriv->dri2.draw_stamp = driDrawPriv->dri2.stamp - 1;
driContextPriv->dri2.read_stamp = driReadPriv->dri2.stamp - 1;
+
+ if (fb->Visual.doubleBufferMode) {
+ intel->ctx.Color.DrawBuffer[0] = GL_BACK;
+ } else {
+ intel->ctx.Color.DrawBuffer[0] = GL_FRONT;
+ }
}
intel_prepare_render(intel);
This doesn't seem right to me. We shouldn't be changing context state
like that during a make-current() call.
During context initialization we call _mesa_init_color() where we set
ctx->Color.DrawBuffer[0] to either GL_BACK or GL_FRONT depending on the
visual's double-buffer flag. You might investigate why that's not doing
the job.
This entire approach is wrong, and it will break GLX. Page 28 (page 34
of the PDF) of the GLX 1.4 spec says (emphasis mine):
No error will be generated if the value of GL DRAW BUFFER in ctx
indicates a color buffer that is not supported by draw. In this
case, *all rendering will behave as if GL DRAW BUFFER was set to
NONE.* Also, no error will be generated if the value of GL READ
BUFFER in ctx does not correspond to a valid color buffer. Instead,
when an operation that reads from the color buffer is executed
(e.g., glReadPixels or glCopyPixels), the pixel values used will be
undefined until GL READ BUFFER is set to a color buffer that is
valid in read. Operations that query the value of GL READ BUFFER or
GL DRAW BUFFER (i.e., glGet, glPushAttrib) use the value set last
in the context, independent of whether it is a valid buffer in read
or draw.
Page 217 of the GL 3.3 spec says:
For the default framebuffer, in the initial state the draw buffer
for fragment color zero is BACK if there is a back buffer; FRONT
if there is no back buffer; and NONE if no default framebuffer is
associated with the context.
This seems a little ambiguous and perhaps in conflict with the GLX text.
I think the right answer is that the context draw buffer (and read
buffer) setting is initialized based on the drawable the first time the
context is bound. It seems like just changing _mesa_init_color to use
the double-buffer setting from the drawable (instead of the context)
should be sufficient.
Yeah, this is all a bit tricky.
Suppose the context is initially in a double-buffered state and we're
drawing to the back buffer. Then we bind the context to a
single-buffered window. We need to make sure we don't try to render
to the non-existant back buffer. There may be a few places in Mesa
where we're not prepared for that.
Anyway, from what I've gathered we should _not_ change the value of
GL_DRAW_BUFFER in this situation either.
I'm putting together a couple piglit GLX tests to exercise this. I
don't plan to send them to the piglit list until I can test them on a
non-Mesa driver. Since I'm still at SIGGRAPH, that won't be until next
week.
I also started on a piglit test for this bug got sidetracked. NVIDIA's
driver seems to do the following:
1. create double-buffered ctx and window, and make current.
2. query GL_DRAW_BUFFER returns GL_BACK
3. create single-buffered window and bind to the context.
4. query GL_DRAW_BUFFER still returns GL_BACK
5. drawing to the single-buffered window has no effect.
-Brian
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