R: another serious issue with OpenGL on Linux

Thanks for testing, Brian.

So you don't see any issue if you copy/paste these lines at the end of your "redraw" function, before buffer swapping?
glReadBuffer(GL_FRONT);
glReadBuffer(GL_BACK);
It's really weird that I have such "flickering", because above lines should not do anything at all.
And instead they trigger this "flickering" that I do not understand.

I cannot provide a complete test program, but as you already experienced, the problem very likely wouldn't be reproducible on a simple test program.
So probably there is something else in my application (BTW, we use QGLWidget as the main graphic window,
and OSG to render inside that window; maybe there is some conflicts between them?).
Anyway, regardless of what we use, I still cannot see why changing and restoring "soon" a graphic attribute (the read buffer)
might affect the general behavior of the application.

Anyway, thanks, I really appreciate the time you spent on that.
Gianluca Natale

-----Messaggio originale-----
Da: Brian Paul <[email protected]> 
Inviato: lunedì 24 settembre 2018 17:48
A: Gianluca Natale <[email protected]>; [email protected]
Oggetto: Re: [Mesa-users] another serious issue with OpenGL on Linux

On 09/24/2018 07:26 AM, Gianluca Natale wrote:
> Hi again.
> 
> I'm very new to Mesa, as I said last week, and I have very serious 
> issues about using OpenGL on Linux.
> 
> The OpenGL code I wrote works fine on Windows, but has several issues 
> on Linux.
> As I have already said, I'm using a VM Linux machine, and when I run 
> my OpenGL application, it prints out this:
> 
> Vendor         :     VMware, Inc.
> 
> Renderer       :     Gallium 0.4 on llvmpipe (LLVM 3.6, 256 bits)
> 
> OpenGL Version :     2.1 Mesa 11.0.7
> 
> So, yes, it is using a software emulation of OpenGL.
> 
> My application creates a QGLWidget with a default rendering context 
> (so, double buffered).
> It works fine, until I try to copy a part of the front buffer into the 
> back buffer. From that point on, it starts flickering, as if it 
> renders in the front buffer directly.
> The significant fragment of my code is this (NOTE THAT I'M EXCLUDING 
> THE REAL COPY OF THE BUFFER HERE BELOW, BECAUSE I'VE DISCOVERED THAT 
> THE ISSUE IS TRIGGERED MUCH SIMPLY EVEN WITHOUT PERFORMING THE COPY).
> 
>          GLint drawBuf[1];
> 
>          glGetIntegerv(GL_DRAW_BUFFER, drawBuf);
> 
>          GLint readBuf[1];
> 
>          glGetIntegerv(GL_READ_BUFFER, readBuf);
> 
> glReadBuffer(GL_FRONT);
> 
>          // HERE THERE SHOULD BE THE CODE THAT COPIES A PART OF THE 
> FRONT BUFFER INTO THE BACK BUFFER
> 
>          glReadBuffer(readBuf[0]);
> 
>          glFinish();
> 
> As you can see, the code is very trivial. I simply change the read 
> buffer to GL_FRONT, and soon change it back to the previous value 
> (which was GL_BACK, as I could verify).
> But, once this code is executed, then on next refresh in graphics the 
> screen starts flickering.

I hacked that code into the Mesa gears demo and tested with llvmpipe but didn't see any issue.

Could you possibly supply a complete test program which shows the problem?  Modifying a Mesa/GLUT demo would probably be easiest.

-Brian

> 
> Basically, the call that causes the issue is that in yellow (if I 
> comment it out, the issue disappears).
> 
> Any idea why? Is it a bug in Mesa?
> 
> Above code works fine on Win, as I said.
> 
> Thanks,
> 
> Gianluca Natale
> 
> 
> 
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