Re: 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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