[vtk-developers] OpenGL error checking in VTK

[burlen]

Hi All,

I'm preparing to merge a patch( 
http://review.source.kitware.com/#/t/2980/) that implements OpenGL error 
checking and reporting in VTK and I'd like to give you a heads up and 
also to know if you have any comments or concerns before I make the 
merge. Here is the summary of the work.

      OpenGL error hunt

       This patch implements OpenGL error checking in VTK.

       OpenGL's error handling implementation error is designed such
       that internal error flags remain set with the first error that
       occurred until they are explicitly checked. With this design
       it's important to check and clear the error flags regularly
       else they become unusable as code checking for errors ends
       up reporting earlier undetected unrelated errors.

       This patch takes the following approach:

       1) at public entry points into code that uses OpenGL clear the
       error flags without reporting errors. This guards against
       reportinig unrealted errors, such as those caused by code
       outside of VTK. See vtkOpenGLClearErrorMacro

       2) before returning from functions that made OpenGL calls check
       for and report OpenGL errors. This detects Open GL errors in the
       function/method where they occurred facilitating debugging and it
       clears error flags so that user code doesn't detect errors caused
       by VTK. See vtkOpenGLCheckErrorMacro

       This patch cleans up a number of bugs that were detected by
       the new error checking and reporting.

       This patch also contains improvements for OpenGL pixel buffers, a
       renderbuffer object, and fast paths through framebuffer objects,
       and texture objects, and fast path for setting uniform variables,
       all of which are needed in vtkSurfaceLICPainter and
       vtkLineIntegralConvolution2D GPGPU code.

After the merge you may see reports for previously undetected OpenGL 
errors in your codes or ctest output. Also, on the Mac if you ask VTK to 
render into an "invalid drawable" you will find that all subsequent 
OpenGL calls fail  (until the drawable becomes valid) with 
INVALID_FRAMEBUFFER_OPERATION reported. The "invalid drawable" issue is 
not new (http://vtk.markmail.org/search/?q=invalid%20drawable). However, 
until now one could ignore this as OpenGL errors were not detected and 
reported. Going forward you will need to fix your code or disable new gl 
error detection and reporting. To fix your code you can make use of the 
new vtkRenderWindow::IsDrawable method to detect invalid 
drawable<http://vtk.markmail.org/search/?q=invalid%20drawable> and avoid 
asking VTK to render until the drawable is valid. Alternatively you may 
disable OpenGL error detection via a cmake variable allowing you to 
ignore the bug.

My motivation for this patch comes from my experience debugging my own 
VTK code and detecting errors that occurred elsewhere in VTK, in some 
instances far away in space and time (measured in lines of code). During 
this process I've noticed a good deal of variability between OpenGL 
implementations, some are very robust and tolerant of errors while 
others more strict/sensitive and can crash and burn on the same code. 
The new error detection and reporting can help quite a bit in tracking 
down cross platform/driver issues and ensures errors are detected at 
their source.

I think it would make sense going forward, that new code contributed to 
VTK that makes use of OpenGL take the approach I described above(with 
exception made for performance sensitive functions) so that OpenGL error 
flags remain usable by all, allowing us to more aggressively detect and 
fix bugs in our OpenGL codes.

Please let me know if you have comments or concerns about this patch.

Thanks
Burlen
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