[vtkusers] Invalidating the OpenGL2 shader cache

[Ken Martin]

Last I heard glGet calls can sometimes stall the pipeline so I have been
trying to avoid them. I usually use the method clearing the shader cache
when mixing opengl codes and that seems to work.

On Tue, Oct 16, 2018 at 9:21 PM, Benjamin Schleimer <BSchleimer at vytronus.com
> wrote:

> Hi,
>
>    I have come across an interesting problem with VTK and I wanted to ask
> if there is another way to work around it.
>
>
> Using the OpenGL2 backend, we have discovered that if another system
> renders to the same opengl context as VTK,
>
> then the shader cache is put into an invalidated state.
>
>
> The root problem is that in Rendering/OpenGL2/vtkOpenGLShaderCache.cxx,
> in vtkOpenGLShaderCache::BindShader(), the lastShaderBound is assumed to
> not change. VTK assumes that it's the only system setting glUseProgram()
> for the opengl context.
>
> So if another system clears the GL_CURRENT_PROGRAM value, then VTK doesn't
> know about it and the following calls to set program uniforms fails with an
> opengl error.
>
>
> A suggested is to add a call to glGetIntegerv(GL_CURRENT_PROGRAM, ...)
> and compare it against this->Handle before returning.
>
>
> I worked around this problem by clearing the shader cache before invoking
> vtkRenderWindow::Render() but this required knowing that the shader cache
> was invalid.
>
>
> Sincerely,
>
> Ben
>
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-- 
Ken Martin PhD
Distinguished Engineer
Kitware Inc.
101 East Weaver Street
Carrboro, North Carolina
27510 USA

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