[vtkusers] Offscreen rendering problems on headless Ubuntu

[Sophonet]

Hi list, happy new year!

I have compiled VTK 8.1.0 on a headless Ubuntu 16.04 with OSMesa support 
(flags: -D VTK_RENDERING_BACKEND="OpenGL2" -D 
VTK_OPENGL_HAS_OSMESA=True).

Using python (for example), I see an error about an unsupported OpenGL 
version (see below). However, Ubuntu comes with mesa 17.2.4 (which 
should be recent enough). Moreover, glxinfo shows "OpenGL core profile 
version string: 3.3 (Core Profile) Mesa 17.2.4.

Any clues what went wrong?

Thanks,

       Sophonet


In [1]: import vtk
In [2]: rw = vtk.vtkRenderWindow()
In [3]: rw.SetOffScreenRendering(1)
In [4]: rw.Render()
ERROR: In 
/home/CODE1/dep14473/SW/source/VTK-8.1.0/Rendering/OpenGL2/vtkOpenGLRenderWindow.cxx, 
line 797
vtkOSOpenGLRenderWindow (0x1991250): GL version 2.1 with the gpu_shader4 
extension is not supported by your graphics driver but is required for 
the new OpenGL rendering backend. Please update your OpenGL driver. If 
you are using Mesa please make sure you have version 10.6.5 or later and 
make sure your driver in Mesa supports OpenGL 3.2.