[vtkusers] Combined on-screen and off-screen possible with VTK-7.0.0 on linux/X?

[Sophonet]

Dear list,

in my application, I would like to display a vtkRenderWindow for user 
interaction, but I also would like to use off-screen functionality 
simultaneously to render pre-defined views / snapshots that are shown as 
static images, like "thumbnails" for different datasets.

This was possible in VTK 5.10 on linux (by creating a dedicated 
vtkRenderWindow per thumbnail, setting OffScreenRenderingOn, rendering 
and capturing the resulting image with a vtkWindowToImageFilter).

In VTK-7.0.0.rc1, using the OpenGL rendering backend on Mesa 9.2.3, I 
now get a segfault with the following backtrace:

(gdb) bt
#0  0x00007fffd045db90 in xcb_glx_get_string_string_length ()
    from /usr/lib64/libxcb-glx.so.0
#1  0x00007fffd0cce605 in ?? () from /usr/lib64/libGL.so.1
#2  0x00007fffd0ccc2b6 in ?? () from /usr/lib64/libGL.so.1
#3  0x00007fffd14cd086 in 
vtkOpenGLExtensionManager::ReadOpenGLExtensions() ()
    from 
/import/QUINTA/non-public/software/linux-install/quinta_gcc_debug/lib/libvtkRenderingOpenGL-7.0.so.1
#4  0x00007fffd14cbd50 in vtkOpenGLExtensionManager::Update() ()
    from 
/import/QUINTA/non-public/software/linux-install/quinta_gcc_debug/lib/libvtkRenderingOpenGL-7.0.so.1
#5  0x00007fffd14cafd7 in 
vtkOpenGLExtensionManager::ExtensionSupported(char const*) ()
    from 
/import/QUINTA/non-public/software/linux-install/quinta_gcc_debug/lib/libvtkRenderingOpenGL-7.0.so.1
#6  0x00007fffd14ec8ea in vtkOpenGLRenderWindow::OpenGLInitContext() ()
    from 
/import/QUINTA/non-public/software/linux-install/quinta_gcc_debug/lib/libvtkRenderingOpenGL-7.0.so.1
#7  0x00007fffd14eae6d in vtkOpenGLRenderWindow::OpenGLInit() ()
    from 
/import/QUINTA/non-public/software/linux-install/quinta_gcc_debug/lib/libvtkRenderingOpenGL-7.0.so.1
#8  0x00007fffd1576932 in 
vtkXOpenGLRenderWindow::CreateOffScreenWindow(int, int) ()

Besides a remark that probably there could be better error handling than 
just letting the app crash without an exception being thrown, I would 
like to know if joint on-screen rendering and off-screen rendering is 
still possible somehow with VTK-7.0.0.

I have made two approaches when configuring via CMake: (i) Setting 
VTK_OPENGL_HAS_OSMESA=True (no effect, I think I remember that in 
earlier versions, this allowed OSMESA_INCLUDE_DIR etc. to be set; this 
is not the case anymore, so does this flag still do something? Note that 
for VTK 5.10 as mentioned above I did not have to set this explicitly), 
(ii) Setting USE_OFFSXCREEN_EGL=True - resulting in an error that this 
could not be combined with VTK_USE_X.

On Windows, combining on-screen and off-screen rendering as described in 
the second paragraph is possible.

Any suggestions would be welcome, the Wiki page I have found on this 
topic is not fully understandable to me 
(http://www.vtk.org/Wiki/VTK/OSMesa_Support; does this mean that I have 
to switch on VTK_OPENGL_HAS_OSMESA or not?...)

Thanks,

       sophonet