• Utkarsh Ayachit's avatar
    Cleanup offscreen rendering support in VTK. · d0e7aab3
    Utkarsh Ayachit authored
    This commit cleans up offscreen rendering support in VTK. This includes
    several changes.
    
    1. It makes EGL support independent of onscreen GL
       support. It's now possible to enable EGL and GLX in same build of
       VTK, for example, so long as the two use shared GL-dispatch
       mechanism.  Previously, EGL could not be enabled together with GLX
       (i.e.  `VTK_USE_X` set to ON). These changes allow for that
       configuration.
    
    2. `VTK_USE_OFFSCREEN_EGL` is replaced by `VTK_OPENGL_HAS_EGL`.
        This is consistent with `VTK_OPENGL_HAS_OSMESA` flag. And also
        in similar vein to changes to `FindOpenGL.cmake` where EGL
        becomes a component of `find_package(OpenGL)`.
    
    3. `VTK_EGL_DEVICE_INDEX` has been replaced by
       `VTK_DEFAULT_EGL_DEVICE_INDEX` since it only affect the default
       value, and doesn't preclude users from manually specifying the
       device index.
    
    4. `VTK_USE_OFFSCREEN` has been replaced by
       `VTK_DEFAULT_RENDER_WINDOW_OFFSCREEN`, since similar to `3`
       this option was only intended to affect the default value.
    
    5. Simplified `FindEGL.cmake`. Apps don't need to link against
       `gldispatch` library and hence removed it and created an
       imported target to simplify use.
    
    6. For all changes to CMake variables, we put out `DEPRECATION`
       messages if old variables are used.
    
    7. When `VTK_USE_X` and `VTK_OPENGL_HAS_EGL` or
       `VTK_OPENGL_HAS_OSMESA` in ON, following things happen:
       i.   The object-factory simply returns the X-based (or
            onscreen) render window i.e. `vtkXOpenGLRenderWindow`.
       ii.  To create one of the offscreen render windows, one can
            manually instantiate them e.g. by explicitly
            instantiating `vtkEGLRenderWindow`.
            `vtkXOpenGLRenderWindow` has implementation to swap to
            OSMesa if `SetUseOffscreen(true)` is called.
       iii. As soon any off-screen features are enabled, `glew`
            uses the offscreen library APIs to get OpenGL function pointers
            irrespective of how the active context was initialized.
    d0e7aab3
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