VTK_RENDERING_BACKEND defaults to OpenGL2 thereby not installing a number of files that are required by other packages (e.g. vtkVolumeTextureMapper3D.h required by ginkgocadx:imaging). Options for the rendering backend are None, OpenGL2, and OpenGL. I would suggest a use flag "opengl2" that is disabled by default. many thanks. Reproducible: Always
vtk-7 is no longer in Gentoo ebuild repo.