Convert np.int8 to VTK_SIGNED_CHAR
VTK currently converts np.int8 to VTK_CHAR, however attempting to render a VTK_CHAR image with the GPU ray caster causes the following error:
[VTK] scalar of type VTK_CHAR is not supported because this type is platform dependent. Use VTK_SIGNED_CHAR or VTK_UNSIGNED_CHAR instead.
So it seems reasonable to convert np.int8 to VTK_SIGNED_CHAR instead.