  Despite the popularity of the Graphics Processing Unit (GPU) for general
purpose computing, one should not forget about the practicality of the GPU for
fast scientific visualisation. As astronomers have increasing access to three
dimensional (3D) data from instruments and facilities like integral field units
and radio interferometers, visualisation techniques such as volume rendering
offer means to quickly explore spectral cubes as a whole. As most 3D
visualisation techniques have been developed in fields of research like medical
imaging and fluid dynamics, many transfer functions are not optimal for
astronomical data. We demonstrate how transfer functions and graphics shaders
can be exploited to provide new astronomy-specific explorative colouring
methods. We present 12 shaders, including four novel transfer functions
specifically designed to produce intuitive and informative 3D visualisations of
spectral cube data. We compare their utility to classic colour mapping. The
remaining shaders highlight how common computation like filtering, smoothing
and line ratio algorithms can be integrated as part of the graphics pipeline.
We discuss how this can be achieved by utilising the parallelism of modern GPUs
along with a shading language, letting astronomers apply these new techniques
at interactive frame rates. All shaders investigated in this work are included
in the open source software shwirl (Vohl, 2017).
