http://cs.swansea.ac.uk/~csmark/PDFS/cgfvoxel.pdf
http://web.cs.wpi.edu/~matt/courses/cs563/talks/powwie/p1/ray-cast.htm#VolumeRendering
http://stackoverflow.com/questions/24029222/rendering-3d-shapes-on-a-voxel
Chapter 39. Volume Rendering Techniques http://http.developer.nvidia.com/GPUGems/gpugems_ch39.html
http://pubs.rsna.org/doi/pdf/10.1148/radiographics.19.3.g99ma14745
Documentation/4.3/Modules/VolumeRendering
http://ivl.calit2.net/wiki/images/5/58/18_VolumeRenderingF13.pdf
https://github.com/huoxudong125/OpenGL_3_3_Tutorial_Translation