An update to Joe Conti's popular vol3d function, allowing voxel colors and alpha values to be defined explicitly. Functionality is increased, but the function is entirely backwards compatible.
In cases where voxels can be any RGB color, use:
vol3d('CData', cdata);
where cdata is an MxNxPx3 array, with RGB color along the 4th dimension. In cases where color and alpha values are highly independent, specify an MxNxP alphamatte as follows:
vol3d('CData', cdata, 'Alpha', alpha);
This function amends the original (as little as possible) with the full permission of Joe Conti. The original function, which came with some additional tools for editing colormaps and alphamaps, is sadly no longer available.

The way this rendering approach works is to render the six sides of each voxel as a translucent plane.