Notes on Transparency
---------------------

The PostScript language originally had no accommodation for transparency.  However, Adobe
added an extension that allows developers to encode some forms of transparency
using the PostScript language model but it is only realized when converting the
PostScript to PDF (and via PDF to any raster image format).
GMT uses this model but there are some limitations: Transparency can only be controlled
on a per-object or per-layer basis.  This means that a color specifications (such as
those in CPTs of given via command-line options) only apply to vector graphic items
(i.e., text, lines, polygon fills) or to an entire layer (which could include items
such as PostScript images).  This limitation rules out any
mechanism of controlling transparency in such images on a pixel level.