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`Notes on Transparency <#toc12>`_
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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.
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