[GRASS-user] ps.map dilemma

[Glynn Clements]

Moritz Lennert wrote:

> > I see the New York Times often has nice bubble plots for their maps.
> > They make the fill color semi-transparent, then stacked bubbles are
> > darker where they overlap. PostScript won't "do" transparency, so
> > perhaps something to look for in the grass7 display drivers.
> 
> Doesn't postscript 3 include transparency ?

No.

PostScript will probably never attempt to include translucency in the
same way that video-oriented graphics systems do.

For video, translucency is implemented by alpha-blending, i.e. 
interpolating between the colour being drawn and what's already in the
frame buffer.

Printers only have 1-bit per component in the frame buffer, i.e. 1bpp
for mono, 4bpp for CMYK, and use halftones to simulate intermediate
shades. Interpolation doesn't work when you only have "off" and "on".

You can simulate translucency to an extent in PostScript using pattern
fills. But this is a lot more complex than with video, as you have to
specifically design all of the patterns to produce the correct result
when overlaid, which means figuring out which parts overlap each other
in advance.

The alternative is to render eveything to a 24-bpp image then print
that. But that creates huge PS/PDF files, which isn't a problem if
you're just sending PS to a printer, but isn't good if you're making
PDFs for download.

For similar reasons, PDF only supports translucency for filled areas,
not for images.

SVG doesn't have this problem, as it's targeted at video rather than
hardcopy.

-- 
Glynn Clements <glynn at gclements.plus.com>