[Rd] re: transparency

[Byron Ellis]

On Fri, 10 Aug 2001, Prof Brian D Ripley wrote:

> On Sat, 11 Aug 2001, Ross Ihaka wrote:
> 
> 
> I had thought of that, but it's a lot more complicated (and software does
> not even agree which of 0 or 255 is transparent).  I think for now I am
> going to look into using 0 and 255 (for solid and transparent) and leave
> open the possibility of implementing the intermediate values. For example,
> it may be my ignorance, but I don't know of support for alpha channels in
> postscript or PDF (at least, PDF 1.3) or Windows Metafiles. PNG does have
> alpha channel support, but simple transparency is more widely implemented.

Transparency is supported in Postscript Level 3 (I don't have my red book
handy, but its there) and will be supported in PDF 1.4, both of those are
full alpha channel transparency models.  WMF doesn't support transparency
in the sense that we're discussing here (it supports a variant of the
region transparency---an all or nothing ordeal). 

As far as interactive displays are concerned-- there probably won't be
true transparency as in the OS X sense where a window can have some
intrinsic alpha value, but most modern canvas packages (the GTK package
obviously and there is support in the Windows GDI API...sort of... I
haven't had a chance to look at GDI+, part of WinXP, but judging from its 
gratitous use in WinXP betas... :-) ) do support some notion of an alpha
channel, though as you point out they might decide 0xFF is opaque rather
than 0x00... This isn't that big of a problem, R would just pick one and
the device implementation should be responsible for subtracting properly
(or doing whatever needs be done to system palettes).

> 
> Brian
> 
> -- 
> Brian D. Ripley,                  ripley@stats.ox.ac.uk
> Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
> University of Oxford,             Tel:  +44 1865 272861 (self)
> 1 South Parks Road,                     +44 1865 272860 (secr)
> Oxford OX1 3TG, UK                Fax:  +44 1865 272595
> 
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Byron Ellis (bellis@hsph.harvard.edu)
"Oook" - The Librarian



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