On Wed, 3 Feb 2010, baptiste auguie wrote:
Thanks for this complementary information. My head itches slightly
when reading about these virtual layers with unidirectional absorption
and reflection properties but I guess that's imputable to my personal
background as a physicist.
It's not unidirectional, it's just infinitesimal thickness -- if the light
didn't hit a pigment particle in the top layer on the way in, it won't on the
way out either. Not realistic, but very physicist ;)
I still have a few questions,
- is this behavior documented? (I'm happy to look at the source, but
I'm not even sure if it's in grDevices, device specific, or somewhere
else altogether)
Several of the devices delegate the computations to the operating system or viewer, so
that's the easiest way to find the algorithm. It's the Porter-Duff compositing
algorithm, the "over" operator.
- have there been any attempts at implementing other options for
colour blending? It would be nice to be able to switch between
additive and substrative colour mixing rules on occasion, but as far
as I understand the scheme we discussed is hard coded deep into R's
internals (correct?). An option not to use the background at all in
the "mixing" / "reflection" process would be great already.
There are some limitations from the ink-on-paper graphics model in R, since the
resulting color from n layers must be a function only of the resulting color
from n-1 layers and the top color (and not, for example, a function of n).
Also, for PDF and SVG we are basically limited to models that PDF implements
automatically, since it isn't feasible to work out which objects overlap and
adjust their colors that way.
-thomas
Thomas Lumley Assoc. Professor, Biostatistics
tlum...@u.washington.edu University of Washington, Seattle
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