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.
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) - 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. Thanks again, baptiste On 3 February 2010 17:04, Thomas Lumley <tlum...@u.washington.edu> wrote: > My mental model for this, which I haven't bothered to check against the > actual algorithms, is that colors are composed of reflective/absorbing > pigment particles and that alpha says how densely they are packed. Alpha=0 > means all the light gets through to bounce of what ever is below, eventually > to the white paper, and alpha=1 means that all the light is reflected from > the top layer of paint. > > With 50% blue over 50% red, you reflect 50% of the blue light and absorb 50% > of the red and green light in the top layer of paint. Of the remaining > light, 50% of the red is reflected and 50% of the green and blue absorbed by > the particles in the bottom layer of paint. Anything that makes it through > will reflect off the white paper. > > There is the additional complication that a transparent background still > behaves as if it had white paper behind it (it's drawn on an acetate sheet > which you lay on paper to see it more clearly). > > -thomas > > > Thomas Lumley Assoc. Professor, Biostatistics > tlum...@u.washington.edu University of Washington, Seattle > > ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.