[R] color blending and transparency

[Ken Knoblauch]

baptiste auguie <baptiste.auguie <at> googlemail.com> writes:
> Adding two semi-transparent colours results in non-intuitive colour
> mixing (a mystery for me anyway). Is it additive (light), substractive
> (paint), or something else? Consider the following example, depending
> on the order of the two "layers" the overlap region is either purple
> or dark red. I have no idea why.
> 
> png("testingOrder.png")
> plot.new()
> 
> # Red below
> rect(0.3, 0.5, 1, 1, col=rgb(1, 0, 0, alpha=0.5))
> rect(0, 0.5, 0.7, 1, col=rgb(0, 0, 1, alpha=0.5))
> 
> # Blue below
> rect(0, 0, 0.7, 0.5, col=rgb(0, 0, 1, alpha=0.5))
> rect(0.3, 0, 1, 0.5, col=rgb(1, 0, 0, alpha=0.5))
> 
> dev.off()
I would expect overlaid transparencies to act like filters and
multiply, producing so-called subtractive color mixing,
so blue and yellow gives green.  Interestingly, however,
overlaying filters is not necessarily a commutative 
operation, since a transparent filter can yield an
additive component (through scatter, for example)
though I suspect that the non-commutativity comes
about in R because these rules apply to physical lights,
filters and surfaces and in R, it is some uncalibrated combination
of frame buffer values that is being used.  

> Best,
> 
> baptiste
> 

Ken

-- 
Ken Knoblauch
Inserm U846
Stem-cell and Brain Research Institute
Department of Integrative Neurosciences
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