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
18 avenue du Doyen Lépine
69500 Bron
France
tel: +33 (0)4 72 91 34 77
fax: +33 (0)4 72 91 34 61
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http://www.sbri.fr/members/kenneth-knoblauch.html
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