[R] color blending and transparency
I am using ggplot and posted this question at that helplist. It was suggested that I try a more general R-help list for a possible solution to this problem. Within ggplot, I am using geom_area with red and blue and expect where they overlap should be purple. But instead, it's dark red. Playing with alpha and with different colors doesn't seem to solve the problem. Here's a very simple reproducible example R --arch x86_64 library(ggplot2) x-c(24,55,69,73) y-c(44,56,12,90) z-c(1,2,3,4) a-data.frame(pos=z, y=y, x=x) ex- ggplot(data=a, aes(pos)) + geom_area(aes(y = y),fill=navyblue, alpha = 0.7, position=identity) + geom_area(aes(y = x), fill= darkred, alpha = 0.7,position=identity ) + opts(panel.background = theme_rect(fill = white)) ex Likewise, with blue and yellow, I would expect overlap to be green. Are there any solutions to this that would allow color overlaps to lead to the expected color? Thank you. sessionInfo() R version 2.10.0 (2009-10-26) x86_64-apple-darwin9.8.0 locale: [1] en_US.UTF-8/en_US.UTF-8/C/C/en_US.UTF-8/en_US.UTF-8 attached base packages: [1] grid stats graphics grDevices utils datasets methods [8] base other attached packages: [1] ggplot2_0.8.3 reshape_0.8.3 plyr_0.1.9proto_0.3-8 [[alternative HTML version deleted]] __ 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.
Re: [R] color blending and transparency
On 02/03/2010 08:43 PM, bluecuttlefish wrote: I am using ggplot and posted this question at that helplist. It was suggested that I try a more general R-help list for a possible solution to this problem. Within ggplot, I am using geom_area with red and blue and expect where they overlap should be purple. But instead, it's dark red. Playing with alpha and with different colors doesn't seem to solve the problem. Here's a very simple reproducible example R --arch x86_64 library(ggplot2) x-c(24,55,69,73) y-c(44,56,12,90) z-c(1,2,3,4) a-data.frame(pos=z, y=y, x=x) ex- ggplot(data=a, aes(pos)) + geom_area(aes(y = y),fill=navyblue, alpha = 0.7, position=identity) + geom_area(aes(y = x), fill= darkred, alpha = 0.7,position=identity ) + opts(panel.background = theme_rect(fill = white)) ex Likewise, with blue and yellow, I would expect overlap to be green. Are there any solutions to this that would allow color overlaps to lead to the expected color? Hi bluecuttlefish, Think ink - should be easy. Only certain devices support transparency. Have you tried this on the pdf device? Jim __ 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.
Re: [R] color blending and transparency
Hi, 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() Best, baptiste On 3 February 2010 11:15, Jim Lemon j...@bitwrit.com.au wrote: On 02/03/2010 08:43 PM, bluecuttlefish wrote: I am using ggplot and posted this question at that helplist. It was suggested that I try a more general R-help list for a possible solution to this problem. Within ggplot, I am using geom_area with red and blue and expect where they overlap should be purple. But instead, it's dark red. Playing with alpha and with different colors doesn't seem to solve the problem. Here's a very simple reproducible example R --arch x86_64 library(ggplot2) x-c(24,55,69,73) y-c(44,56,12,90) z-c(1,2,3,4) a-data.frame(pos=z, y=y, x=x) ex- ggplot(data=a, aes(pos)) + geom_area(aes(y = y),fill=navyblue, alpha = 0.7, position=identity) + geom_area(aes(y = x), fill= darkred, alpha = 0.7,position=identity ) + opts(panel.background = theme_rect(fill = white)) ex Likewise, with blue and yellow, I would expect overlap to be green. Are there any solutions to this that would allow color overlaps to lead to the expected color? Hi bluecuttlefish, Think ink - should be easy. Only certain devices support transparency. Have you tried this on the pdf device? Jim __ 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. __ 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.
Re: [R] color blending and transparency
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 portable: +33 (0)6 84 10 64 10 http://www.sbri.fr/members/kenneth-knoblauch.html __ 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.
Re: [R] color blending and transparency
On 03/02/2010 8:50 AM, Ken Knoblauch wrote: 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)) I think it's a fairly simple calculation. In the first example: We are writing red (1,0,0) at alpha=0.5 onto white (1,1,1), so we get a mixture of half existing and half new, i.e. (1,0.5,0.5). Then we write blue (0,0,1) at alpha 0.5 onto that, giving (0.5, 0.25, 0.75). In the second pair, the first write yields (0.5,0.5,1), and the second yields (0.75, 0.25, 0.5). So this is like mixing paints: you don't get the same colour if you mix equal parts red and white, then take equal parts of that mixture with blue, as you get if you put the blue in first. You've got less red in the first mixture than in the second. You would get the same color in both mixtures if you didn't mix the white in: # Red below rect(0.3, 0.5, 1, 1, col=rgb(1, 0, 0, alpha=1)) 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=1)) rect(0.3, 0, 1, 0.5, col=rgb(1, 0, 0, alpha=0.5)) Duncan Murdoch 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 __ 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.
Re: [R] color blending and transparency
That makes perfect sense, thank you, except that I'm not sure where the white comes from when I set the background to transparent? png(testingOrder.png, bg = transparent) plot.new() par(bg=transparent) 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)) 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() Still produces two different overlap colours, although I *think* only two colours are involved. What I have I missed here? Thanks, baptiste On 3 February 2010 15:17, Duncan Murdoch murd...@stats.uwo.ca wrote: On 03/02/2010 8:50 AM, Ken Knoblauch wrote: 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)) I think it's a fairly simple calculation. In the first example: We are writing red (1,0,0) at alpha=0.5 onto white (1,1,1), so we get a mixture of half existing and half new, i.e. (1,0.5,0.5). Then we write blue (0,0,1) at alpha 0.5 onto that, giving (0.5, 0.25, 0.75). In the second pair, the first write yields (0.5,0.5,1), and the second yields (0.75, 0.25, 0.5). So this is like mixing paints: you don't get the same colour if you mix equal parts red and white, then take equal parts of that mixture with blue, as you get if you put the blue in first. You've got less red in the first mixture than in the second. You would get the same color in both mixtures if you didn't mix the white in: # Red below rect(0.3, 0.5, 1, 1, col=rgb(1, 0, 0, alpha=1)) 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=1)) rect(0.3, 0, 1, 0.5, col=rgb(1, 0, 0, alpha=0.5)) Duncan Murdoch 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 __ 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. __ 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.
Re: [R] color blending and transparency
On 03/02/2010 9:38 AM, baptiste auguie wrote: That makes perfect sense, thank you, except that I'm not sure where the white comes from when I set the background to transparent? You'd have to check the png device documentation or source code to find out what it does when you mix half red with half transparent. When I view a .png file in the default viewer in Windows 7, setting the background to transparent displays it as white, I don't see the things behind the window showing through. I don't know if it's the viewer or the file determining that. Duncan Murdoch png(testingOrder.png, bg = transparent) plot.new() par(bg=transparent) 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)) 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() Still produces two different overlap colours, although I *think* only two colours are involved. What I have I missed here? Thanks, baptiste On 3 February 2010 15:17, Duncan Murdoch murd...@stats.uwo.ca wrote: On 03/02/2010 8:50 AM, Ken Knoblauch wrote: 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)) I think it's a fairly simple calculation. In the first example: We are writing red (1,0,0) at alpha=0.5 onto white (1,1,1), so we get a mixture of half existing and half new, i.e. (1,0.5,0.5). Then we write blue (0,0,1) at alpha 0.5 onto that, giving (0.5, 0.25, 0.75). In the second pair, the first write yields (0.5,0.5,1), and the second yields (0.75, 0.25, 0.5). So this is like mixing paints: you don't get the same colour if you mix equal parts red and white, then take equal parts of that mixture with blue, as you get if you put the blue in first. You've got less red in the first mixture than in the second. You would get the same color in both mixtures if you didn't mix the white in: # Red below rect(0.3, 0.5, 1, 1, col=rgb(1, 0, 0, alpha=1)) 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=1)) rect(0.3, 0, 1, 0.5, col=rgb(1, 0, 0, alpha=0.5)) Duncan Murdoch 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 __ 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. __ 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. __ 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.
Re: [R] color blending and transparency
On Wed, 3 Feb 2010, baptiste auguie wrote: That makes perfect sense, thank you, except that I'm not sure where the white comes from when I set the background to transparent? png(testingOrder.png, bg = transparent) plot.new() par(bg=transparent) 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)) 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() Still produces two different overlap colours, although I *think* only two colours are involved. What I have I missed here? 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.eduUniversity 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.
Re: [R] color blending and transparency
On MacOSX I can tell Preview or Photoshop not to use a white background yet the mixing still shows a difference (with either pdf or png for that matter). So I guess it's something to do with mixing colours with the transparent channel as you say. I'll try to find the reason in the source code later (I haven't found any documentation on this so far). Thanks, baptiste On 3 February 2010 16:38, Duncan Murdoch murd...@stats.uwo.ca wrote: On 03/02/2010 9:38 AM, baptiste auguie wrote: That makes perfect sense, thank you, except that I'm not sure where the white comes from when I set the background to transparent? You'd have to check the png device documentation or source code to find out what it does when you mix half red with half transparent. When I view a .png file in the default viewer in Windows 7, setting the background to transparent displays it as white, I don't see the things behind the window showing through. I don't know if it's the viewer or the file determining that. Duncan Murdoch png(testingOrder.png, bg = transparent) plot.new() par(bg=transparent) 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)) 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() Still produces two different overlap colours, although I *think* only two colours are involved. What I have I missed here? Thanks, baptiste On 3 February 2010 15:17, Duncan Murdoch murd...@stats.uwo.ca wrote: On 03/02/2010 8:50 AM, Ken Knoblauch wrote: 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)) I think it's a fairly simple calculation. In the first example: We are writing red (1,0,0) at alpha=0.5 onto white (1,1,1), so we get a mixture of half existing and half new, i.e. (1,0.5,0.5). Then we write blue (0,0,1) at alpha 0.5 onto that, giving (0.5, 0.25, 0.75). In the second pair, the first write yields (0.5,0.5,1), and the second yields (0.75, 0.25, 0.5). So this is like mixing paints: you don't get the same colour if you mix equal parts red and white, then take equal parts of that mixture with blue, as you get if you put the blue in first. You've got less red in the first mixture than in the second. You would get the same color in both mixtures if you didn't mix the white in: # Red below rect(0.3, 0.5, 1, 1, col=rgb(1, 0, 0, alpha=1)) 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=1)) rect(0.3, 0, 1, 0.5, col=rgb(1, 0, 0, alpha=0.5)) Duncan Murdoch 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 __ 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. __ 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. __ 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.
Re: [R] color blending and transparency
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.
Re: [R] color blending and transparency
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.eduUniversity 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.
