[R] How to draw a transparent polygon

[Boris Steipe]

Hex-codes are ubiquitously used on the Web - they are a short-hand version of specifying rgb triplets. The first two digits specify the red value, the second two are for green, then blue. R accepts a fourth pair of digits to optionally specify the transparency, the semantics of the code is thus "#RRGGBB" or "#RRGGBBAA".

I prefer hex codes because they are compact, and convenient to make up colors on the fly. But there are many ways to define colors rigorously and then convert them.

E.g. if you go with one of the internally defined "named" colors:
R provides col2rgb() to convert different types of colors to rgb values, and rgb() to convert triplets of rgb values to hex-code. (Since col2rgb creates rows with values between 0 and 255, and rgb expects columns with intensities from 0 to 1, you have to transpose and divide).

rgb(t(col2rgb("red"))/255)        # "#FF0000"
rgb(t(col2rgb("peachpuff"))/255)  # "#FFDAB9"

But the real advantage is when using color matching and palette tools like:
   https://kuler.adobe.com/

Or you can extract color palettes from images like here:
   http://www.pictaculous.com/

Or you can use the "eye-dropper" tool of standard image editing software to access to color value of a particular pixel, or online here ...
   http://html-color-codes.info/colors-from-image/
   http://imagecolorpicker.com/

Many options.
Hope this helps,
Boris



On 2014-05-21, at 1:59 PM, Jun Shen wrote:

> Thanks everyone who replied. 
> 
> Boris, could you explain a bit more how to obtain this hex-code for a specific color? Thanks.
> 
> Jun
> 
> 
> On Wed, May 21, 2014 at 1:37 PM, Boris Steipe <boris.steipe at utoronto.ca> wrote:
> ... I just specify alpha values directly in the hex-code for a color:
> 
> set.seed(112352)
> plot(runif(20),runif(20), pch=16)
> polygon(runif(3), runif(3), col="#FF000022")
> polygon(runif(3), runif(3), col="#00FF0022")
> polygon(runif(3), runif(3), col="#0000FF22")
> 
> B.
> 
> 
> On 2014-05-21, at 12:05 PM, William Dunlap wrote:
> 
> > You can use adjustcolor() to do some of the arithmetic for you.  E.g.,
> > the following draws red squares with 10 opacities ('alpha's) from 5%
> > to 95%:
> >   plot(1:10)
> >   square <- 0.45 * cbind(c(-1,1,1,-1),c(-1,-1,1,1))
> >   for(i in 1:10) polygon(square+i, col=adjustcolor("red", alpha=(i-.5)/10))
> > Bill Dunlap
> > TIBCO Software
> > wdunlap tibco.com
> >
> >
> > On Wed, May 21, 2014 at 8:47 AM, David L Carlson <dcarlson at tamu.edu> wrote:
> >> The standard colors in R are opaque, but you can add an alpha value to make them semi-transparent. In this example we set alpha halfway between 0 and 255 to define a semi-transparent red:
> >>
> >>> set.seed(42)
> >>> x <- runif(10)*10
> >>> y <- runif(10)*10
> >>> plot(x, y, pch=16)
> >>> col2rgb("red", alpha=TRUE)
> >>      [,1]
> >> red    255
> >> green    0
> >> blue     0
> >> alpha  255
> >>> redtrans <- rgb(255, 0, 0, 127, maxColorValue=255)
> >>> polygon(c(2, 5, 8), c(2, 10, 2), col=redtrans)
> >>
> >> -------------------------------------
> >> David L Carlson
> >> Department of Anthropology
> >> Texas A&M University
> >> College Station, TX 77840-4352
> >>
> >>
> >>
> >>
> >>
> >> -----Original Message-----
> >> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of Jun Shen
> >> Sent: Wednesday, May 21, 2014 10:32 AM
> >> To: R-help
> >> Subject: [R] How to draw a transparent polygon
> >>
> >> Hi everyone,
> >>
> >> How do I draw a transparent ploygon overlaying with a scatter plot?
> >>
> >> Let's say, we call plot() to have a scatter plot, then call polygon() to
> >> add a polygon. I was hoping the polygon can be transparent so the scatter
> >> plot is still visible. I can't find any argument in polygon() for such a
> >> feature. Is there another way to do it? Thanks.
> >>
> >> Jun Shen
> >>
> >>        [[alternative HTML version deleted]]
> >>
> >> ______________________________________________
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> >> and provide commented, minimal, self-contained, reproducible code.
> >>
> >> ______________________________________________
> >> R-help at 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 at 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 at 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.
>