[R] png() resolution problem {was "Silhouette ..."}

[Uwe Ligges]

Martin Maechler wrote:
> Hallo Sebastian,
> 
>>>>>> "SP" == Sebastian Pölsterl <sebp at k-d-w.org>
>>>>>>     on Sun, 14 Jun 2009 14:04:52 +0200 writes:
> 
>     SP> Hello Martin,
>     SP> I plotting the silhouette of a clustering and storing it as png. When I 
>     SP> try to store the image as png the bars are missing. The bars are plotted 
>     SP> when I use x11 or postscript as device. In addition, it seems to work 
>     SP> when I use a smaller matrix (e.g. ruspini).
> 
>     SP> Would be great if you have look at this issue.
> 
> Hmm, I've been at a conference in Italy...
> The silhouette plot only uses standard R plotting functions,
> so any problem with it exposes problems in standard R
> graphics.
> 
> --> Such a message should really go to R-help.
> to which I CC now.
> 
> ----------
> 
>      library(cluster)
>      nmat <- matrix(rnorm(2500*300), ncol=300, nrow=2500)
>      rmat <- matrix(rchisq(1000, 300, 50), ncol=300, nrow=1000)
>      mat <- rbind(nmat, rmat)
>      pr <- pam(mat, 2)
>      sil <- silhouette(pr)
>      png("sil.png")
>      #postscript("sil.ps")
>      plot(sil)
>      dev.off()
> 
> ----------
> 
> Anyway, I can confirm the "problem",
> but of course, it has not much to do with the silhouette
> function, but rather with the png() device which produces a
> bitmap, and the lines you draw are too fine (in the bitmap
> resolution) and so are "rounded to invisible".
> 
> You can reproduce the problem much more simply:
> 
> set.seed(1); x <- rlnorm(5000)
> 
> png("bar.png");barplot(x,col="gray",border=0,horiz=TRUE);dev.off()
> system("eog bar.png  &")
> 
> ## which is also empty, and the completely analogue, replacing 
> ## png [bitmap] with  pdf [vector graphic]
> 
> pdf("bar.pdf");barplot(x,col="gray",border=0,horiz=TRUE);dev.off()
> system("evince bar.pdf &")
> 
> ## gives a very nice plot, into which you can zoom and see all details.
> 
> ----------------
> 
> Now in principle you should be able to use  png() with a much
> higher resolution than the default one,
> but replacing the above
>     png("bar.bng")
> with
>     png("bar.bng", res = 1200)
> 
> did not help, as we now get the infamous
>     Error in plot.new() : figure margins too large
> 
> Other R-help readers will be able to make the png() example work
> for such cases, where you need so many lines.
> {but let's stick with barplot(*, border=0, *)}


Well, it's pretty hard, but one of the few cases where conversion from 
some vector image to the bitmap produces fair results, hence I'd use 
bitmap() with ghostscript to generate the png as in:

   bitmap("sil.png", type="pnggray", res=300)
   plot(sil)
   dev.off()

Best,
Uwe





> Regards,
> Martin Maechler, ETH Zurich
> 
> ______________________________________________
> 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.