[Rd] readPNG gives warnings and doesn't execute sample code from help files

[Simon Urbanek]

Joris,

On Feb 14, 2011, at 10:05 AM, Joris Meys wrote:

> Dear all,
> 
> I noticed in the latest R version (R.2.12.1) that the readPNG gives
> following warning when running the example code in the help file (or
> when using any other png for that matter) :
> 
> 50: In rasterImage(img, 1.2, 1.27, 1.8, 1.73) :
>  Per-pixel alpha not supported on this device
> 
> No picture is shown, and code I used to be able to run, doesn't run any more.
> 

You may want to use a device that supports alpha. The R logo in the example uses alpha so are probably the images you are using. If you don't want to (or can't) use a device that supports alpha, you'll have to flatten the alpha, - i.e. plot just img[,,1:3]
However, most images don't have color where alpha is zero, so you'll have to replace it with the background color, e.g.:
r = as.raster(img[,,1:3])
r[img[,,4] == 0] = "white"

Cheers,
Simon



>> sessionInfo()
> R version 2.12.1 (2010-12-16)
> Platform: i386-pc-mingw32/i386 (32-bit)
> 
> locale:
> [1] LC_COLLATE=English_United States.1252  LC_CTYPE=English_United
> States.1252    LC_MONETARY=English_United States.1252
> [4] LC_NUMERIC=C                           LC_TIME=English_United
> States.1252
> 
> attached base packages:
> [1] grDevices datasets  splines   graphics  stats     tcltk     utils
>   methods   base
> 
> other attached packages:
> [1] png_0.1-2       svSocket_0.9-51 TinnR_1.0.3     R2HTML_2.2
> Hmisc_3.8-3     survival_2.36-2
> 
> loaded via a namespace (and not attached):
> [1] cluster_1.13.2  grid_2.12.1     lattice_0.19-13 svMisc_0.9-61
> tools_2.12.1
> 
> 
> -- 
> Joris Meys
> Statistical consultant
> 
> Ghent University
> Faculty of Bioscience Engineering
> Department of Applied mathematics, biometrics and process control
> 
> tel : +32 9 264 59 87
> Joris.Meys at Ugent.be
> -------------------------------
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>