R help - Jul 2009 - Read PNG file and display with more than 256 colors RGDAL

Hello list

I am trying to use a Googlemaps tile (png file, 640 X 640 px tile) as a
background to a plot and have been using the rgdal library to read in the PNG
file (modified from code in the RGoogleMaps package).  This works OK.  My
problem is is that the SGDF2PCT function in rgdal seems to be limited to 256
colors and this is introducing (I think) some degradation to the image which
appears on the output graph as speckling.

Is there a way around this?  I have been reading about the EBImage package but
it (a) seemed to have too many dependencies on other software for my purposes
and (b) wouldn't load properly in any case on my machine (couldn't find
dll's that it was looking for....)


Ideally I would like to read in the png image and display in its original form
as a plot background, hopefully with all its colors represented rather than
being converted to a 256 color scale.


A code example is below:

  png(file="file.png", 4000,4000)
  par(mar=c(0,0,0,0))
  myTile <- readGDAL("basemap.png",silent=TRUE); #basemap.png is a
PNG file returned from googlemaps
  myTile at data <- myTile at data[,1:3]
  col <- SGDF2PCT(myTile,ncolors=256) ## myTile is a spatialGridDataFrame
with 3 bands
  myTile$ind <- col$idx ## add the colour index to the data frame
  myTile <- as.image.SpatialGridDataFrame(myTile["ind"],1,2)$z;
  attr(myTile, "COL") <- col$ct;
  attr(myTile, "type") <- "rgb";
  myTile <- list(lat.center, lon.center,NA, myTile)

  image(z=myTile[[4]], col = attr(myTile[[4]], "COL"))

  dev.off()
> sessionInfo()
R version 2.9.1 (2009-06-26) 
i386-pc-mingw32 

locale:
LC_COLLATE=English_Australia.1252;LC_CTYPE=English_Australia.1252;LC_MONETARY=English_Australia.1252;LC_NUMERIC=C;LC_TIME=English_Australia.1252

attached base packages:
[1] grid      stats     graphics  grDevices utils     datasets  methods   base

other attached packages:
[1] abind_1.1-0    rgdal_0.6-9    sp_0.9-37      gridBase_0.4-3

loaded via a namespace (and not attached):
[1] lattice_0.17-25 tools_2.9.1    


Paul Rustomji

If you read the help page for the image method you are actually using:

?image.SpatialGridDataFrame

you see that it supports red=, green=, and blue= arguments. This suggests
that you could try:

myTile <- readGDAL("basemap.png",silent=TRUE)
class(myTile)
summary(myTile)
# assuming the positions are 1, 2, 3
image(myTile, red=1, green=2, blue=3)

On the other hand, the "colours" in these images are arbitrary
manipulations
of the sensors, and have already been warped and resampled, so decimating
the colour representation isn't inappropriate, and 256 is not a small
number.

Roger Bivand

PS. Detailed rgdal questions like this may get a faster response on
R-sig-geo


Paul.Rustomji wrote:
> 
> Hello list
> 
> I am trying to use a Googlemaps tile (png file, 640 X 640 px tile) as a
> background to a plot and have been using the rgdal library to read in the
> PNG file (modified from code in the RGoogleMaps package).  This works OK. 
> My problem is is that the SGDF2PCT function in rgdal seems to be limited
> to 256 colors and this is introducing (I think) some degradation to the
> image which appears on the output graph as speckling.
> 
> Is there a way around this?  I have been reading about the EBImage package
> but it (a) seemed to have too many dependencies on other software for my
> purposes and (b) wouldn't load properly in any case on my machine
> (couldn't find dll's that it was looking for....)
> 
> 
> Ideally I would like to read in the png image and display in its original
> form as a plot background, hopefully with all its colors represented
> rather than being converted to a 256 color scale.
> 
> 
> A code example is below:
> 
>   png(file="file.png", 4000,4000)
>   par(mar=c(0,0,0,0))
>   myTile <- readGDAL("basemap.png",silent=TRUE); #basemap.png
is a PNG
> file returned from googlemaps
>   myTile at data <- myTile at data[,1:3]
>   col <- SGDF2PCT(myTile,ncolors=256) ## myTile is a
spatialGridDataFrame
> with 3 bands
>   myTile$ind <- col$idx ## add the colour index to the data frame
>   myTile <-
as.image.SpatialGridDataFrame(myTile["ind"],1,2)$z;
>   attr(myTile, "COL") <- col$ct;
>   attr(myTile, "type") <- "rgb";
>   myTile <- list(lat.center, lon.center,NA, myTile)
> 
>   image(z=myTile[[4]], col = attr(myTile[[4]], "COL"))
> 
>   dev.off()
> 
>> sessionInfo()
> R version 2.9.1 (2009-06-26) 
> i386-pc-mingw32 
> 
> locale:
>
LC_COLLATE=English_Australia.1252;LC_CTYPE=English_Australia.1252;LC_MONETARY=English_Australia.1252;LC_NUMERIC=C;LC_TIME=English_Australia.1252
> 
> attached base packages:
> [1] grid      stats     graphics  grDevices utils     datasets  methods  
> base     
> 
> other attached packages:
> [1] abind_1.1-0    rgdal_0.6-9    sp_0.9-37      gridBase_0.4-3
> 
> loaded via a namespace (and not attached):
> [1] lattice_0.17-25 tools_2.9.1    
> 
> 
> Paul Rustomji
> 
> ______________________________________________
> 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.
> 
> 
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