[R-sig-Geo] writeRaster and RGB vs Gray

[MacQueen, Don]

Thanks, Barry, this is very helpful.

-Don

-- 
Don MacQueen

Lawrence Livermore National Laboratory
7000 East Ave., L-627
Livermore, CA 94550
925-423-1062





On 4/26/13 12:53 PM, "Barry Rowlingson" <b.rowlingson at lancaster.ac.uk>
wrote:

>On Fri, Apr 26, 2013 at 8:25 PM, MacQueen, Don <macqueen1 at llnl.gov> wrote:
>> Here's a little example script using functions from the raster package,
>>
>> r1 <- r2 <- r3 <- raster(ncol=25,nrow=25,
>>      xmn=0, ymn=0, xmx=1, ymx=1)
>> r1[] <- round(runif(25^2,0,255))
>> r2[] <- round(runif(25^2,0,255))
>> r3[] <- round(runif(25^2,0,255))
>> s <- stack(r1,r2,r3)
>>
>> plotRGB(s)
>>
>> writeRaster(s,'s.tif', format='GTiff', overwrite=TRUE,
>>             bandorder='BSQ')
>>
>>
>> When I open the file s.tif in QGIS, it's fine; I see the same image as
>>in
>> R.
>>
>> But the Mac platform's default viewer, Preview.app, thinks the file's
>> color model is "Gray", rather than "RGB", and it does not render
>>properly.
>> Preview does recognize, for example, a jpeg photo as having an RGB color
>> model.
>> (Adobe Reader (for Mac) wouldn't open it at all.)
>>
>> Is there some better set of arguments to give writeRaster that will
>> somehow make the file more broadly recognizable as RGB?
>
> Hmmm. I think QGIS just makes a guess that any 3-band image is R,G
>and B. But there must be some tag in a tiff file that specifies this
>explicitly...
>
>  If I create a tif as above, and also create one with gimp, then I
>can run 'tiffinfo' on them:
>
> The R one has "Photometric Interpretation: min-is-black", whereas the
>Gimp one has "RGB Color". The problem is now how to change that tag in
>the R output tiff!
>
> This can be done on the command line:
>
>tiffset -s 262 2 s.tif
>
> where 262 is the id of the photometric interpretation tag and 2 means
>'rgb': 
>http://www.awaresystems.be/imaging/tiff/tifftags/photometricinterpretation
>.html
>
> but I can't see how to do this in writeRaster...
>
> ah ha.
>
>writeRaster([all that other stuff], options="PHOTOMETRIC=RGB")
>
>got from the list of gdal driver options:
>http://www.gdal.org/frmt_gtiff.html
>
>sorted.
>
>Barry