[gdal-dev] gdalbuildvrt -init 255

[Even Rouault]

Greg,

I don't think the issue is really about black or white, but more about 
setting nodata appropriately. Basically, if I've well understood your 
needs, you want to make some black padding in images transparent. 
Assuming that your imagery is RGB, you could try adding -srcnodata 0 to 
the gdalbuildvrt command line, so that pixels with value 0 in source 
imaginery are not drawn in the VRT buffer. You will also need to specify 
a -vrtnodata XXX (which is about like -init XXX of gdal_merge.py, but 
does more than -init as it sets that value as nodata), where XXX can be 
255 if you want it to be nodata white, but 0 should also do, but the 
value should not matter much to QGIS : it should just ignore the pixels 
matching the nodata value whichever it is.

You must be aware of the limitation of the nodata concept when working 
with RGB imagery however. Nodata is considered band per band, and not as 
a triplet. I mean if you have a blue pixel (0,0,255), and you set a 
nodata value of 255 for each of the 3 bands, the blue component of 
(0,0,255) being 255, it will be considered as nodata for that 
component... The only proper way to deal with that is to work with RGBA 
imagery.

Best regards,

Even
> By default, gdal_merge.py starts by creating an output image with all black pixels, and then replaces these black pixels with pixels from the images being read in. Passing to gdal_merge.py -init 255 pre-initializes the output image to all white, instead of all black, pixels.
> Similar to gdal_merge.py, gdalbuildvrt fills with black pixels. I am seeking for gdalbuildvrt, the -init 255 option that gdal_merge.py supports, but do not see -init 255 as a supported option on the gdalbuildvrt man page. In GDAL 1.7.0, will gdalbuildvrt support -init 255? If not, then in GAL 1.7.0 can the option -vrtnodata be used in a way similar to how gdal_merge.py supports -init 255? I need areas where there is no image to be white, rather than the black that gdalbuildvrt is by default setting them to be.
> Greg
>
> gdal_merge.py man page
> http://www.gdal.org/gdal_merge.html
> gdalbuildvrt man page
> http://www.gdal.org/gdalbuildvrt.html
> gdalbuild_black image
> http://homepage.mac.com/gregcoats/jp2/images/gdalbuildvrt_black.png
> gdalbuildvrt_white image (simulated)
> http://homepage.mac.com/gregcoats/jp2/images/gdalbuildvrt_white.png
>
> _______________________________________________
> gdal-dev mailing list
> gdal-dev at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/gdal-dev
>
>
>
>