[gdal-dev] How to make VRT offsite areas white?

[Rahkonen Jukka]

Even Rouault wrote:
> 
> Selon Jukka Rahkonen <jukka.rahkonen at mmmtike.fi>:
> 
> > Hi,
> >
> > I thought this would be easy but it was not.
> >
> > I have a mosaic of images which do not fill the whole 
> bounding box of the
> > area.
> > I made a .VRT file the images and converted the virtual 
> raster into geotiff.
> > Now
> > I have a nice mosaic but it has black boxes in the areas 
> where I do not have
> > source data. An example of the result is here
> > http://latuviitta.org/documents/nodata_3.tif
> >
> > I would rather paint nodata areas with white instead of 
> black. But how can I
> > make it? I thought that gdalwarp -dstnodata would do it but 
> areas of missing
> > source data remains black
> > gdalwarp -dstnodata "255,255,255" nodata.vrt nodata_3.tif
> >
> > The following command does make the black box white but 
> unfortunately is is
> > also
> > changing everything that was black in the orininal images, 
> including all the
> > texts, into white:
> > gdalwarp -dstnodata "255,255,255" -srcnodata "0,0,0" 
> nodata.vrt nodata_3.tif
> >
> > So what is the correct way to do this simple task?
> 
> Jukka,
> 
> Several options :
> 
> - use the -addalpha option of gdalbuildvrt

Doing 
gdalbuildvrt -addalpha nodata_alpha.vrt *.tif

and then
gdalwarp -dstnodata "255,255,255" nodata_alpha.vrt white_nodata.tif

makes the desired result.


> - use nearblack -setalpha ( http://gdal.org/nearblack.html ) ?

Did not try this one.

> - cheat a bit and create a white image that you add as the 
> first position in
> your VRT.
> 
> This can be a single pixel image that you artifically expand 
> to full VRT size,
> like this :
> 
>       <SrcRect xOff="0" yOff="0" xSize="1" ySize="1"/>
>       <DstRect xOff="0" yOff="0" xSize="replace_by_vrt_x_size"
> ySize="replace_by_vrt_y_size"/>

I have nothing against cheating and I will try this with my real dataset. Dataset is pretty big and running it through gdalwarp takes several hours more time than running it through gdal_translate.


> Best regards,

Thank you once again.

-Jukka-

> 
> Even
>