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

[Even Rouault]

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

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

- 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"/>

Best regards,

Even