[gdal-dev] <MaskBand>...</MaskBand> Usage in vrt files.

[ajuvolkov]

Hello. I can't find any information about the using mask bands in virtual
datasets of Gdal (*.vrt files).
I wan't to mask region in my source dataset.

For example, I have the Earth map in Plate Carree projection. Map resolution
is 1000x500 pixels. I want to define a valid area inside the rectangle: x =
400, y = 200, width = 100, height = 50, and reproject all map to another
projection. 
I found this article:
http://www.gdal.org/gdal_vrttut.html, that says that I should use the
<MaskBand> options.

I wrote a vrt file:

<VRTDataset rasterXSize="1000" rasterYSize="500">
   <SRS>GEOGCS[...]</SRS>
   <GeoTransform>...</GeoTransform>
   <VRTRasterBand dataType="Byte" band="1">
   ...
   </VRTRasterBand>
   <VRTRasterBand dataType="Byte" band="2">
   ...
   </VRTRasterBand>
   <VRTRasterBand dataType="Byte" band="3">
   ...
   </VRTRasterBand>
   <MaskBand>
    <VRTRasterBand dataType="Byte">
      <SimpleSource>
        <SourceFilename relativeToVRT="1">plate.png</SourceFilename>
        <SourceBand>mask,1</SourceBand>
        <SrcRect xOff="0" yOff="0" xSize="1000" ySize="500" />
        <DstRect xOff="0" yOff="0" xSize="1000" ySize="500" />
      </SimpleSource>
    </VRTRasterBand>
  </MaskBand>  
</VRTDataset>

What does it mean?:
<SrcRect xOff="0" yOff="0" xSize="1000" ySize="500" />
<DstRect xOff="0" yOff="0" xSize="1000" ySize="500" />

What I should past to this parameters? If I write 
<SrcRect xOff="400" yOff="200" xSize="100" ySize="50" />
I got unexpected results.


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