[gdal-dev] Transparency using gdalbuildvrt

[Brent Fraser]

Jason,

  I'm doing some similar things with vrt and DEMs.  I can't comment about the alpha/RGBA thing but I was able to get the no-data setting used for the source data by editing the vrt file.  

  gdalbuildvrt creates a "SimpleSource" type of VRT:
-----------------
<VRTDataset rasterXSize="3601" rasterYSize="3601">

  <VRTRasterBand dataType="Int16" band="1">
    <NoDataValue>-3.27670000000000E+004</NoDataValue>
    <SimpleSource>
      <SourceFilename relativeToVRT="0">082\082a13_0102_deme.dem</SourceFilename>
      <SourceBand>1</SourceBand>
      <SourceProperties RasterXSize="1201" RasterYSize="1201" DataType="Int16" BlockXSize="1201" BlockYSize="1201"/>
      <SrcRect xOff="0" yOff="0" xSize="1201" ySize="1201"/>
      <DstRect xOff="2400" yOff="2400" xSize="1201" ySize="1201"/>
    </SimpleSource>
      :

-----------------

I edited the VRT to be a "ComplexSource" type, and added a <NODATA> tag for each source entry:
-----------------
<VRTDataset rasterXSize="3601" rasterYSize="3601">

  <VRTRasterBand dataType="Int16" band="1">
    <NoDataValue>-3.27670000000000E+004</NoDataValue>
    <ComplexSource>
      <SourceFilename relativeToVRT="0">082\082a13_0102_deme.dem</SourceFilename>
      <SourceBand>1</SourceBand>
      <SourceProperties RasterXSize="1201" RasterYSize="1201" DataType="Int16" BlockXSize="1201" BlockYSize="1201"/>
      <SrcRect xOff="0" yOff="0" xSize="1201" ySize="1201"/>
      <DstRect xOff="2400" yOff="2400" xSize="1201" ySize="1201"/>
      <NODATA>-32767</NODATA>
    </ComplexSource>
      :

-----------------

Best Regards, 
Brent Fraser


Jason Beverage wrote:
> Hi all,
> 
> I've been playing around with gdalbuildvrt lately and it has proven to 
> be a very useful tool:)
> 
> What I'm attempting to do is take multiple RGB images and treat them as 
> a single image using the VRT.  This works great, but since the images 
> are RGB, areas where there is no data are coming back as black.  Is 
> there a way that I can modify the VRT or the code to treat the VRT as an 
> RGBA dataset and somehow say "If there is no data in any bands, the 
> alpha is 0"?
> 
> Thanks!
> 
> Jason
> 
> 
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