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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