QGIS automatically set transparency for (#000000) pixels and save *.qlr

Question

Starting with FME, i convert some hundreds of geotiffs to jpeg2000. The jpeg2000 writer does not support nodata-values. Adding the data to QGIS gives me these black (#000000) areas when rasters are rotated. With raster>properties i can set a transparency for #000000 and save a layer definition (*.qlr) for that raster which can be loaded on demand with the desired transparency.

Question: Since I have to do this for hundreds of rasters, is there a way (i.e. Python, GraphicalModeler) to automatically set transparency for a raster and save it to a layer definition?

Answer 1

I would read the JPEG2000 files through GDAL virtual format http://www.gdal.org/drv_vrt.html.

Convert JPEG2000 into VRT and assign nodata value with gdal_translate:

gdal_translate -of vrt -a_nodata 0 input.jp2 output.vrt

Check with gdalinfo

gdalinfo output.vrt

Band 1 Block=128x128 Type=Byte, ColorInterp=Red
  NoData Value=0
  Overviews: 6000x6000, 3000x3000, 1500x1500, 750x750, 375x375, 187x187
Band 2 Block=128x128 Type=Byte, ColorInterp=Green
  NoData Value=0
  Overviews: 6000x6000, 3000x3000, 1500x1500, 750x750, 375x375, 187x187
Band 3 Block=128x128 Type=Byte, ColorInterp=Blue
  NoData Value=0
  Overviews: 6000x6000, 3000x3000, 1500x1500, 750x750, 375x375, 187x187

Looks good with NoData. Now just keep both the .vrt and .jp2 files available when you open .vrt with QGIS.

The GDAL VRT file is a small XML file which has a pointer to the real image file and additional instructions about how to handle it. In this snippet "read file input.jp2 and consider 0 as NoData for band 1".

<VRTRasterBand dataType="Byte" band="1">
    <NoDataValue>0</NoDataValue>
    <ColorInterp>Red</ColorInterp>
    <SimpleSource>
      <SourceFilename relativeToVRT="1">input.jp2</SourceFilename>
      <SourceBand>1</SourceBand>
      <SourceProperties RasterXSize="12000" RasterYSize="12000" DataType="Byte" BlockXSize="1024" BlockYSize="1024" />
      <SrcRect xOff="0" yOff="0" xSize="12000" ySize="12000" />
      <DstRect xOff="0" yOff="0" xSize="12000" ySize="12000" />
    </SimpleSource>
  </VRTRasterBand>

Answer 2

Just a quick thought off the top of my head: maybe its possible to create the .qlr file by programmatic means without using qgis? Look at a generated qlr and see if you can make sense of it.

Answer 3

That worked fine for me. I put it in a for-do-done like

for f in *.jp2; do gdal_translate -of vrt -a_nodata 0 $f $f.vrt; done

Besides messages 'FAILURE: Too many command options.' i get the desired results as well.

JPEG2000:

VRT: