[GIS] How Does QGIS Read ‘nodata’ in 3 Band Raster Data

gdalgdalbuildvrtnodataqgistransparency

I have 3 band GeoTiffs of aerial imagery. The imagery is byte data with values of 0-255 on each band. A 'nodata' value has not been assigned on any band. I am assembling these images using a VRT, with no additional arguments, and displaying them in QGIS.

gdalbuildvrt out.vrt *.tif

Is it possible to tell QGIS or gdalbuildvrt to ONLY render transparency in those instances where:

b1 = 0
b2 = 0
b3 = 0

Currently, if you set transparency to zero in QGIS or use:

gdalbuildvrt -srcnodata "0 0 0" out.vrt *.tif

many instances outside of these conditions are rendered transparent. For example, if:

b1 = 0
b2 = 0
b3 = 15

Am I missing something in defining transparent/nodata values in QGIS or with gdal?

Best Answer

QGIS can handle this under the Layer Properties/Transparency.

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[GIS] Virtual Raster Mosaic – gdalbuildvrt – data gaps

Without -srcnodata or changing it to -3.39999999999999996e+38 didn't help and yielded the same faulty mosaic with data gaps.

However, changing the -srcnodata to "-3.3999999521443642e+38" (as displayed in QGIS) worked, and now I can obtain a proper mosaic. I understand that this is just a workaround and rather should be fixed by converting the source data. Thank you @user30184 !

QGIS 3 – Optimal Method to Convert Raster from 32bit Float to 8bit Byte

The resulting file is bigger because it has no compression. JPEG compression resulted in image degradation because it is a lossy compression method, LZW and DEFLATE are lossless (input value will always = output value). Lossy compression methods are good for data where filesize is more important than absolute data integrity, such as a stretched aerial photo purely for display.
You can set any value (within your data type range) as NoData, you don't have to use 0. In the example below, I use gdalbuildvrt to change the NoData value from -3.4e+38 to 255 which is within the Byte data range 0-255. (Note the -3.39999999999999996e+38 is just because gdalinfo is printing -3.4e+38 with floating point representation error, your NoData is -3.4e+38 which is the minimum value of the Float32 range)

The options for GDAL Translate (convert format) in the GUI window are somewhat hidden. To use the LZW compression method (or other lossless method), choose Profile : Default and then add a compression method with the big green + and enter manually COMPRESS under Name and LZW under Value.

For full functionality of GDAL Translate, use OSGeo4W Shell in Windows (or a terminal shell in Linux/MacOS). It is opened separately from QGIS Desktop, a different program available from the start-up menu. Prior to running the code below, set the path for files with cd followed by path (e.g. cd "C:/Users/yourname/gis files").

E.g (in Linux/MacOS Bash shell):

# Create an LZW compressed version
gdal_translate -co compress=LZW -co tiled=YES test.tif test_lzw.tif

# Convert NoData to 255
# the first file with .vrt is the virtual output, the second is the input
gdalbuildvrt -srcnodata -3.4E+38 -vrtnodata 255 test.vrt test.tif

# Write out a Byte with LZW compression TIFF file
# the first file with .vrt is the input (from buildvrt), the second is the output
gdal_translate -co compress=LZW -co tiled=YES -ot Byte test.vrt test_byte_lzw.tif

# Write out a Byte with no compression TIFF file
gdal_translate -co tiled=YES -ot Byte test.vrt test_byte.tif

# Check file info
gdalinfo test.tif -stats
gdalinfo test_byte_lzw.tif -stats

# Check file size
du -sh test.tif  # Uncompressed
du -h test_lzw.tif
du -h test_byte.tif  # Uncompressed 
du -h test_byte_lzw.tif  #My file uses random 0-31 values so will always be much bigger compressed than "real" data which compresses better.

Output:

Driver: GTiff/GeoTIFF
Files: test.tif
       test.tif.aux.xml
Size is 21768, 12920
<snip>
Band 1 Block=21768x1 Type=Float32, ColorInterp=Gray
  Min=0.000 Max=20.000 
  Minimum=0.000, Maximum=20.000, Mean=9.999, StdDev=8.165
  NoData Value=-3.39999999999999996e+38

Driver: GTiff/GeoTIFF
Files: test_byte_lzw.tif
Size is 21768, 12920
<snip>
Band 1 Block=256x256 Type=Byte, ColorInterp=Gray
  Minimum=0.000, Maximum=20.000, Mean=9.999, StdDev=8.165
  NoData Value=255

1.1G    test.tif
99M test_lzw.tif
275M    test_byte.tif
64M test_byte_lzw.tif

E.g (in OSGeo4W shell):

REM Create an LZW compressed version
gdal_translate -co compress=LZW -co tiled=YES test.tif test_lzw.tif

REM Convert NoData to 255
REM the first file with .vrt is the virtual output, the second is the input
gdalbuildvrt -srcnodata -3.4E+38 -vrtnodata 255 test.vrt test.tif

REM Write out a Byte with LZW compression TIFF file
REM the first file with .vrt is the input (from buildvrt), the second is the output
gdal_translate -co compress=LZW -co tiled=YES -ot Byte test.vrt test_byte_lzw.tif

REM Write out a Byte with no compression TIFF file
gdal_translate -co tiled=YES -ot Byte test.vrt test_byte.tif

REM Check file info
gdalinfo test.tif -stats
gdalinfo test_byte_lzw.tif -stats

REM Check file size
dir test.tif 
dir test_lzw.tif
dir test_byte.tif
dir test_byte_lzw.tif

Best Answer

Related Solutions

[GIS] Virtual Raster Mosaic – gdalbuildvrt – data gaps

QGIS 3 – Optimal Method to Convert Raster from 32bit Float to 8bit Byte

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