How can I retrieve private tag GDAL_NODATA from a TIFF using libtiff?

Question

In an earlier post I reported about my attempts to read TIFF with data stored in tiles using python and libtiff. First I tried to use libtiff's function TIFFReadEncodedTile but finally I tried function TIFFReadTile. I have been able to make progress now. The question I have today: how can I retrieve the value of private / custom tag GDAL_NODATA. Both GDAL and ArcGIS make use of this tag. I've seen examples of how to do this in C++ and C#. I still wonder how to do this in Python though. I guess it would be possible by means of the class TIFFExtender and the function add_tags - both defined in module libtiff_ctypes. Could anybody provide an example?

Answer 1

I've now figured out how to go about it. Before "libtiff.libtiff_ctypes.add_tags(extra_tags)" and before opening the TIFF file, I added this code:

from libtiff.libtiff_ctypes import TIFFFieldInfo, TIFFDataType, FIELD_CUSTOM
extra_tags = [
    TIFFFieldInfo(42113, -1, -1, TIFFDataType.TIFF_ASCII, FIELD_CUSTOM, True, False, "GDAL_NODATA"),
    TIFFFieldInfo(33550, 3, 3, TIFFDataType.TIFF_DOUBLE, FIELD_CUSTOM, True, False, "ModelPixelScaleTag"),
    TIFFFieldInfo(33922, 6, 6, TIFFDataType.TIFF_DOUBLE, FIELD_CUSTOM, True, False, "ModelTiepointTag"),
    TIFFFieldInfo(42112, -1, -1, TIFFDataType.TIFF_ASCII, FIELD_CUSTOM, True, False, "GDAL_METADATA")
]

I have added 3 other tags which are added when ArcGIS exports TIFF. You can then call: GetField("GDAL_NODATA")