RFC 58: Removing Dataset Nodata Value

Authors: Sean Gillies

Contact: sean at mapbox.com

Status: Adopted, implemented

Implementation version: 2.1

Summary

This RFC concerns addition of a DeleteNoDataValue() function to the C++ GDALRasterBand API. This function removes the nodata value of a raster band. When it succeeds, the raster band will have no nodata value. When it fails, the nodata value will be unchanged.

Rationale

The nodata value has accessors GetNoDataValue() and SetNoDataValue(). For GeoTIFFs, the value is stored in a TIFFTAG_GDAL_NODATA TIFF tag. Newly created GeoTIFF files can have no nodata value (no tag), but once a nodata value is set and stored it can only be given new values, it can not be removed. Nor can it be set to a value outside the range of the data type; for 8-bit data passing nan, -inf, or 256 to GDALSetNoDataValue() has the same effect as passing 0.

The problem with un-removable nodata values is this:

Nodata masks (see GDAL RFC 15) can cover up a nodata value but if the .msk file gets lost (and this is ever the problem with sidecar files), the nodata value you were hiding is exposed again.
Nodata masks are not available everywhere in GDAL, nodata values are the only definition of valid data in some parts of GDAL.

The current recommended practice for removing a nodata value is to copy the GeoTIFF using gdal_translate, specifying that the nodata tag not be copied over along with the data. By making the nodata value fully editable and removable we could avoid copying unnecessarily.

Changes

The virtual CPLErr DeleteNoDataValue() method will be added to the GDALRasterBand method in gdal_priv.h (C++ API), and CPLErr GDALDeleteRasterNoDataValue() to gdal.h (C API)

Updated drivers

The following drivers will be updated: GTiff, MEM, VRT and KEA.

For GTiff, the TIFFTAG_GDAL_NODATA TIFF tag is unset if GDAL is built against libtiff 4.X, which is the appropriate behavior. For libtiff 3.X where TIFFUnsetField() does not exist, the tag is set to the empty string : GDAL 2.0 will detect that as the absence of a nodata value, older versions will parse it as 0.

The GDALPamRasterBand class will also be updated (for drivers that have no built-in mechanism nodata mechanism and rely on .aux.xml sidecars). Note that this only removes the tag from the .aux.xml, so in the situation where a driver would have a internal way of storing nodata, but would be opened in read-only mode (so defaulting to PAM), DeleteNoData() would have no effect.

SWIG bindings (Python / Java / C# / Perl) changes

The DeleteNoDataValue() method is added to the RasterBand object.

Utilities

The gdal_edit.py script is enhanced with a -unsetnodata option.

Documentation

The new method and function are documented.

Test Suite

The tests of the updated drivers test the effect of the new method.

Compatibility Issues

None for the C API. ABI change for the C++ API because of introduction of a new virtual method.

Implementation

Implementation will be done by Even Rouault and be sponsored by Mapbox.

A proposed implementation is available in https://github.com/OSGeo/gdal/compare/trunk...rouault:rfc58_removing_nodata_value

Voting history

+1 from EvenR, HowardB, DanielM and JukkaR