[Qgis-developer] gdal provider changes data type in the absence of nodata value

Etienne Tourigny etourigny.dev at gmail.com
Mon Jun 18 13:03:12 PDT 2012

A raster that does not contain a nodata value, a default value is
selected that is outside of the range of the raster's data type.

This is fine and a requirement for many operations, explained in

      // But we need a null value in case of reprojection and BTW also for
      // aligned margines

However, the data type is changed (to include the new nodata value)
which leads to unexpected results.

For example, calling QgsContrastEnhancement::minimumValuePossible() on
a raster with Byte data returns -32768 instead of 0.
You can see the effect of this if you load a 1-band Byte raster and
open the Layer Properties dialog (with no contrast enhancement),
min/max values are -32768/32767 - but they should be 0/255

Would it be possible to change this, perhaps adding a flag
"mNoDataSet" like in the GDAL GTiff driver, and keep the out-of-range
nodata value?


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