[Gdal-dev] wrong nodata returned in GDAL dependent software

[Frank Warmerdam]

Maciej Sieczka wrote:
> Hi,
> 
> Although my GeoTIFF has a value range 0-255 with nodata set to 256 [1],
> both QGIS 0.8.1 and OpenEV output '255' when I query the area supposed
> to be nodata. Any ideas what could be the culprit? I *suspect* the
> issue might be in GDAL itself, as I can reproduce the problem in two
> different viewers. Example GeoTiff attached. My specs:

Maciej,

Your gdalinfo report listed the file as data type Byte.  The largest value
that can be represented in a Byte is 255.  So there is no way to read back
values of 256 from such a file.

If the file was written with GDAL from a buffer bigger than Byte but
to a file with data type Byte, then GDAL would clamp a value of 256 to
the maximum representable value, 255, while writing.  Perhaps this happened?

In any event, it is not meaningful to have a nodata value of 256 for a byte
data file.

Best regards,
-- 
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGeo, http://osgeo.org