[Gdal-dev] sample scaling

Alan Stewart astewart at TerraGoTech.com
Mon Oct 30 14:09:08 EST 2006

Thanks for the response, Frank.

I didn't know about the -mm flag for gdalinfo. However, it does reveal
the sample values as lying in the 0-2047 range. I was using Global
Mapper, but OpenEV has the same issue. I also have been provided a JPEG
overview; the overview color balance matches the display of the 16 bit
per sample image.

In looking more closely at tiffinfo output, I see one difference other
than the expected change in bits per sample. The 16 bit per sample image
has a Photometric Interpretation of "min-is-black" and has an Extra
Samples tag not present in the 8 bit per sample image. The 8 bit per
sample image has a Photometric Interpretation of "RGB color".

Frank Warmerdam wrote:
> Alan Stewart wrote:
>> I've got NITF files that have three bands @ 16 bits per sample. I 
>> need to produce a GeoTIFF with 8-bit samples, JPEG compressed. In my 
>> first attempts the resulting image appeared black. I discovered a 
>> GDAL metadata item. "NITF_USE00A_DYNAMIC_RANGE=02047", which I take 
>> it means that sample values range from 0 to 2047. If I use "-scale 0 
>> 2047 0 65535" the resulting TIFF looks fine, but if I use "-scale 0 
>> 2047 0 255 -ot Byte" the image is much darker and greenish. I thought 
>> this would just rescale the sample values to fit in the range 0-255 
>> and create a TIFF with 8-bit samples. I didn't expect the color 
>> balance of the image to change. Is "Byte" actually a signed value or 
>> something? If so, how do I get an unsigned 8-bit sample?
> Alan,
> Your understanding of how things should work sound right.  I don't know
> why the results are bad.  How are you checking the results?  What viewer
> did you use with the -scale 0 2047 0 65536 output tiff that looked good?
> You might want to view your image in OpenEV, and inspect various pixels
> to make sure that the dynamic range info of 02047 is accurate.  Or
> use "gdalinfo -mm your.ntf" to actually scan the data for min and max 
> values.
> Best regards,


Alan Stewart
Senior Software Architect
TerraGo Technologies, Inc.
astewart at TerraGoTech.com
678-391-9700 x9615

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