[gdal-dev] Trouble handling HFA histogram (.img file)

[Frank Warmerdam]

On 11-01-05 04:09 PM, Michael Buchoff wrote:
> We are trying to convert a 16-bit img into an 8-bit tif. Unfortunately, the
> colors appear VERY dark (not downconverted, but just plain dark). I suspect
> that this has something to do with the histogram. The gdalinfo of the original
> image can be found here <http://pastebin.com/cmVxRJyE>.
>
> When I tell GDAL to convert straight from 16-bit to 8-bit with:
>
> gdal_translate -ot Byte -of GTiff 28jan09_worldview_tucumcari_nm.img output.tif
>
> I get what looks like the original image with clouds covering it… almost solid
> white. (See the gdalinfo here <http://pastebin.com/ike52mMp>)

Michael,

Note that the above gdal_translate is just truncating all values
above 255 to 255 so it is not surprising that you got bad results.

> When I scale around 16-bits:
>
> gdal_translate -ot Byte -of GTiff *-scale 0 65535 0 255*
> 28jan09_worldview_tucumcari_nm.img output.tif
>
> I get a solid black image. (See the gdalinfo here <http://pastebin.com/XpJL6AQY>)

Not surprising given that the data is all in the range
117 to 2047.

> When I scale around the extents specified by the histogram:
>
> gdal_translate -ot Byte -of GTiff *-scale 117 2047* 0 255
> 28jan09_worldview_tucumcari_nm.img output.tif
>
> I get the dark image described above. (See the gdalinfo here
> <http://pastebin.com/Ju4azd47>)
>
> I suspect if we can get GDAL to process and remove the histogram, we will not
> have these issues. Any help in doing this or further diagnosing the issue would
> be greatly appreciated.

I don't believe the histogram is coming into it.  gdal_translate always
does a linear scaling.  I think you need to either fool around with the
scaling range to get a more pleasing result or consider moving to a
non-linear scaling - perhaps a histogram equalization stretch.  For
a non-linear scaling you will need to consider another tool than
gdal_translate.  I produced a histogram equalization and downscaler
from 16bit to 8bit as "gdal/apps/gdalenhance.cpp" but I didn't really
finish to a broadly useful level.  Nevertheless you might find it
of some use, or a good starting point if you want to write something
yourself.

Of course, other software like GRASS, and OSSIM no doubt have
non-linear scaling options available for your use case.  The
world is bigger than GDAL.

Best regards,
-- 
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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    | Geospatial Programmer for Rent