Re: Tiff with signed samples

On Mon, 17 Aug 2020, Tom Lechner wrote:
>
> I'm trying to process this image, a 2G tiff with apparently signed samples:
> https://astrogeology.usgs.gov/search/map/Mars/GlobalSurveyor/MOLA/Mars_MGS_MOLA_DEM_mosaic_global_463m

The 2GB download size seems somewhat prohibitive to easily test with. 
:-)

> If I try to "gm convert" to a smaller size so I can actually edit on my 
> computer, it converts, but the samples seem to be read as unsigned, making 
> very light and very dark areas.
>
> I can mess with the pixel values to get something that seems right, but is 
> there some option I'm missing to convert properly with gm?

It is indeed true that GM currently lacks direct support for signed 
TIFF since it only uses unsigned types for its pixel quantum storage. 
The signed values will be parsed as if they are unsigned.  Values less 
than zero will become very large positive values since the top-bit is 
set in 2's-complement notation.  As long as the Quantum size in the GM 
build is at least the size of the original samples, then no actual 
loss should have occured yet.

A problem with signed values is that it is necessary to know the range 
of the values in order to make sense of the image.  Often the range is 
suggested by tags in the TIFF file.

If the range is all positive values, then the solution is relatively 
easy since the image will appear darker (by half) as normal.  If the 
range includes negative values, then things are more tricky.

GM's -operator command can be useful such as using the logical And 
operator, as well as the LShift, RShift, and Multiply operators to try 
to convert the data into useful unsigned values.

Software such as GDAL may be better suited for dealing with these 
images.

Bob
-- 
Bob Friesenhahn
[email protected], http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/
Public Key,     http://www.simplesystems.org/users/bfriesen/public-key.txt