Am 22.03.2011 12:42, schrieb gregjohn:
> In SEM work, it's not that 99.9% of PIXELS exist between GL64 and 192. Instead,
> there's a distribution of electron counts (current?), and some bloke arbitrarily
> decided to make 64 the min and 192 the max. Why didn't they decide to make it
> like GL32 and 224?   I can say with some authority that failure to use the full
> grey scale range in SEM work in in many cases cause one to lose information that
> is present in the imaging conditions. I'm finding it annoying that people write
> auto contrast brightness routines that throw away information.

Leaving the brightest and darkest pixel values unused does have the 
advantage that even on poorly calibrated displays the brightest and 
darkest regions still show contrast. Keeping clear of the minimum & 
maximum representable values also has benefits when it comes to certain 
image processing, as some intermediate steps might otherwise 
over-/underflow. While these were probably bigger issues some decades 
ago, when calibrated displays were sparse and image processing was 
almost invariably done on bytes to conserve memory, tradition and 
backward compatibility may have preserved it to this date.

Using a range from 64 to 191, rather than 32 to 223, has the advantage 
that you get exactly 128 (2^7) brightness levels. Computers love powers 
of 2.

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