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Re: filmscanners: ColorSteps?

On 05 Apr 2001 11:01:44 EDT  Richard Starr (Richard.Starr@valley.net) 

> I don't know if color steps is the right term but it seems to be a 
> display
> problem.  In several high resolution scans, I've seen some odd areas of 
> color
> that should be continuous appear to step from one tone to another as 
> though
> displayed in 256 colors or fewer. 

Usually called 'contouring' (like a map) or 'posterisation' (like a poster 
printed without contones).

You will be able to chack whether it is the file, or a problem with the 
display/graphics system, by viewing the histogram. Contouring shows up as 
missing bit values, leaving the histogram looking like a mangy dog's comb.

What file type is this, and what processing has been done (and by what) en 
route to the screen? And what scanner/software?

If at any stage the file has been manipulated as an 8 bit file, 
posterisation is easily possible. This can include adjustments within the 
scanner driver, at acquisition time - especially if the s/w performs 
adjustments after reduction to 8 bits from the scanner's native bit depth. 
The more severe the adjustments (eg gamma), the more likely this is to 


Tony Sleep
http://www.halftone.co.uk - Online portfolio & exhibit; + film scanner 
info & comparisons


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