Re: Clipping

[Kennedy McEwen <rkm@xxxxxxxxxxxxxxxxxxxx>]

In article <00e001c1dcd6$1d11c360$ee51313f@ewxm9>, Gene DeVol 
<fedevol@mindspring.com> writes
>I've been following this group for about three months now, and have found it
>to be very educational.
>
>I have a question. Frequently, I see references to "clipping." Does that
>refer to a histogram that does not extend completely to either end of the
>scale, "0" at the shadow end, "255" at the highlight end? I have a Nikon
>SuperScan 4000, and I often see voids on both ends of the histogram.
>Recently, I have been scanning some Fuji Provia 100 slides - there, the void
>is mostly confined to the shadow side.
>
>Any insight would be appreciated in terms of defination, causes, fixes etc.
>
>Regards, Gene
>
Actually Gene, clipping can appear in at least two separate ways in the 
histogram, however in either case it is characterised by a sudden 
transition of the histogram from a large population of pixels to zero 
within one bin on the histogram.

Usually that transition occurs at the extreme limits of the histogram 
window - indicating that the image density range exceeds the range that 
the scanner has been set to accept: poor setting of black and white 
points can cause this, or poor manipulation of the image after scanning.

In some cases, the histogram can end abruptly within the displayed range 
- again caused by clipping somewhere in the scanning process.  What is 
happening here is that the scanner has been set to expect a much wider 
density range than it is capable of reproducing.  It is difficult to see 
why a properly functional scanner and driver should produce this effect, 
however there have been various reports of such histogram profiles.

Simply having a histogram which does not extend over the full range of 0 
to 255 does not, in itself, indicate clipping anywhere in the system. If 
the histogram gradually tapers off to zero population at either end then 
it is more likely to be just the scanner expecting a wider range of 
densities than actually are present (black or white points set too 
extreme for the image).
-- 
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's pissed.
Python Philosophers
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