On Fri, 09 May 2008 17:33:36 GMT, Norm Dresner wrote:
> I asked a few months ago about the difference between manually doing
Levels
> and using the Auto Levels command because when the program does it
> automatically there are no holes in the histogram and whenever I did it
> manually I got a lot of spikes with spaces around them. At the time I
was
> told -- and it sounded convincing -- that the program does it
individually
> on each of the channels separately and then merges the result which
produces
> a continuous histogram.
>
> WELL ... I've tried doing it channel-by-channel (R/G/B) manually about a
> dozen times now and I still get a combined RGB histogram that shows many
> missing levels, much as it did when I just leveled the whole image.
>
> SO ... I'm asking again: what does the program do when applying Auto
Levels
> that results in a continuous histogram that can't be duplicated
manually?
> Does it intentionally spread out the levels so that there are no holes?
It doesn't actually do this. Changing the individual channels in levels,
automatically or not, always results in a degree of histogram combing and
gaps. These are masked if you look at the composite histogram, which is a
weighted average of the other channels.
The histogram is often incorrectly used as an indicator of quality, or
lack
thereof, or as a warning as to when banding might occur. This
conventional
wisdom has submerged any usefulness that the histogram might offer, and
the
end result is a tool that has caused far more harm than good. I recommend
not using it. I because convinced of this after reading Dan Margulis.
Adding noise to individual channels is the best way to eliminate banding,
should it occur.
If you have time to spend, curves is a better tool for improving the
image,
since it can do every thing levels can, and more.
--
Mike Russell - http://www.curvemeister.com
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