Re: AW: [APML] Photoshop and bit depth (was: RG200+SHQ100 Composite Posted)

[Matt BenDaniel <matt@starmatt.com>]

Andreas Roerig wrote:
> How exactly do you adjust the levels to avoid loss of significant signal?

[Folks please feel free to correct me or add on]

Ultimately you will probably need to convert to 8-bit for your output
device/format.  If you do that at the latest possible step, you will
absolutely minimize information loss.  However, if you convert to 8-bit
early in the process, the way I do, it is important to adjust the levels
first.

In Photoshop, the basic step is to open a levels dialog, and for each of
the three color channels (red, green, blue), adjust the left and right
triangles to line up with the edges of the histogram.  You may also want
to also adjust the center triangle to make the image look closer to your
desired contrast and color balance.

The purpose is to spread the intensity values across the entire domain
(0-255), utilizing the maximum dynamic range.  Even if you are not
changing bit depth, adjusting the levels can make the image easier to
curve, especially when only a small portion of the domain was being
used.  If you adjust levels before reducing bit depth, you will reduce
(but not eliminate) information loss.

What happens if you don't do this?  Let's say you have an image with
narrow histograms.  Then you convert it to 8-bit.  Then you curve it in
a way that it looks pretty good, probably medium contrast.  Now look at
the histograms.  They will probably have a lot of "missing teeth". This
represents unused values.  The number of colors displayed will be
limited.  If the image contains content which is inherently very smooth
(e.g. a twilight sky), then you may see color banding artifacts, because
the colors don't change as gradually as they could across the image.
-- 
Matt BenDaniel
http://starmatt.com

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