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8-bit and 16-bit images

Raphael Bustin
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On 22 Jun 2006 17:28:03 -0700, "Bill Hilton" <(E-Mail Removed)>

>>>>Raphael Bustin wrote:

>> Nor would I claim that a single conversion from 16 bits/chan
>> to 8 bits/chan would "ruin" any image. That's poppycock.

>I'm not sure what you mean here ... I've never seen anyone claim
>converting from one mode to the other damages an image.

I'm saying that extra bit-depth is useful to have before and
during the major color moves. Once you stop shifting tones
around, 8 bits per color will hold any sensible end-result.

IOW, I think we mostly agree.

rafe b
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Raphael Bustin
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On 22 Jun 2006 17:28:03 -0700, "Bill Hilton" <(E-Mail Removed)>

>OK, I ran a quick test with these steps and you are right, you see
>posterizing on the 8 bit image but not the same image in 16 bit mode.
>Here's the example I ran, starting with a tiff, converting to jpeg with
>8 level quality (which would make it more likely to posterize but
>that's a reasonable quality level with Photoshop's jpeg conversion),
>duplicating the 8 bit version and changing to 16 bit mode and running
>the same steps on both six times.

And notice that the effect is only apparent in the unfocused
background. You'd never notice the difference in the
brightly colored foreground detail.

Roger Clark and I had this same discussion a while back
and I was arguing your side. It became obvious after a
few go-arounds that Roger was doing much more
radical color moves than I typically do. If you're going
to wail on the colors, the bit depth is useful. If you get
the scan right in the first place and limit the extent
(and number) of color moves, you can get by with less.

rafe b
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Dave Martindale
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"Bill Hilton" <(E-Mail Removed)> writes:

>If you entered "2" for levels I'd expect 8 colors if you are right (3
>bits total), but according to the Help files for Posterize (CS version)
>this is not so ... the Help file says "For example, choosing two tonal
>levels in an RGB image gives six colors: two for red, two for green,
>and two for blue."

Maybe the writer wasn't counting white and black, which should also be
available with 2 levels per colour.

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Robert Feinman
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In article <(E-Mail Removed) .com>, Removed) says...
> At any rate, if you can provide an example of an 8 bit jpeg that looks

I have two discussions about 8 vs 16 bits in the tips section of my
web site:

They show *differences* between the two images, whether one is
better than the other or the differences are "noticeable" is in
the eye of the beholder.

I see no harm in keeping to more than 8 bits for most of the editing
process (except for slowing down the editing) and then converting
to 8 bit for output (print or online). For digital cameras I can
see speed and storage space factors, but once again each person
will have to decide for themselves. If you have an important image
and you capture it in 8 bits or in jpeg format and sometime in the
future you want to reprocess it to display with some miracle new
technology, your options will be limited. If you keep all the data
available you will have more options later.

Robert D Feinman
Landscapes, Cityscapes and Panoramic Photographs
mail: (E-Mail Removed)
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