[filmscanners] RE: 8bits vs. 16bits/channel: can the eye see the difference

["Karasev, Alexander" <alexander.karasev@gs.com>]

I think it is quite an assertion you are making there, Paul, that "the level
of noise in a real-world image, either from film grain or CCD noise, is
always greater than a least-significant-bit of an 8-bit value." There are
situations when this is not the case. One perhaps is scanning a very
fine-grained emulsion at a medium resolution; another, dealing with
downsampled images (from medium or large format film?), and yet another,
applying a smoothing algorithm to the sky and/or other areas that do not
carry details or patterns. These are just a few off the top of my head.

Besides, once it is established that the eye *can* see the difference
between any two RGB24 levels, which this experiment does, the doubt as to
the advantage of the greater than 8 bits color depth for input and output as
well (not just processing) goes out the window. If your input is noisy, it
could mask the limitations of 8 bits/channel representation - OR perhaps
even lesser representations, depending on how bad the image noise is. But
the better the images, the more obvious will be the proven-to-be-visible
advantage of the 16 bits / channel representation.

Alex

>Date: Fri, 14 Mar 2003 17:16:19 -0800
>From: "Paul D. DeRocco"
>But the level of noise in a real-world image, either from film grain or CCD
>noise, is always greater than a least-significant-bit of an 8-bit value.
>This means that finer gradations are indeed represented in an 8-bit image
>through dithering. Your test isn't a fair test.
----------------------------------------
>> From: Karasev, Alexander
>> Actually most observers can, in smooth areas (sky, etc.) particularly in
>> midrange tones.
>>
>> Create in Photoshop, Paintbrush, CorelDraw, or other software, two large
>> adjacent rectangles or other figures, one with a solid RGB24 fill of
>> [128,128,128] (i.e. neutral grey on the scale of 0-255), and
>> another either
>> [129,129,129] or [128,128,129]. Most people who used to argue for lack of
>> benefit [if only in final output] of higher color depths than 8 bits /
>> channel, are usually quite stunned at their ability to pick up the
>> difference, often at a glance.
>>
>> That means, the same limitations can apply to photo reproductions of
>> substantially high quality and low noise, of smooth areas (such as a
>> clear blue or uniformly "grey" sky).

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