At 3:49 PM +0000 12/6/03, olympus-digest wrote:
>Date: Fri, 5 Dec 2003 21:58:46 -0800
>From: Stephen Scharf <scharfsj@xxxxxxxxxxxxx>
>Subject: [OM] Re: Different way of seeing...
>
>[snip]
>
>Earl, with all due respect, our eyes are more like a digital camera
>than they are anything like film. They are, in fact, R/G/B sensors
>(much like a digital camera) capable of distinguishing over
>12,000,000 colors. Moreover, our brain uses what is effectively a
>"look-up table" to map the colors that we see; just a computer does.
>For more in formation on how our brain sees color; check out Edwin
>Land's Retinex Theory of Color Perception (Land, Edwin H. "Recent
>Advances in Retinex Theory and Some Implications for Cortical
>Computations: Color Vision and the Natural Image", Proc. Natl. Acad.
>Sci. USA, Vol. 80 pp. 5163-5169; August 1983 Physics).
A gamut of 12,000,000 colors. Let's see what this implies about the biological
RGB sensors of the eye:
There are three colors, so this 12,000,000-color space is the product of the
dynamic ranges of the three kinds of color sensors. If we assume that all
three colors have the same dynamic range (it isn't quite true, but never mind),
then each color's sensors have an instantaneous dynamic range of the cube root
of 12,000,000, which is 228.94 to one, or 7.84 bits, call it 8 bits. This is
another aspect of the fact that humans can detect brightness differences of
about 1%. Humans are in fact somewhat more sensitive to color differences than
brightness differences, which shows in the implied 1/228.94= 0.44 hange in one
color component needed to make a distinguishible color.
So, why do we need more than 8 bits per color in photography? Because, unlike
in a standard color-matching experiment, the brightness of the scene varies
from place to place, even if the scene is monochrome. The eye's response is
closer to logarithmic than linear, and in addition the eye's overall
sensitivity adjusts over a wide range to match average scene brightness.
How many bits one would need in the ideal is the subject of "How many
bits/pixel does a digital image need?" posted 19 September 2003, which
concluded that 16 to 18 bits per color per pixel would achieve perfection in
linear sensors, in the sense that more bits would not help. So, we are at 12
bits, and counting...
Joe Gwinn
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