[futurebasic] Re: [FB] Trillions of Colors

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From: michael evans <michael.evans@...>
Date: Wed, 26 May 2004 10:52:46 -0400
on 5/26/04 9:57 AM, tedd at tedd@... wrote:

> Hi gang:
> 
> Can anyone tell me what's going on with the "new" (new to me that is)
> color values. The old colors were pretty simple, RBG with values that
> ranged from 0-255 (00-FF) -- three bytes making millions of colors
> (i.e., 255 x 255 x 255 = 16+ million).

Actually, the range 0-255 contains 256 values (i.e., 256 x 256 x 256 = 16+
million).
> 
> However, the new colors range (six bytes) of RGB are from 0-65535
> (0000 -- FFFF) and not 0-255  -- what's up with that? Anyone have any
> reference or explanation as to why -- the new color thing is easily
> going beyond human perception.
> 
> Thanks.
> 
> tedd

tedd:

When one manipulates color images, toning, contrast adjustments, etc, the
three-byte color space is very likely to suffer from rounding errors (eg
122.47 becomes 122) which can show up in manipulated as visual 'artifacts'
(artificial facts) or a lack of smoothness in tonal gradations.

The six-byte color space provides much more mathematical 'room' to
manipulate images thus eliminating, or vastly lessening artifacts or a lack
of smoothness in tonal gradations.

There are thousands of faq's and sites:

Try <http://www.poynton.com/ColorFAQ.html> and
<www.poynton.com/GammaFAQ.html> to start with.

Any thing authored by charles poynton has biblical authority!

Excerpt from:
<http://www.faqs.org/faqs/graphics/colorspace-faq/>

====================================================================
G-12  HOW MANY BITS DO I NEED TO SMOOTHLY SHADE FROM BLACK TO WHITE?

At a particular level of adaptation, human vision responds to about a
hundred-to-one contrast ratio of intensity from white to black. Call these
intensities 100 and 1. Within this range, vision can detect that two
intensities are different if the ratio between them exceeds about 1.01,
corresponding to a contrast sensitivity of one percent.

To shade smoothly over this range, so as to produce no perceptible steps,
at the black end of the scale it is necessary to have coding that
represents different intensity levels 1.00, 1.01, 1.02 and so on. If linear
light coding is used, the "delta" of 0.01 must be maintained all the way up
the scale to white. This requires about 9,900 codes, or about fourteen bits
per component.

If you use nonlinear coding, then the 1.01 "delta" required at the black
end of the scale applies as a ratio, not an absolute increment, and
progresses like compound interest up to white. This results in about 460
codes, or about nine bits per component. Eight bits, nonlinearly coded
according to Rec. 709, is sufficient for broadcast-quality digital
television at a contrast ratio of about 50:1.

If poor viewing conditions or poor display quality restrict the contrast
ratio of the display, then fewer bits can be employed.

If a linear light system is quantized to a small number of bits, with black
at code zero, then the ability of human vision to discern a 1.01 ratio
between adjacent intensity levels takes effect below code 100. If a linear
light system has only eight bits, then the top end of the scale is only
255, and contouring in dark areas will be perceptible even in very poor
viewing conditions.
====================================================================

Warning exploration of digital imaging issues can be a black hole of your
time...

Cheers,

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Michael Evans
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