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Antwort: Re: grayscale displays and human vision

Posted by Joachim Wesner on Jul 10, 2006; 3:48pm
URL: http://imagej.273.s1.nabble.com/grayscale-displays-and-human-vision-tp3702214p3702227.html

Hi there,

JTS wrote
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I'm not an authority on human vision but I read somewhere that the human
eye could distingusih approximately 16 different gray levels. This is far
less than 256 of 8 bit. In fact it is only 3 bit.

I tested this in a small demonstration with a group of colleagues who spend
a lot of time reading x-ray films of the hands and feet and 16 levels
appeared to be about what the best could do.

Try it your self. You can compose images with gray scales of anything
between  0 and 256. Code them, mix them up in
random fasion and ask your colleagues to tell you which is whiter (or
blacker ) comparing A to B, A to C, A to D etc.

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There seems to be a confusion here between smalled difference in brightness
one can distinguish (say a change of x
percent) and the total number of such small differences that can appear in
ONE scene (any dark adaption would NOT count here, because then we are
comparing diffent scenes) with having a brightness too low to discern or
too bright for the eye (causes glare etc.).

I do not know what the brightness differences were that you used, but only
16 levels within the one-scene dynamic
range is definitely too low! Take any BW picture in an image processing
progamm and reduce the number of gray levels
to 64 and later to only 16, you will start to see a difference at 64 levels
and definitely at 16 levels!

(HOWEVER, trying to surely identify a certain level my be a different
thing!)

The gamma faq

http://www.poynton.com/notes/colour_and_gamma/GammaFAQ.htm

has a good discussion on that, (I hope I will not get sued by some
underemployed advaocate for citing from it):

<cite

15. 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 luminance from white to black. Call
these luminance values 100 and 1. Within this range, vision can detect that
two luminance values 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 luminance 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 luminance 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

\cite>

JW


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