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Re: LStar

Hi David,

here you find a plot against gamma curves:

http://homepage.mac.com/hanspeterharpf/pictures/PhotoAlbum52.html

As you can see, with any gamma over 1.8 there is an uneven distribution of tone values in the shadows. 
L* is the simples curve you can draw - it´s a straight line. Of course you cannot define it as a single number like gammas. But you have to keep in mind, that a single number on itself doesn´t say anything at all. This single number has to be seen in it´s context of an exponential function, which doesn´t make it real simple. 
L* can be described in a very simple equation: L*=(RGB)/2.55
And, L* is the visually equidistant like in L*a*b*.
Since TFT monitors do nat have a native hardware gamma, it is nonsense to force them into a (artificial) gamma behavior that does not represent the human perception of brighness steps at all. Gamma can always be only an approximation, as you can see in the graph. So, why not go for the real thing?

Regards,

Karl

 
Am 18.05.2005 um 16:50 schrieb David Tobie:

LStar is not really a gamma but a gray scale constructed so that each step

VISUALLY appears to be an even increase in brightness from the step before

it. This is why it is sometimes referred to as the "Equal brightness" scale.



Right, but if you plot it against a gamma curve in the 2.3 - 2.4 range, the 

differences will be relatively small. So the oft repeated marketing statement 

that "L* is not a curve" should really be worded, for educational purposes, 

that "L* is not a simple curve that can be defined by a single number... but is 

relatively close to such a curve".


C. David Tobie


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