[filmscanners] Re: Vuescan upper histogram

["Bob Shomler" <bob@shomler.com>]

> Weighted green is the most common method for evaluating lightness ...
>however, since Vuescan's histogram is most commonly used for evaluating the
>endpoints, black & white, I wonder if equal weighting is more appropriate(?)

Equal -- probably not.  There are some comments on this in Charles Poynton's 
"ColorFAQ -- FREQUENTLY ASKED QUESTIONS ABOUT COLOR" at

  www.inforamp.net/~poynton/notes/colour_and_gamma/ColorFAQ.txt

According to Poynton's text, the contribution of green to luminance is more 
than two thirds of the total while red and blue contribute about 21 and 7 
percent respectively.  I think this is what Photoshop uses to compute its 
luminosity histogram, though I cannot find a reference for that right now.

Extract from Poynton's FAQ item 7 follows.

That said, I have no idea what Ed uses for his vuescan luminance histogram 
calculation.  The above might be a starting point.  From looking at some of my 
images, that histogram's Y values do seem to be quite green-weighted.

Bob Shomler
www.shomler.com

------------
 9.   WHAT WEIGHTING OF RED, GREEN AND BLUE CORRESPONDS TO BRIGHTNESS?

 Direct acquisition of luminance requires use of a very specific spectral
 weighting. However, luminance can also be computed as a weighted sum of
 red, green and blue components.

 If three sources appear red, green and blue, and have the same radiance in
 the visible spectrum, then the green will appear the brightest of the three
 because the luminous efficiency function peaks in the green region of the
 spectrum. The red will appear less bright, and the blue will be the darkest
 of the three. ...

 Contemporary CRT phosphors are standardized in Rec. 709 [8] ... The weights
 to compute true CIE luminance from linear red, green and blue ... are these:

   Y = 0.212671 * R + 0.715160 * G + 0.072169 * B;

 ...
 Some computer systems have computed brightness using (R+G+B)/3. This is at
 odds with the properties of human vision, as will be discussed under What
 are HSB and HLS? in section 36.


------------
At 03:46 PM 1/10/2002 -0330, you wrote:
>Ken writes ...
>
>> ...  I think I understand that the bottom histogram in Vuescan
>> is the RGB channels, but what is the upper one, and why does
>> it not clip sometimes when the lower ones do?
>
>  It is Ed's best calculation of the "lightness" of a pixel.  If I remember
>correctly, it is weighted to the green channel value, and least weight goes
>to blue ... I believe you can see a similarity.
>
>  Weighted green is the most common method for evaluating lightness ...
>however, since Vuescan's histogram is most commonly used for evaluating the
>endpoints, black & white, I wonder if equal weighting is more appropriate(?)
>
>cheerios ... shAf  :o)

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