Re: [Gimp-user] Histogramm -- values

[Ofnuts <ofnuts laposte net>]

On 11/25/2013 01:51 PM, Elle Stone wrote:
> On 11/25/2013 12:16 AM, scl wrote:
> > Hi,
> >
> > thank you, Wolfgang, for your reply and sorry I couldn't answer earlier.
> >
> > On 19.11.2013 at 5:14 AM Wolfgang Hugemann wrote:
> >  >> I'm for using the words value, tonal value, brightness, lightness 
> > etc.
> >  >> consistently to avoid further confusion.
> >  >> What are the 'officially defined' or 'most commonly used' names and
> >  >> meanings in these contexts for you, the users?
> >  >
> >  > Do you think it is the right approach to ask the users? I would 
> > rather
> >  > suggest asking someone who has a thorough understanding of colour 
> > spaces
> >  > and alike. I could ask Fred Weinhaus 
> > (www.fmwconcepts.com/imagemagick)
> >  > to give that a look, as he has a deep understanding of the matter.
> >
> > Asking someone with a thorough understanding of the topic is the
> > rationale behind asking the users. The large amount of GIMP users
> > and the assumption that there must be at least some among them with
> > a thorough knowledge led me to my proposal. Yes, it's not perfect,
> > but at least one step into the right direction. If you have an expert
> > I'd be glad if you asked him and reported back.
> > Another point that needs clarification is the use and meaning of the
> > word 'Value' in the Layer and Painting modes.
> >
> > Elle Stone and Nicolas Robidoux, can you please also have a
> > look on this topic? Thank you in advance.
>
> This post is only about the word "Value" as used in Gimp's Histogram 
> and Levels dialog. I don't know if the "Value" in the blending mode is 
> the same as the Value in the histogram and Levels. Apologies in 
> advance for a long post.
>
> XYZ is a model of how humans actually perceive color.
> RGB refers to a convenient subset of all possible XYZ values. The 
> simplest possible subset of all possible XYZ values is determined by 
> picking red, green, and blue primaries in XYZ space, plus a black 
> point and a white point, plus a tone response curve (TRC) to create a 
> matrix RGB color space such as sRGB.
>
> Above is covered in http://ninedegreesbelow.com/photography/xyz-rgb.html
>
> So RGB is mathematically derived from XYZ. In turn the various 
> HSL/HSV/HSI color space models are mathematically derived from RGB. So 
> first you pick an RGB color space. Then you calculate HSL/HSV/HSI 
> values. The question is "why"? Why not just use the RGB values directly?
>
> According to Wikipedia 
> (https://en.wikipedia.org/wiki/HSV_color_space#Motivation), the answer 
> is because apparently RGB isn't intuitively obvious to us end-users of 
> graphics programs, and also (here's the real reason) HSL/HSV 
> calculations are computationally cheaper than RGB calculations, which 
> made a difference back in the 1970s and 80s and 90s, but not so much 
> today because computer hardware is exponentially faster than it used 
> to be.
>
> The question is Gimp's term "Value". The HSL/HSV/HSI models start with 
> RGB and then define more or less adequate measures of:
>
> *Hue (whether it's red, blue, green, or some in-between color)
>
> *Saturation (whether it's very close to or far from the XYZ neutral 
> axis connecting the RGB color space's white point and black point)
>
> *Some measure of "Luminance". "Luminance" is a physical measure (based 
> on real properties of actual light) of how bright something is 
> (speaking loosely), or how much light reflects off of it in the 
> direction of the viewer's eyeballs (speaking more precisely). Luminace 
> happens to be equal to the "Y" of the equivalent XYZ value for the 
> given RGB values in any specified RGB color space.
>
> If your goal is to represent what people see out there in the world, 
> rather than to make computationally cheaper calculations, none of the 
> HSX models are any good. See the nice illustration in the 
> "disadvantages" section of the Wikipedia article: 
> https://en.wikipedia.org/wiki/HSV_color_space#Disadvantages, and then 
> read the paragraph following the illustration.
>
> To illustrate the problem with numbers, let's pick an RGB color space, 
> sRGB. Then pick some colors and calculate Value, Lightness, and 
> Intensity. Then divide each value by 255 to put them on a scale from 0 
> to 1 so we can compare them to the equivalent XYZ "Y" value. The L, I, 
> and V equations are from the Wikipedia article:
>
> Lightness L = The sum of the maximum ("M") and minimum ("m") RGB 
> values for the RGB values for the color, divided by two = (M + m)/2
>
> Intensity I = one third of the sum of R, G, and B = (R + G + B)/3.
>
> Value V = the maximum of R, G, and B = M. This is what Gimp uses in 
> the Histogram and Levels. I'm not sure what Curves does.
>
> Below the word "value" (lower case) means "the numerical value" of the 
> RGB coordinates of a color as specified in a particular RGB color space.
>
> Let's pick the reddest possible red in the sRGB color space, at 
> 8-bits, which is (255,0,0). The maximum RGB value of reddest sRGB red 
> is 255. The minimum RGB value is 0.
>
> The Lightness is (0+255)/2 =127.5.  Divide by 255= 0.500000
> The Value is 255.  Divide by 255= 1.000000
> The Intensity is (255+0+0)/3 = 85.  Divide by 255= 0.333333
>
> Now let's pick sRGB middle gray: (119,119,119). The maximum value of 
> sRGB middle gray is 119. The minimum value is also 119.
>
> Lightness=(119+119)/2 = 119.  Divide by 255= 0.466667
> The Value is 119.  Divide by 255=  0.466667
> The Intensity is (119+119+119)/3 = 119. Divide by 255=  0.466667
>
> So how close are V, L, and I to the actual XYZ "Y" luminance?
>
> xicclu -s255 argyll-sRGB.icm
> 255 0 0 [RGB] -> 0.436035 0.222443 0.013901 [XYZ]
> Y (Luminance) = 0.222443
>
> 119 119 119 [RGB] -> 0.233189 0.207916 0.428216 [XYZ]
> Y (Luminance) = 0.207916
>
> So all of the HSX models do a bad job of estimating actual real world 
> Luminance.
>
> There's another problem with these HSX models: Change the color space 
> and the HSL/HSV/HSI values stay the same, but of course the 
> **meaning** of the RGB values from which HSL/HSV/HSI are calculated 
> changes drastically. Here are the Luminance values for the ProPhotoRGB 
> color space, using the same RGB values as above:
>
> xicclu -s255 Rimm-elleV2-g180.icc
> 255 0 0 [RGB] Y = 0.288040
> 119 199 199 RGB] Y = 0.254242
>
> So it sounds like I'm saying that Gimp's use of Value from HSV is a 
> bad thing for Histograms and Levels. Actually, I'm not. The question 
> is what is the point of a histogram, and what is the editing purpose 
> of Levels?
>
> The point of a histogram is to show the distribution of RGB values in 
> the image, in the RGB color space that the image is in. When I edit an 
> image, when I adjust Levels, I want to know when I'm clipping RGB 
> values. I'm not so concerned about what the equivalent "Luminance/Y" 
> value is. Adjusting the left and right sides of the Levels, the 
> image's "white point" and "black point", is arguably the most 
> important step in image editing, and where you start to clip pixels at 
> the top and bottom of the histogram is a crucial piece of information.
>
> If Gimp used Intensity or Lightness on the histogram and on the Levels 
> dialog, that would be singularly useless information from an editing 
> point of view.
>
> But Gimp's "Value" gives the maximum of R, G, and B. So I know that I 
> can adjust an image's white point using Gimp Levels upper right hand 
> slider all the way to the right edge of the histogram and not clip any 
> pixels.
>
> I wish Gimp also had a Minimum histogram, too. At present we don't 
> have a histogram that shows the minimum of R, G, and B. So sliding the 
> left hand upper Levels slider to set the image black point just by 
> looking at the Value (Maximum) histogram is misleading. Sometimes very 
> misleading. It can result in a whole lot of clipped pixels, whereas 
> switching to Curves would allow to compress rather than clip, if the 
> user knew they were about to clip pixels.
>
> If Gimp calculated the XYZ "Y" Luminance value, that would be useful 
> as a blending mode, and also useful in a histogram as an overview of 
> the image's distribution of light and dark, but not as useful for 
> actually setting the image black and white point as the mathematically 
> simpler "Minimum" (which we don't have at present) and "Maximum" 
> (which we do have, only it's called "Value").
>
> If I were designing the Levels and Histogram interfaces, I'd change 
> the word "Value" to "Maximum RGB" and I'd add the equivalent "Minimum 
> RGB" Histogram. And I'd offer the option to view the RGB channels in 
> the Levels dialog just like you can in the Histogram, so you could 
> move the sliders and see which channels are being clipped.
>
> Having a Luminance histogram would also be nice to show the actual 
> distribution of Luminance values. A Luminance histogram would be 
> constant regardless of the color space the image was in.
>
> Elle
>
> __

Fascinating... of course this kind of small details also lurks in 
Color/Desaturate, but where I miss them the most is in the layer blend 
modes. Color/Hue/Saturation/Value are very hard to work with...