Re: [Gimp-user] Histogramm -- values

[Elle Stone <ellestone ninedegreesbelow com>]

On 11/25/2013 02:10 PM, Wolfgang Hugemann wrote:
> > > Elle Stone and Nicolas Robidoux, can you please also have a
> > > look on this topic? Thank you in advance.
>
> Wow, this was a very elaborate answer ...
The original question was whether Gimp uses the term "Value" correctly, 
at least that was the part of the question I tried to answer.

The answer is "Yes" at least for the Histogram, Levels, and Curves 
dialogs. I looked at the actual Levels code, Curves emulates Levels, and 
the Histogram accurately reflects adjustments made using Levels and 
Curves. So logically they must all use the same algorithm, which is 
Value, which is the maximum of R, G, and B.

Being assured that the term 'Value' is used correctly doesn't say much 
unless you understand what "Value" in Gimp really means. If I erred on 
the side of too much information (almost certainly yes!), my apologies!


> Fred Weinhaus also answered me and he is pointing to some transfer
> functions that ImageMagick uses, see
> http://www.imagemagick.org/script/command-line-options.php#intensity.

Quoting from the Imagemagick page:
   Rec709Luma        0.212656R' + 0.715158G' + 0.072186B'
   Rec709Luminance   0.212656R + 0.715158G + 0.072186B
   Brightness        max(R', G', B')

The Imagemagick page says that Primed (eg R') values are non-linear 
sRGB. Unprimed are linear, presumably linear sRGB although the immediate 
text doesn't make this clear. Imagemagick "Brightness" is the same as 
Gimp "Value".

Rec709Luma and Rec709Luminance both give the appropriate values for 
calculating what to send to an D65 sRGB-compatible monitor screen, in a 
NON-color-managed application. Gimp is color-managed.

The right values for calculating sRGB Luma and Luminance for a 
color-managed application like Gimp are 0.2225*red + 0.7169*green + 
0.0606*blue (to 4 decimal places). If you want real Luminance ("Y" of 
XYZ), then you need to use the linear sRGB RGB values, not the regular 
sRGB RGB values. See 
http://ninedegreesbelow.com/photography/srgb-luminance.html for details.


> He also gives some helpful illustrations what effects different transfer
> functions have on the image:
> http://www.fmwconcepts.com/imagemagick/color2gray/index.php
Fred's scripts fall into the category of "instant pretty" algorithms 
(click a button or run a script and the image might be prettier). "How 
to manipulate the RGB values to make the image instantly prettier" is a 
very different topic than the question of what Gimp's "Value" actually 
means. As an aside, Fred's scripts license isn't floss-compatible.


> I understand what Elle says in regard to histogram clipping, i.e. max(R,
> G, B) being very helpful for finding the white point -- while min (R, G,
> B) in regard to the black point is missing.
>
> Perhaps one should give the user just a few more options what to display
> in the histogram and how to transfer R, G, B into a single 'value'. I
> would be very lucky with 'Rec709Luma' as defined by ImageMagick. I
> understand that GIMP internally uses the sRGB colour space at the moment
> (?).

"Single measure" histograms are very useful, and which measure to use is 
an important question. But Imagemagick's "Rec709Luma" isn't particularly 
useful (IMHO), having a mathematically convoluted and physically 
meaningless relationship to the image's actual Luminance ("Y" from XYZ) 
values.


> A standalone feature for GIMP would be the possibility to display a
> cumulated histogram,

https://en.wikipedia.org/wiki/Histogram#Cumulative_histogram

> which would for instance demonstrate the effect of
> equalisation better than an ordinary histogram. This should be easy to
> implement, I guess, and would be a simple check box for the user.

> BTW: Besides equalisation, one could also try to meet other cumulative
> histogram distribution, like a Gaussian bell curve, see
> http://www.fmwconcepts.com/imagemagick/redist.

Fred's non-floss "REDIST" algorithm script is an "instant pretty" way to 
manipulate the image's RGB values to produce an image with a 
predetermined histogram shape. The resulting image histogram will be 
different because the image's actual RGB values have changed by the 
algorithm.

Two different questions are easy to mix up:

One question is how to helpfully display the image histogram, based on 
the image's current RGB values. This involves things like what type of 
histogram ("ordinary" seems more generally useful; "cumulative" would 
have a specialized use) and what measure(s) (eg Value, Luminance, Min, 
Max, R, G, B, etc) should be displayed.

A second question is how to instantly prettify an image by applying 
algorithms that are designed to alter the image's distribution of RGB 
values, which of course means the image's histogram will change, 
regardless of which type of histogram you use. If I understand him 
correctly, Wolfgang says the cumulative histogram might make choosing an 
algorithm easier.


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.

Gimp uses the term "Lightness" as in Hue/Saturation/Lightness. Here's an 
excerpt from the code:

  max = gimp_rgb_max (rgb);
  min = gimp_rgb_min (rgb);
  hsl->l = (max + min) / 2.0;

That's exactly the definition of Lightness from HSL. I'm going to bet 
that the Gimp interface always uses the right terminology for whatever 
HSX model is being used. So it's not terminological inconsistency but 
rather different HSX models for different purposes.

Gimp uses "Brightness" as in "Colors/Brightness-Contrast", which based 
on a glance at the code is just a call to Levels. Brightness and Value 
mean the same thing, not just in Gimp but as far as I can tell, in 
general, when referring to HSB and HSV. If Gimp used the terminology 
"Value-Contrast" that might confuse people who use to other image 
editors, as "Brightness-Contrast" seems to be used universally. So 
again, the Gimp terminology is accurate. As an aside, there is nothing 
you can do with Brightness-Contrast that you can't do with more control 
using Levels.

In "Colors/Desaturate", looking at the code, Lightness and Average are 
used correctly. Luminosity isn't "gamma"-corrected in Gimp 2.8 (so the 
term is technically incorrect), but it is "gamma"-corrected in Gimp 2.9, 
so when the next release of Gimp comes along the terminology will be 
100% accurate.

I am very far from being an expert in using Gimp. Where is "tonal value" 
used in the user interface?

> Another approach would be to have a look on what these things are
> called in Photoshop

Photoshop seems have its own version of HSX: 
http://stackoverflow.com/questions/12121393/please-explain-this-color-blending-mode-formula-so-i-can-replicate-it-in-php-ima

Elle