Re: [Gimp-docs] Feedback on first pushed commits

[Marco Ciampa <ciampix libero it>]

On Fri, Apr 28, 2017 at 10:44:25AM -0400, Elle Stone wrote:
> On 04/28/2017 04:35 AM, Marco Ciampa via gimp-docs-list wrote:
>
> > Is this right?
> >
> > src/menus/colors/desaturate/desaturate.xml:108(phrase)
> >
> >        <para>The shades of gray will be calculated as</para>
> >          <informalequation>
> >            <mediaobject>
> >              <textobject>
> >                <phrase>
> >                  Lightness (HSL)) = &frac12; &times; (max(R,G,B) + min(R,G,B))
> >                </phrase>
> >              </textobject>
> >            </mediaobject>
> >          </informalequation>
> >        </listitem>
>
> If you mean the formula for Lightness, it's correct:
>
> From 2.8 docs:
> Lightness
>
>     The graylevel will be calculated as
>     Lightness = ½ × (max(R,G,B) + min(R,G,B))
>
> From posted 2.9 docs:
> Lightness (HSL)
>
>     The shades of gray will be calculated as
>     Lightness (HSL)) = ½ × (max(R,G,B) + min(R,G,B))
>
> From Wikipedia (https://en.wikipedia.org):
> In the HSL "bi-hexcone" model, lightness is defined as the average of the
> largest and smallest color components (fig. 12c).
>
> From GIMP code (app/operations/gimpoperationdesaturate.c):
>     case GIMP_DESATURATE_LIGHTNESS:
>       /* This is the formula for Lightness in the HSL "bi-hexcone"
>        * model: https://en.wikipedia.org/wiki/HSL_and_HSV
>        */
>       while (samples--)
>         {
>           gfloat min, max, value;
>
>           max = MAX (src[0], src[1]);
>           max = MAX (max, src[2]);
>           min = MIN (src[0], src[1]);
>           min = MIN (min, src[2]);
>
>           value = (max + min) / 2;
>
> Not to be confused with LAB/LCH Lightness, which is calculated from XYZ, not
> from RGB (http://brucelindbloom.com/index.html?Eqn_XYZ_to_Lab.html).

Many thanks Elle! Sorry for the noise!

Just a very minor typo: I think there is a spurious parenthesis after HSL
...

bye

--


Marco Ciampa

I know a joke about UDP, but you might not get it.

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 GNU/Linux User #78271
 FSFE fellow #364

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