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)) = ½ × (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).
Best,
Elle
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