Re: [Gimp-docs] Colors/Desaturate/Desaturate documentation

[Elle Stone <ellestone ninedegreesbelow com>]

On 03/26/2017 10:58 AM, Elle Stone wrote:
>     2- Mode
>             * Luminance
>             * Luma
>             * Lightness
>             * Average
>             * Value
>     3- Advanced options
>             * Apply filter to the layer's RAW pixels: with an example

If the above means that there should be documentation for the five Modes 
and *also* for "Advanced options/Apply filter to the layer's RAW 
pixels", I don't think adding documentation for "Advanced options Apply 
filter to the layer's RAW pixels" to the desaturate help (or any other 
help page for a specific editing option) is a good idea.

Just one example wouldn't do as it would be misleading. Between the 
gamma hack and the "color managed" option there are four possible 
combinations for the user to choose among, for each desaturate mode:

* The precise effect of each combination of "mode+using or not using the 
advanced options" does vary depending on whether the operation in 
question requests linearized RGB or RGB encoded using the sRGB TRC. So 
for example the effect when choosing Luminance is different from the 
effect when using Luma.

*The precise effect also varies depending on whether the image is at 
"linear" or "perceptual" precision.

* It seems, though I'm not 100% certain, that the effect also depends on 
whether there has been any previous "making of a new profile from a 
pre-existing non-GIMP-built-in profile", which in turn depends on 
whether the image had an embedded linear gamma ICC profile and whether 
the image has been promoted to 32f with linear precision and then 
possibly back to gamma precision, and on whether the user has chosen to 
automatically promote to 32f in Preferences.

In other words, accurately explaining the possible results from using 
the Advanced Color Options in the Desaturate dialog would require a flow 
chart in order to explain what happens when using which combination of 
the Advanced Color Options given X current precision, Y previous 
precision changes and Z original non-sRGB embedded ICC profile, combined 
with the choice amongst the various modes for converting to black and 
white that the user has made.

I'd very much prefer to not create such a flow chart, especially since 
the code for the relevant factors affecting the outcome of any given set 
of choices+previous-precision-changes is still in a state of flux.

Best,
L