Re: [Gimp-user] Luminance and luma for an sRGB image using GIMP 2.9

[Elle Stone <ellestone ninedegreesbelow com>]

On 09/07/2015 01:52 PM, Elle Stone wrote:

> Here are two examples comparing luminance and luma conversions to black
> and white:
> http://ninedegreesbelow.com/photography/luminance-luma-color-to-black-white.html

Here is why it matters how to get "luminance" vs "luma" for an sRGB 
image when using GIMP 2.9:

* "Luminance" is what you get when making a "luminosity" conversion to 
black and white while operating on linear gamma RGB.

* Luminance conversions to black and white produce a more faithful 
rendition of the tonality of the original color image color image. This 
is just one of many artistic advantages to editing in linear gamma color 
spaces (or in GIMP 2.9, operating on linearized RGB).

* Sometimes converting to luminance or luma is just one step in a more 
complicated series of editing steps. Whether you start with luminance or 
luma makes a difference in the final image. And many times you might 
well prefer the result of operating on linear RGB for all steps in the 
procedure.

* The procedure for getting luminance or luma in GIMP 2.9 is very 
different than the procedure for getting luminance or luma in other 
image editors. For example, here's one to get "luminance" vs "luma" for 
Krita:

* To get luma using Krita, open a regular sRGB image and use 
Filter/Adjust/Desaturate/Desaturation method, and pick Luminosity (ITU-R 
BT.709).

* To get luminance using Krita, open a regular sRGB image and convert it 
to a linear gamma sRGB profile from disk, using 16-bit floating point or 
higher precision, and then use Filter/Adjust/Desaturate/Desaturation 
method, and pick Luminosity (ITU-R BT.709).

And here's one way to get "luminance" vs "luma" for showFoto:

* To get luma using showFoto, open a regular sRGB image and use 
Color/Channel Mixer, click the Monochrome button, and dial in 
22.2/71.7/6.1 (same settings as for GIMP's monochrome, but multiplied by 
100).

* To get luminance using showFoto, open a regular sRGB image, convert it 
to 16-bit integer precision, and then convert it to a linear gamma sRGB 
profile from disk. Then use Color/Channel Mixer, click the Monochrome 
button, and dial in 22.2/71.7/6.1 (the same settings as for GIMP's 
monochrome operation, but multiplied by 100).

Notice that in Krita and showFoto, when you want to switch between 
luminance and luma you have to make an actual ICC profile conversion.

For most RGB operations, GIMP 2.9 currently allows the user to choose 
between operating on linearized vs perceptually uniform RGB, without 
making an ICC profile conversion, which could be a very nice capability. 
But the user interface for doing is complicated.

Best,
Elle