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

[Elle Stone <ellestone ninedegreesbelow com>]

In response to a question that was sent to me by a GIMP 2.9 user, I 
wrote up instructions on how to use GIMP 2.9 to convert from color to 
black and white to get "relative luminance" ("luminance" for short).

I'm posting the instructions here because converting from color to 
luminance is often very useful when editing an image, and right now 
using GIMP 2.9 to get luminance for an sRGB color image requires using a 
somewhat complicated and nonobvious series of steps.

In a color-managed image editor like GIMP, the luminance of an sRGB 
color is calculated using this formula: Luminance = R*0.222 + G*717 + 
B*0.061, where R, G, and B refer to a pixel's RGB channel values. The 
"catch" is that you have to operate on linearized RGB. Otherwise you get 
"luma" instead of luminance.

GIMP 2.9 is a development version of GIMP, and so these instructions 
might not be valid for very long. But as of September 7th, 2015, here 
are 7 ways to use GIMP 2.9 to get luminance for a regular sRGB image, 
plus 3 ways to get luma:

7 ways to get luminance for an sRGB image

1. Assign the GIMP built-in sRGB profile, even if the image already has 
an embedded sRGB profile. Then convert to 32-bit floating point *linear* 
precision, and use Colors/Desaturate/Luminosity, and make sure you *DO* 
check the gamma hack.

2. Or instead of assigning the GIMP built-in sRGB profile, if there is 
an embedded sRGB profile in your image, first convert to 32-bit floating 
point *gamma* precision, then convert from the embedded sRGB profile to 
GIMP's built-in sRGB profile, and then convert to 32-bit *linear* 
precision, and then use Colors/Desaturate/Luminosity, and make sure you 
*DO* check the gamma hack.

3. Or if your image has an embedded sRGB profile, stay at 32-bit 
floating point *gamma* precision, and convert your image from the 
embedded regular sRGB color space to a linear gamma version of the sRGB 
color space, using a linear gamma sRGB profile from disk. Then use 
Colors/Desaturate/Luminosity, and make sure you *DON'T* check the gamma 
hack.

4. Or else stay at 32-bit floating point *gamma* precision and use 
Colors/Desaturate (the second Desaturate)/Mono mixer, with channel 
values Red: 0.222 / Green: 0.717 / Blue: 0.061, and *DON'T* use the 
gamma hack. Using this method, if your image already has an embedded 
sRGB profile, you don't have to assign or convert to the GIMP built-in 
sRGB profile.

5. Or stay at 32-bit floating point *gamma* precision, convert your 
image to a linear gamma sRGB profile from disk, and use 
Colors/Desaturate (the second Desaturate)/Mono mixer, with channel 
values Red: 0.222 / Green: 0.717 / Blue: 0.061, and *DO* use the gamma hack.

Or if you want to produce a true grayscale (single channel) image:

6. Assign or convert to the GIMP built-in sRGB profile, then convert to 
grayscale using "Image/mode/convert to grayscale", and you have 
automatically get relative luminance. It doesn't matter whether you use 
linear or gamma precision. Both give the same result.

7. Or if your image has an embedded regular sRGB profile, stay at 
*gamma* precision (because currently linear precision doesn't work with 
images with embedded ICC profiles), and use "Image/mode/convert to 
grayscale".

3 ways to get luma for an sRGB image:

1. Assign the GIMP built-in sRGB profile, even if the image already has 
an embedded sRGB profile. Then convert to 32-bit floating point gamma OR 
linear precision (it doesn't matter which), and use 
Colors/Desaturate/Luminosity, and make sure you *DON'T* check the gamma 
hack.

2. Or if you don't want to assign or convert to the GIMP built-in sRGB 
profile, and instead prefer to use an sRGB profile that's already 
embedded in the image (or if you have assigned an sRGB profile from 
disk), then convert to 32-bit floating point *gamma* precision (*don't* 
select linear precision), and then use Colors/Desaturate/Luminosity, and 
make sure you *DON'T* check the gamma hack.

3. Or stay at 32-bit floating point *gamma* precision and use 
Colors/Desaturate (the second Desaturate)/Mono mixer, with channel 
values Red: 0.222 / Green: 0.717 / Blue: 0.061, and *DO* use the gamma 
hack. As long as you are at gamma precision, it doesn't matter whether 
you use GIMP's internal sRGB profile or a regular sRGB profile from disk 
(but of course don't convert the image to a linear gamma sRGB profile 
from disk), either way checking the gamma hack will produce luma instead 
of luminance.

Well, hopefully I didn't make any mistakes writing these instructions up.

Best,
Elle Stone
--
http://ninedegreesbelow.com
Color management and free/libre photography