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
_______________________________________________
gimp-user-list mailing list
List address:    gimp-user-list@gnome.org
List membership: https://mail.gnome.org/mailman/listinfo/gimp-user-list
List archives:   https://mail.gnome.org/archives/gimp-user-list

Reply via email to