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

[Pat David <patdavid gmail com>]

Hi Elle!

Did you happen to have some visual examples that demonstrate the
differences between the operations (luma vs luminance for instance) for a
user to see why they might want one over the other?
On Mon, Sep 7, 2015 at 9:24 AM Elle Stone <ellestone ninedegreesbelow com>
wrote:

> 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
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