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

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