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In my opinion, the main specialty of sRGB is its nonlinear gamma (to emulate perceptual brightness), while RGB, XYZ, and LAB have linear gamma (to match photon

In my opinion, the main specialty of sRGB is its nonlinear gamma (to emulate perceptual brightness), while RGB, XYZ, and LAB have linear gamma (to match photon intensity). Accordingly, the conversion formulas given in the article will be wrong for 8bit sRGB images like they are usually used in JPEG or PNG, because they lack the gamma conversion.

pezezin 5y ago
CIELAB is non-linear, it has a gamma of 3 (a cubic root actually): https://en.wikipedia.org/wiki/CIELAB_color_space#Converting_between_CIELAB_and_CIEXYZ_coordinates https://en.wikipedia.org/wiki/CIELAB_color_space#Converting_...
formerly_proven 5y ago
Gamma is by definition non-linear so a CSC matrix can never account for it. Pretty much all color spaces intended for content delivery use some kind of gamma correction because as you say brightness is perceived on a log-scale and you don't want to waste any bits on stuff nobody is going to perceive.
- fxtentacle 5y ago
  I agree with you, but I see that as an omission in the article that they treat sRGB like RGB when one is logarithmic and the other is linear gamma.
  - zokier 5y ago
    "RGB" does not really specify any particular colorspace, s I don't know if it could really be said to have any well-defined gamma, linear or not.
    - fxtentacle 5y ago
      Agree again, but the article assumes linear gamma for RGB. Also if I remember correctly, the CIE-RGB is specified to be linear as to be compatible to CIE-XYZ. Since the article omitted the CIE prefix on XYZ, I believe they also meant to refer to the CIE RGB variant. Or at least, that's where that conversation formula came from.
CarVac 5y ago
The conversion formulas are applied after you linearize though…
Dave_Rosenthal 5y ago
As sibling comments are pointing out, yes, any non-1.0 gamma (i.e. intensity^gamma) is a non-linear transformation, but what I think you are trying to point out instead (and is weird/unique) is that sRGB has a "non-constant gamma". Or, said another way, the non-linear intensity transformation cannot be represented by a simple gamma value. The transformation is, in fact, piecewise, with a linear transformation in the region from 0 to [some low intensity] and a more traditional gamma-like curve from [some low intensity] to 1.0. (In practice this curve is a pain, so many systems over the years have just settled to approximate it as a 2.2 gamma curve.)

pezezin 5y ago

CIELAB is non-linear, it has a gamma of 3 (a cubic root actually): https://en.wikipedia.org/wiki/CIELAB_color_space#Converting_between_CIELAB_and_CIEXYZ_coordinates https://en.wikipedia.org/wiki/CIELAB_color_space#Converting_...

formerly_proven 5y ago

Gamma is by definition non-linear so a CSC matrix can never account for it. Pretty much all color spaces intended for content delivery use some kind of gamma correction because as you say brightness is perceived on a log-scale and you don't want to waste any bits on stuff nobody is going to perceive.

fxtentacle 5y ago
I agree with you, but I see that as an omission in the article that they treat sRGB like RGB when one is logarithmic and the other is linear gamma.
- zokier 5y ago
  "RGB" does not really specify any particular colorspace, s I don't know if it could really be said to have any well-defined gamma, linear or not.
  - fxtentacle 5y ago
    Agree again, but the article assumes linear gamma for RGB. Also if I remember correctly, the CIE-RGB is specified to be linear as to be compatible to CIE-XYZ. Since the article omitted the CIE prefix on XYZ, I believe they also meant to refer to the CIE RGB variant. Or at least, that's where that conversation formula came from.

CarVac 5y ago

The conversion formulas are applied after you linearize though…

Dave_Rosenthal 5y ago

As sibling comments are pointing out, yes, any non-1.0 gamma (i.e. intensity^gamma) is a non-linear transformation, but what I think you are trying to point out instead (and is weird/unique) is that sRGB has a "non-constant gamma". Or, said another way, the non-linear intensity transformation cannot be represented by a simple gamma value. The transformation is, in fact, piecewise, with a linear transformation in the region from 0 to [some low intensity] and a more traditional gamma-like curve from [some low intensity] to 1.0. (In practice this curve is a pain, so many systems over the years have just settled to approximate it as a 2.2 gamma curve.)