Several dropdown menus offer a choice of HSL, HSL, and All (RGB). Is All calculated as luminance? The documentation for Color Curves might mention that.
Assuming it is luminance, another choice of curve could be Luma, for greater separation of contrast and saturation changes.
Or is HSL-L essentially the same value, and does it separate contrast and saturation as well? If so, the documentation might mention that. Some raw developers offer both RGB and "Luma" curves without offering an entire suite of Lab tools.
Luma or luminance curve?
Moderator: jsachs
Re: Luma or luminance curve?
HSV V (V stands for Value) is computed as the max(R,G,B) and HSL L (L stands for Lightness) is computed as (min(R,G,B) + max(R,G,B))/2
The third component in HSV or HSL is hue, not contrast.
Luminance and color spaces such as Lab which is based on luminance are not well suited for adjusting image brightness. The problem is that luminance is a weighted average of R, G and B, with G having a much greater weight than B. Thus for example a 100% pure blue has a very small luminance. If you try to make it brighter by increasing its luminance using an adjustment curve, the color you get is out of gamut -- potentially way out of gamut. Forcing it back into gamut requires changing its saturation and/or hue, so you can't adjust the three components independently.
In HSV, 100% pure blue has a value of 100% and cannot be made any lighter.
In HSL, 100% pure blur has a value of 50% and adjusting it to 100% produces pure white.
In either HSL or HSV, any of the three channels can be adjusted without affecting the other two, so a set of three color curves makes sense since the set of all possible colors is a cube. In space like Lab, the space of all possible RGB colors is a complicated shape and cannot be edited using a set of three independent curves.
Much of this is described in some detail under Color Spaces in the help file.
The third component in HSV or HSL is hue, not contrast.
Luminance and color spaces such as Lab which is based on luminance are not well suited for adjusting image brightness. The problem is that luminance is a weighted average of R, G and B, with G having a much greater weight than B. Thus for example a 100% pure blue has a very small luminance. If you try to make it brighter by increasing its luminance using an adjustment curve, the color you get is out of gamut -- potentially way out of gamut. Forcing it back into gamut requires changing its saturation and/or hue, so you can't adjust the three components independently.
In HSV, 100% pure blue has a value of 100% and cannot be made any lighter.
In HSL, 100% pure blur has a value of 50% and adjusting it to 100% produces pure white.
In either HSL or HSV, any of the three channels can be adjusted without affecting the other two, so a set of three color curves makes sense since the set of all possible colors is a cube. In space like Lab, the space of all possible RGB colors is a complicated shape and cannot be edited using a set of three independent curves.
Much of this is described in some detail under Color Spaces in the help file.
Jonathan Sachs
Digital Light & Color
Digital Light & Color