Effect of Luminosity Blending Mode on Curve Adjustment Layers in Photoshop

[DanK]

According to one article in Wikipedia, the odd convention of asterisks in L*a*b* is to distinguish between the current and an earlier version of the model, called Hunter's Lab. Since Hunter's is older, I assume Adobe is using L*a*b* and just simplifying notation.

I found another Wikipedia article, https://en.wikipedia.org/wiki/HSL_and_HSV, to be clearer than some other websites in clarifying the differences in terminology. Here's the relevant part:

Colorfulness: The "attribute of a visual sensation according to which the perceived color of an area appears to be more or less chromatic".[16]

Chroma: The "colorfulness relative to the brightness of a similarly illuminated white".[16]

Saturation: The "colorfulness of a stimulus relative to its own brightness".[16] [This explains why darkening with a luminosity blend or y using the L channel increases saturation]

Brightness and colorfulness are absolute measures, which usually describe the spectral distribution of light entering the eye, while lightness and chroma are measured relative to some white point, and are thus often used for descriptions of surface colors, remaining roughly constant even as brightness and colorfulness change with different illumination. Saturation can be defined as either the ratio of colorfulness to brightness, or that of chroma to lightness.

This is consistent with a few other sources I found and suggests to me that when we talk about tonality adjustments affecting "saturation", what we really mean is "colorfulness". Luminosity-blend adjustments do affect saturation by changing the denominator, but they don't affect "colorfulness". Ditto, adjustments to the L channel. Normal-blend adjustments in RGB mode do affect colorfulness, as the photos above clearly show.

[Manfred M]

L*a*b* is the CIELAB 1976 colour space; a device independent colour model established by the Commission internationale de l'éclairage, its official French name that translates to the International Commission of Illumination. CIE is the abbreviation of the French name. That being said, the commission is actually located in Vienna, Austria, in spite of its French name. CIELAB 1976 builds on previous work.

http://cie.co.at/

Adobe is just using the "official" name.


Colours and colour spaces are all about the details. As an example, in photography we use sRGB, but if you look at video, the equivalent colour space is REC. 709. They are quite similar, but in photography we use a transfer function (Gamma) of 2.2 whereas in video, the transfer function value is 2.4. For historical reasons Apple used a Gamma of 1.8 for many years, but has also standardized the the 2.2 value the Windows users are familiar with.

Colours and their definitions have a lot of nuance that really have little to no practical impact on how we assess the colours we are working wiht.