But Steve, after scouring the web, I've come across several resources contradicting your statement. These sources claim that there is a common misconception about luminance/luma.
Basically, they all claim that Luma is gamma corrected luminance. Below are a few quotes from a few websites I've come across.
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Charles Poynton: Frequently Asked Quesitions about Color
It is useful in a video system to convey a component representative of luminance and two other components representative of color. It is impor- tant to convey the component representative of luminance in such a way that noise (or quantization) introduced in transmission, processing and storage has a perceptually similar effect across the entire tone scale from black to white. The ideal way to accomplish these goals would be to form a luminance signal by matrixing RGB, then subjecting luminance to a nonlinear transfer function similar to the L* function.
There are practical reasons in video to perform these operations in the opposite order. First a nonlinear transfer function – gamma correction – is applied to each of the linear R, G and B. Then a weighted sum of the nonlinear components is computed to form a signal representative of luminance. The resulting component is related to brightness but is not CIE luminance. Many video engineers call it luma and give it the symbol Y’. It is often carelessly called luminance and given the symbol Y. You must be careful to determine whether a particular author assigns a linear or nonlinear interpretation to the term luminance and the symbol Y.
The coefficients that correspond to the “NTSC” red, green and blue CRT phosphors of 1953 are standardized in ITU-R Recommendation BT. 601-4 (formerly CCIR Rec. 601). I call it Rec. 601. To compute nonlinear video luma from nonlinear red, green and blue:
Y ′ = 0.299R′ + 0.587G′ + 0.114B′ 601
The prime symbols in this equation, and in those to follow, denote nonlinear components.
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Charles Poyton: YUV and Luminance Considered Harmful
This is a plea for precise terminology in video. The notation YUV, and
the term luminance, are widespread in digital video. In truth, digital video almost never uses Y’UV color difference components, and never directly represents the luminance of color science. The common terms are almost always wrong. This note explains why. I urge video engi- neers and computer graphics specialists to use the correct terms, almost always Y’CBCR and luma.
Conclusion: A plea
Using the term luminance for video Y’ is tantamount to using the word cement instead of concrete to describe the main construction material of a bridge. Lay people don’t care, and experts can live with it, but people in the middle – in this case, the programmers and engineers who are reimplementing video technology in the computer domain – are liable to draw the wrong conclusions from careless use of terms. Users suffer from this, because the exchange of images is compromised.
I urge video engineers and computer graphics specialists to avoid YUV and luminance, and to use the correct terms, Y’CBCR and luma.
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Wikipedia:
In a similar vein, the term luminance and the symbol Y are often used erroneously to refer to luma, which is denoted with the symbol Y'. Note that the luma (Y') of video engineering deviates from the luminance (Y) of color science (as defined by CIE). Luma is formed as the weighted sum of gamma-corrected (tristimulus) RGB components. Luminance is formed as a weighed sum of linear (tristimulus) RGB components.
In practice, the CIE symbol Y is often incorrectly used to denote luma. In 1993, SMPTE adopted Engineering Guideline EG 28, clarifying the two terms. Note that the prime symbol ' is used to indicate gamma correction.
Similarly, the chroma/chrominance of video engineering differs from the chrominance of color science. The chroma/chrominance of video engineering is formed from weighted tristimulus components, not linear components. In video engineering practice, the terms chroma, chrominance, and saturation are often used interchangeably to refer to chrominance.
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From Apple Color User Manual: "Basic Color and Imaging Concepts"
"Luma Explained
Luma (which technically speaking is gamma-corrected luminance) describes the exposure (lightness) of a video shot, from absolute black, through the distribution of gray tones, all the way up to the brightest white. Luma can be separated from the color of an image. In fact, if you desaturate an image completely, the grayscale image that remains is the luma.
Luma is measured by Color as a digital percentage from 0 to 100, where 0 represents absolute black and 100 represents absolute white. Color also supports super-white levels (levels from 101 to 109 percent) if they exist in your shot. While super-white video levels are not considered to be safe for broadcast, many cameras record video at these levels anyway"
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What is the difference between Luma and Luma?
By Daniel Metz
In the standard sRGB or REC 709, the luminance was replaced by a distribution of brightness suitable for reproduction on monitors. This particular distribution roughly corresponds to what is called a gamma 2.2. This sRGB gamma is truncated in the first values to simplify calculations and the rest of the curve resembles a gamma 2.2.
We apply exactly the same sRGB gamma coding Y'CbCr. To avoid confusion with the luminance (Y), we add an apostrophe (Y ') this distribution normalized luminance. And replaces the word luminance Luma. This is the standard REC 601.
To summarize, we can say that the luma corresponds to the weighted luminance of a gamma 2.2. The proportion of primary to get the white is thereby slightly modified: the luma is composed of 59% green, 30% red and 11% blue. The other two channels of the chrominance Y'CbCr describe one for red and one for blue.
By abuse of language, many people continue to call the luminance parameter Luma. So be careful and always try to find the direction in which an author uses the word luminance. | 