Talk:Luma (video)

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I don't think this word was created in the NTSC (~1953) era. For example, some of the members of the original NTSC committee published a book (under Hazeltine laboratories) explaining how NTSC works. They use the term luminance, and make note of the incorrect order of operations.

also see [1] Glennchan 06:06, 16 November 2006 (UTC)

[edit] 601 vs 709 numbers

The coefficients listed even before Glennchan's big edit are from the 601 gamma-compressed luma versus the 709 linear colorimetric luminance. I think the latter numbers are probably not what we want here, and are unnecessarily confused with the former. Is this just a misinterpretation of what the 709 spec is saying, or what? Dicklyon 23:48, 19 November 2006 (UTC)

Rec. 709 also uses gamma-compressed components in calculating luma. The transfer functions on Rec. 709 are different; in practice Rec. 709 and Rec. 601 material is interchanged without translating/converting the transfer functions. I'm not sure what you're saying about the latter numbers confused with the former. They are clearly two different sets of numbers????Glennchan 01:17, 20 November 2006 (UTC)

I'm saying that the weights shown associated with 709 are the weights for linear R,G,B. Can you point me at the 709 documents that suggest using such numbers with gamma-compressed R, G, and B? Dicklyon 01:28, 20 November 2006 (UTC)

The Rec. 709 documents would have it, although you gotta pay for em. What you're looking for is implicit in the information presented at [2]. Rec 601 <--> 709 is interchangeable if you use a matrix to account for the different luma co-efficients. Consumer devices also omit this matrix. This wouldn't be possible unless Rec. 709 used gamma corrected components. Do you believe me now?

Perhaps you are confused since those weightings are correct for linear light RGB components. Rec 709 luma simply uses those same numbers, but with the *wrong* order of operations. This can cause very incomplete luminance/color separation for fully saturated colors (like an error of 60%). Poynton shows the calculations at [3].Glennchan 08:09, 20 November 2006 (UTC)

Forgive me if I remain somewhat incredulous. You may be right, but I'd like to see it from someplace other than an old Poynton rant. Dicklyon 15:55, 20 November 2006 (UTC)

[4] I have no idea if that document is supposed to be on the internet, but it is. If you look carefully, it details the Rec. 709 transfer function as well as Y'CbCr encoding.Glennchan 04:21, 22 November 2006 (UTC)

Fascinating. So maybe we should list all the coefficients from Table 6-9. What is 709-5? A section? or a variant of 709? Is this really the specification used in HDTV? Go ahead and fix it as you see fit. Dicklyon 05:49, 22 November 2006 (UTC)

As each standard is updated, they change the number they stick at the end. Rec. 709 is a set of recommendations about HD formats AFAIK. Most HD acquisition formats and broadcast formats do follow it. I think the transitional 1035i formats were created before Rec. 709, and they do not follow Rec. 709.Glennchan 05:06, 24 November 2006 (UTC)