<p>I fully understand the confusion (I was also a bit confused when I saw it). So, I decided to look at the ACES implementation of the ST 2084 EOTF. <a href="https://github.com/ampas/aces-dev/blob/v1.0.3/transforms/ctl/lib/ACESlib.Utilities_Color.ctl#L402" rel="nofollow noreferrer">https://github.com/ampas/aces-dev/blob/v1.0.3/transforms/ctl/lib/ACESlib.Utilities_Color.ctl#L402</a></p>
<p>From there it is very clear: You perform the transfer function on the R, G and B values separately and not by first calculating the luminance. The confusion most likely comes from the fact that we are talking about two different kinds of luminance.</p>
<p>We have <a href="https://en.wikipedia.org/wiki/Relative_luminance" rel="nofollow noreferrer"><em>relative luminance</em></a>, which is the luminance we use when talking about colours and can be calculated with <span class="math-container">$Y=0.2126R+0.7152G+0.0722B$</span></p>
<p>We also have <a href="https://en.wikipedia.org/wiki/Luminance" rel="nofollow noreferrer"><em>luminance</em></a> for which the SI unit is cd/m2 and basically means how bright something is.</p>
<p>You, and I when I first read your post, were thinking about <em>relative luminance</em>, but the Wikipedia article was talking about just <em>luminance</em>. In this case, it talks about the luminance of red light, green light and blue light separately (since a camera captures it separately and a monitor produces it separately). I'm pretty sure the ST 2048 spec is more clear about this, but that's behind a paywall.</p>

CC BY-SA 4.0
2020-10-25T00:01:20.957Z
I think spectral radiance (from Radiometry) might be the better term to refer to the values used in the EOTF for L. Luminance (or relative luminance if it's normalized 1 or 100) from photometry is the one that I was incorrectly thinking of. Correct me if I'm wrong just so that I don't throw someone off out there :) thanks.
Rafael Sabino
CC BY-SA 4.0
2020-10-22T11:31:15.807Z
2020-10-22T11:31:15.807Z
bram0101
<p>I need a point clarified.  According to <a href="https://en.wikipedia.org/wiki/High-dynamic-range_video" rel="nofollow noreferrer">wikipedia</a>, SMPT ST 2084 EOTF is this:  <a href="https://i.sstatic.net/M8d5a.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/M8d5a.png" alt="enter image description here" /></a></p>
<p>I've seen this equation implemented in game engines where for <strong>L</strong> they feed the channels <strong>RGB</strong>, each of which is 0 to 10,000 and normalize them.  Am confused because luminance from what I understand is supposed to be one quantity instead of 3.  I understood luminance to be kind'a like speed in physics, and the RGB is the &quot;direction&quot; vector. Is it really correct to plug the RGB linear channels in into <strong>L</strong> (the &quot;linear optical luminance&quot;) in this equation, or am I misunderstanding something? The way I think of it, we need to compute the luminance from the RGB triplet, and then plug that value into L, then from V we compute colors.</p>
<p>Where is my thinking wrong?</p>
<p>Thanks.</p>

<colorspace>
Luminace, RGB, and SMPT ST 2084 EOTF
Rafael Sabino
# Luminace, RGB, and SMPT ST 2084 EOTF

(Asked by: username_0 on 2020-10-22)

I need a point clarified. According to [wikipedia](https://en.wikipedia.org/wiki/High-dynamic-range_video), SMPT ST 2084 EOTF is this: [![enter image description here](https://i.sstatic.net/M8d5a.png)](https://i.sstatic.net/M8d5a.png)

I've seen this equation implemented in game engines where for **L** they feed the channels **RGB**, each of which is 0 to 10,000 and normalize them. Am confused because luminance from what I understand is supposed to be one quantity instead of 3. I understood luminance to be kind'a like speed in physics, and the RGB is the "direction" vector. Is it really correct to plug the RGB linear channels in into **L** (the "linear optical luminance") in this equation, or am I misunderstanding something? The way I think of it, we need to compute the luminance from the RGB triplet, and then plug that value into L, then from V we compute colors.

Where is my thinking wrong?

Thanks.

**Tags:** colorspace


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## 1 answers

### Answer by username_1 on 2020-10-22. Score: 1 | Accepted answer

I fully understand the confusion (I was also a bit confused when I saw it). So, I decided to look at the ACES implementation of the ST 2084 EOTF. <https://github.com/ampas/aces-dev/blob/v1.0.3/transforms/ctl/lib/ACESlib.Utilities_Color.ctl#L402>

From there it is very clear: You perform the transfer function on the R, G and B values separately and not by first calculating the luminance. The confusion most likely comes from the fact that we are talking about two different kinds of luminance.

We have [*relative luminance*](https://en.wikipedia.org/wiki/Relative_luminance), which is the luminance we use when talking about colours and can be calculated with $Y=0.2126R+0.7152G+0.0722B$

We also have [*luminance*](https://en.wikipedia.org/wiki/Luminance) for which the SI unit is cd/m2 and basically means how bright something is.

You, and I when I first read your post, were thinking about *relative luminance*, but the Wikipedia article was talking about just *luminance*. In this case, it talks about the luminance of red light, green light and blue light separately (since a camera captures it separately and a monitor produces it separately). I'm pretty sure the ST 2048 spec is more clear about this, but that's behind a paywall.

> **Comments:**
> **username_0**: I think spectral radiance (from Radiometry) might be the better term to refer to the values used in the EOTF for L. Luminance (or relative luminance if it's normalized 1 or 100) from photometry is the one that I was incorrectly thinking of. Correct me if I'm wrong just so that I don't throw someone off out there :) thanks.  

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