Resolve Gamma 2.4 vs Rec709 (Scene)

[Anton Meleshkevich]

Why default Resolve timeline colorspace gamma curve is 'gamma 2.4'?
But all the Resolve Log to Rec709 LUTs, ACES Rec709 and so on are look like 'Rec709 (SCENE)' gamma curve.
What's the point of 'gamma 2.4' curve as a default gamma curve? Well, I know what gamma 2.4 is. But Resolve also has 'gamma 2.2' curve and 'Rec709 (Scene)' curve.
So we have 3 curves for rec709 delivery. And the last one is look like regular, typical and familiar rec709 curve. So why 'gamma 2.4' is the default?
What am I missing?

Edited November 22, 2018 by Anton Meleshkevich

[Scott Stacy]

2.4 was determined the be the broadcast standard by the BBC some time ago.  It used to be 2.35.  2.2 is for the web - bright environments.

Regarding Scene  See this article by Steve Shaw: https://www.lightillusion.com/aces_overview.html

 

[Jussi Rovanperä]

In Resolve, rec709 gamma is bt.1886 formula, srgb is the srgb formula, gamma 2.2 and gamma 2.4 are pure gamma functions, like gamma 2.2 is linear^(1/2.2) and gamma 2.4 is linear^(1/2.4).

gamma 2.2 is very close to srgb formula, but gamma 2.4 is quite different from bt1886.

 

Edited December 9, 2018 by Jussi Rovanperä

[Jannik Altgen]

2.4 is basically TV

2.2 for web, if you find 2.4 to low. We deliver for TV and web with a 2.4 Rec709 and clients are fine with it.

[Anton Meleshkevich]

I know that 2.4 monitor gamma for dark environment broadcast, 2.2 for lighter room, etc.
I know what bt 1886 is.
My question is about WHY default resolve colorspace gamma is 2.4, while all rec709 LUTs have 1.90 gamma. Which is darker than 2.4 and 2.2. And this is not strange, because this isn't monitor gamma (which would be brighter) but colorspace gamma. So it is darker.
Resolve has rec709 (Scene) gamma in RCM settings. Which is similar to, for example, ARRI rec709 LUT or all BMD rec709 LUTs. As far as I know these LUTs are 1.90. Not 2.2 or 2.4. And of course not bt1886 which is only for screens. It's gamma depends from black level. It's 2.4 only if black level of a screen is 0.00000000 nits. Otherwise it has lower (brighter) gamma in shadows.

Edited December 10, 2018 by Anton Meleshkevich

[Anton Meleshkevich]

Just want to add some useful info from wiki.
 

Quote

 

Rec. 709 specifies the OETF (opto electrical transfer function) of HDTV encoding in reference to the camera, known as camera gamma (sometimes indicated as "scene-referred" gamma). The Rec. 709 transfer function from the linear signal (luminance) to the nonlinear (voltage) is linear in the bottom part and then transfers to a power function for the rest of the [0..1] range.

The power function of the majority of the gamma curve is 0.45, but because it is offset by the linear section the resulting equivalent gamma is more approximate to 0.50-0.53 (the inverse of which is approximately gamma 1.9-2.0 to convert back to linear).

While Rec. 709 does not specify the display referred gamma, display gamma is discussed in EBU Tech 3320 and specified in ITU-R BT.1886 as a gamma of 2.4. This is a higher gamma than the 2.0 the math shown above would indicate, because the television system has been deliberately designed with an end-to-end system gamma of about 1.2, to provide compensation for the ‘dim surround’ effect. Therefore, the monitor gamma is not the inverse of the camera gamma.

 

 

[Cary Knoop]

 

 

1

On 12/10/2018 at 10:34 AM, Anton Meleshkevich said:

all rec709 LUTs have 1.90 gamma. 

Rec709 has a linear and a gamma part, the gamma is 2.4, taken together (linear + gamma) gives a resulting gamma of approximately 1.9.

See the attached mage for a comparison.

[cameronrad]

On 4/12/2019 at 10:08 AM, Cary Knoop said:

Rec709 has a linear and a gamma part, the gamma is 2.4, taken together (linear + gamma) gives a resulting gamma of approximately 1.9.

See the attached mage for a comparison.

sRGB has the 2.4 gamma part in the middle. Rec.709 is 1/0.45.

In any case this is just the encoding/transfer function. https://www.image-engineering.de/library/technotes/714-color-spaces-rec-709-vs-srgb

Which is different than the decoding/display gamma. (Gamma 2.4). The combination results in an end-to-end system gamma that isn't 1.0 but rather around 1.2 to compensate for viewing environment.

Quote

While Rec. 709 does not specify the display referred gamma, display gamma is discussed in EBU Tech 3320 and specified in ITU-R BT.1886 as a gamma of 2.4. This is a higher gamma than the 2.0 the math shown above would indicate, because the television system has been deliberately designed with an end-to-end system gamma of about 1.2, to provide compensation for the ‘dim surround’ effect. Therefore, the monitor gamma is not the inverse of the camera gamma

https://en.wikipedia.org/wiki/Rec._709#Transfer_characteristics

https://www.chromapure.com/colorscience-gamma-new.asp