Regarding the gamma 2.2 transfer function

[dantmnf]

TL;DR

• Gamma 2.2 is a popular calibration target but unfortunately not supported in Windows MHC/ACM/HDR pipelines.
• You can't change color reproduction of color-managed apps by simply changing the calibration target tone curve.

The Problem

Think of a random color-managed app, given specified content (e.g. an image) and display profile, tell the rendering behavior.

Content Profile	Display Profile	Rendering Behavior
Unspecified	Unspecified	❓
sRGB	Unspecified	❓
Apple Display P3	Unspecified	❓
Unspecified	sRGB	❓
Unspecified	Rec. 709 with gamma 2.2	❓
sRGB	Rec. 709 with gamma 2.2	❓
Apple Display P3	Rec. 709 with gamma 2.2	❓
sRGB	Apple Display P3	❓

Note

• Chromium discards TRCs in display profiles and always assumes sRGB TRC for SDR.
• Few content is encoded in Rec. 709 (or sRGB, if you like) gamut with gamma 2.2
• Apple Display P3 consists of DCI-P3 gamut, D65 white point and sRGB transfer function.

Facts

• In the current implementation, if an MHC2 transform or ACM/HDR mode is in effect, the display is always tagged with either the original MHC ICC profile or a pseudo profile with sRGB TRC.
• For the MHC CSC pipeline to work properly with color-managed apps, tone curves in MHC ICC profiles should be sRGB TRC to ensure encoded RGB values are correctly converted back to linear XYZ.
• For good old non-MHC ICC profiles, if you choose gamma 2.2 as calibration target, the resulting profile will have a gamma 2.2 TRC. When the profile is associated with a display, CMMs will convert sRGB-encoded contents from sRGB piecewise TRC to pure-power gamma 2.2 TRC before sending them to the display.
  • No known CMM will passthrough sRGB-encoded content, without conversion, to a gamma-2.2 tagged display.

My Views

• Given the pipelines are fixed to sRGB TRC, it doesn't make sense to use gamma 2.2 in some stages in the pipeline.
  • Yes, gamma 2.2 can be used for non-color-managed apps/contents, but this is unfortunately not supported in current Windows MHC/ACM/HDR pipelines.
• Don't treat sRGB TRC special - it's just like some other gamma curve with different power.
  • For SDR display: either ignore the curve and accept different OETF/EOTF combination (just like BT.1886-encoded video), or follow the full source EOTF - linear XYZ - destination OETF pipeline
  • Avoid explicitly tagging your content with sRGB TRC, especially if it's not created under sRGB TRC (like gamma 2.2)

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Feel free to drop your thoughts here, or upvote the related Feedback Hub ticket¹.

Footnotes

1. https://aka.ms/AAnpmrt ↩

[Whitiger3000]

I think (learned from internet) that the movies should be displayed with certain gamma not encoding gamma,the latest document is ITU-R BT.1886.In this case,the content displayed with color-managed app or ACM ON,will be "correct" gamma but not the creators of content intended to show you.

For the problem,my opinion,color-managed app (same as ACM) will display all contents(2.0/2.2/srgb gamma) correctly regardless of monitor`s gamma.If content is sRGB,display profile is Rec. 709 with gamma 2.2,monitor real gamma is sRGB.
(1)content displayed without color-managed,will be correct.
(2)content displayed with color-managed.monitor real gamma,will be incorrect,because display profile for gamma 2.2.
(3)content displayed with color-managed,monitor real gamma,VCGT(base real gamma and transfer target 2.2),will be correct.

[dm-u]

Here estimation of max error of CSC of DCI P3 to sRGB gamma 2.2. Max error is 2.2DE. May be it's acceptable for most users who prefer dark environment.