HDR to SDR Conversion for Windows' "Stream HDR" Function Is Incorrect
Content Type: Troubleshooting | Article ID: 000088801 | Last Reviewed: 05/03/2024
HDR (high-dynamic-range) to SDR (standard-dynamic-range) conversion done for Windows* Stream HDR function. Instead of converting PQ EOTF (perceptual quantizer - electro-optical transfer function) to sRGB gamma (the default windows SDR transfer function), PQ EOTF is converted to 2.4 gamma, causing a washed-out picture on sRGB calibrated displays. The issue is observed using HDR video from apps like Netflix* and Movies & TV*.
This problem cannot be fixed on built-in displays because most have no ability to change SDR transfer function without complicated workarounds such as applying a 3D LUT through Windows DWM.
The issue affects both external and built-in displays. Because I am unable to change the gamma on built-in displays, there is no workaround for the issue. External displays usually offer the ability to change gamma so it is not really an "issue," but, it is still wrong. The Windows default SDR gamma is sRGB and HDR video should be tone-mapped to sRGB gamma instead of 2.4 to reflect that.
The function used on the Stream HDR feature is a software function controlled by Windows*, and it uses the industry's default value for gamma: 2.2.
The viewing conditions where HDR is consumed may vary the optimal gamma value for the best viewing experience. In general, the gamma 2.2 setting is generally best suited for brighter viewing conditions, such as an office setting with bright overheads and lots of indirect daytime light from exterior windows. In contrast, the gamma 2.4 setting is generally used for darker viewing conditions, such as watching prime-time television at night in a living room. This is where the issue may originate: the viewing condition.
A lot of discussion about the “best” gamma value can be found online, but most of those discussions conclude that it comes to personal preference. However, it should be noted that the vast majority of HDR content is recorded using this gamma value, and as such, 2.2 is the ubiquitous recommendation of a baseline gamma from where to then calibrate the display to the personal liking.
Using some software (such as DWM LUT) to apply a custom 3DLUT and force the gamma to 2.4 may seemingly correct the issue. This “correction” that the user perceives may be due to the viewing conditions where the content is consumed. This is a complicated workaround. However, Windows does feature the option to adjust the gamma value to get the best viewing experience for the user according to the viewing conditions.
Users may try these steps from third-party websites. Either option allows changing the gamma value to get the best viewing experience for their conditions without using any complicated workaround:
| Note | The gamma correction and HDR to SDR tools included with Windows* are developed and maintained by Microsoft. Only Microsoft can actually correct any issue with these tools. If the user is not satisfied with the options included with Windows, this should be reported to Microsoft Support*. |
It is unrealistic to expect the same quality from displays listed as NOT supporting HDR as from fully HDR-capable displays. The Windows HDR to SDR tool is included to help improve the consumption of HDR content on SDR displays, but it cannot be expected that this tool will make SDR panels display the same quality as HDR panels.