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Related APIs: CAMetalLayer, WantedExtendedDynamicRangeContent, potentialEDRHeadroom, currentEDRHeadroom, [UIScreen mainScreen].brightness.
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Premise: I am a Video Player developer. The information I got from Apple’s development documentation about currentEDRHeadroom is that if I set WantedExtendedDynamicRangeContent = YES on a View, then the value of currentEDRHeadroom will be increased according to the current system brightness by a ratio that is not less than the brightness of potentialEDRHeadroom.
This is not a problem when at least ios16.
- Question: Device: iPad pro 12.9 2022 model, iPadOS 17.2. turn off Reference Mode, turn off Night Shift mode, turn off automatic brightness (not turning it off actually has the same result),
use the same picture or video.
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step: (1): Set [UIScreen mainScreen].brightness to 1.0 (or use system tools to adjust the system brightness to 100%), create the UIView where CAMetalLayer is located and set its parameter WantedExtendedDynamicRangeContent=YES, Start rendering. At this time, the potentialEDRHeadroom is always equal to 16 (this has always been the case in the iPad Pro 12.9 2022 model, which should be set by the hardware). At this time, the value of the currentEDRHeadroom is about 3.7-3.9, which means it can provide standard 100% brightness 3-4 times the brightness, My picture at this time is that there are no over exposure of pixels in the entire picture.
(2): Set [UIScreen mainScreen].brightness to 0.8 (or use system tools to adjust the brightness to 80%) or lower, such as 50%, create the UIView where CAMetalLayer is located and set its parameter WantedExtendedDynamicRangeContent=YES, At this time, I set [UIScreen mainScreen].brightness to 1.0 (or use system tools to adjust the brightness to 100%). Now the maximum value of currentEDRHeadroom can only be about 1.79. My pictures is that the entire picture or video has over exploded pixels.
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Bug or Feature: I have the impression that this problem did not exist in iPadOS 16, and from Apple's documentation and the courses in Developer.app, there is no mention that the range of currentEDRHeadroom is related to the system brightness before creating CAMetalLayer. The documentation and tutorials talk about real-time system brightness. I want to know if this is a bug in iOS 17.2 or will it be designed this way in the future?