Pixel Envy

Written by Nick Heer.

Extended Dynamic Range in Metal

Stu Maschwitz on Twitter:

After noticing that my iPhone 12 Pro Max and my Pro Display XDR happily display HDR values in an SDR context, I have since discovered that the same thing happens on my iMac Pro and 16″ MBP, neither of which claims to have an HDR display. How is no one talking about this?

This is the exact same demo as in the previous tweet in the thread, but on my iMac from 2017. Apple has quietly added HDR display capabilities to devices that long predate the Pro Display XDR, and I cannot find a single bit of information about this.

I shot a short HDR video on my iPhone and transferred it to my 2017 iMac, and was able to replicate Maschwitz’s findings: I saw an higher dynamic range than the display is allegedly capable of. When the HDR video was selected in Finder and I started a screenshot, the white lines of the screenshot crop area were clearly visible against the apparently white Finder background and a white webpage. Those crop lines were a brighter shade of white, if you will. When I selected a text file in Finder and tried the same screenshot test, the white crop lines of the screenshot tool became invisible against the Finder background and the webpage.

This is all on a 2017 5K iMac — not even the Pro model — running Big Sur. According to Apple, my iMac does not support HDR video, but I see the same effect as Maschwitz all the same.

Apple calls this capability Extended Dynamic Range, which is different from High Dynamic Range and Extreme Dynamic Range. Unlike those more hardware-based features, EDR appears to be a software-oriented implementation added in a recent version of Metal. Apple has been rewriting its graphic layers in Metal for years; High Sierra featured a rewritten WindowServer which affects every application in MacOS. This is an intriguing discovery that seems scarcely documented. Kudos to Maschwitz for finding this.