Brighter than White — EDR on Macs

Stu Maschwitz observes that Apple has enabled the display HDR content on many older Macs models. Until now I thought this was a feature only for the very pricy Apple XDR Display hardware, but it looks like support also exists for the far more “ordinary” screens of some older Macs.

I downloaded the two HDR clips he shared and did a little experiment. And I think there’s one detail wrong in this paragraph from Stu’s blog:

Think of it this way: This EDR display philosophy is so important to Apple that they are willing to spend battery life on it. When you map “white” down to gray, you have to drive the LED backlight brighter for the same perceived screen brightness, using more power. Apple has your laptop doing this all the time, on the off chance that some HDR pixels come along to occupy that headroom.

I’m pretty sure it’s wrong to say they’re making the LED backlight brighter all the time. My observations support that the display brightness and color mappings change dynamically when some HDR content appears on screen. Those are reverted back to normal once the HDR image disappears. So no compromise on battery life until you put HDR content on your screen: the display is only made brighter while HDR content is visible.

Here’s how I came to that conclusion.

First of all, I have a late 2014 Retina 5K iMac, which appears to support EDR. (Wow!)

When I select one of Stu’s HDR video with the Finder in column view, it shows a preview in the rightmost column. When the preview first appears on screen, it is rendered normally, in SDR. It then progressively becomes brighter over the span of one or two seconds. Brighter than the surrouding white. This appears to be the EDR system firing up: slowly cranking up the display brightness at the same time as it darkens the standard white point for everything but the video. Those two operations are done in tandem so well that you don’t perceive any change on screen other than the video becoming brighter.

There are however some small side effects you can observe. I’ll take my free app Sim Daltonism to do some observations. Set it so it does not filter anything (the “normal vision” setting), and all it’ll do then is continuously take a screenshot and render it its window. As EDR is firing up you can observe those screenshots of the video in the window slowly becoming overexposed. That’s because Sim Daltonism screenshots comes from an EDR image that gets truncated to SDR. Here’s what it looks like:

Another thing you might notice while EDR is activating (or when it’s shutting down) is occasionally some of the frames in Sim Daltonism can be black, sometimes only for some windows in the screenshot. It’s as if the window server fails to produce a screenshot during the process of activating or shutting down EDR.

Note that for EDR to work (on this hardware) you can’t have your screen at full brightness. And thus when at full brightness, Sim Daltonism will not show you overexposed screenshots and you won’t see any black glitches. EDR cannot do anything at full brightness on this monitor, so there are no side effects so see.

On macOS Big Sur, when in dark mode, I noticed some white text labels are sensitive to EDR. (This looks like a bug.) When the video becomes visible on screen and EDR activates, those white text labels will slowly brighten at the same time as the video. I’ve located two of those labels for now: Safari’s active tab text (while the window is frontmost), and Gamma Control’s text label below the tabs (when the palette is set to translucent dark appearance). There’s also the checkmark for checked items in menus that appears affected. When you close the window with the video, they’ll slowly fade back to normal white. It can be a bit hard to see though.

Another Big Sur issue: running an app that tweaks the gamma curve can break this EDR system temporarily. If you open and then quit Gamma Control, or do the same with Apple’s Calibration Assistant, the image of the HDR suddenly lose its EDR-enhanced brightness. It’ll regains its brightness the next time you open an app (any app). At least it does that on my iMac. If you’re at Apple, see FB8929660.

This has been tested on macOS Big Sur 11.0.1 and Catalina 10.15.7.