Dear Apple, please make sure this bug gets delivered to whoever is responsible. That's all I ask. Please don't let it sit for months unassigned. This is, by far, the worst bug I've ever found with the macOS wallpaper system.
FB21532401
If you own a 13" 2020 or newer MacBook pro model, set to the default resolution, and are running macOS Tahoe, macOS will significantly degrade the quality of any image set as wallpaper.
When a still image is set as the wallpaper on macOS Tahoe, on some display configurations, the systems downscales the image to an incorrect size, resulting in pixelated wallpaper. The problem is exacerbated by the fact that macOS Wallpaper Agent appears to be using a less than ideal downscaling algorithm, which results in Super Mario Bros’ type pixelation (nearest neighbor) as opposed to any other reasonable modern method (like bicubic.) The issue does not repro on macOS Sequoia.
Every model MacBook we’ve tested offers some resolutions with some form of this problem, but the 13” is the only one where it is notably awful. The most evident default case of this is the 13” MacBook Pro models with a 2560x1600 physical display (for example, 2020 MacBook Pro 13” (17,1.)) These models have a physical display resolution of 2560x1600, and a default scaled resolution of 1440x900. The relationship between the physical resolution and scaled resolution is not an even ratio (1:1 or 2:1), which seems to be the common condition under which this issue occurs.
Repro steps:
- Set the systems display resolution to the default resolution - ideally on the model described above (see details on this below)
- Set a high resolution image (in this example 5120x2880) as the system wallpaper using any method
Results:
On the model described above, Wallpaper Agent will generate and display a 1440x810 image as the wallpaper. It should be generating and displaying at a minimum of 2560x1600, or more appropriately at 2880x1800 which is the proper 2X resolution. This can be confirmed by viewing the properties of the generated images in the macOS wallpaper cache here:
~/Library/containers/com.apple.wallpaper.agent/Data/Library/Caches/com.apple.wallpaper.caches/extension-com.apple.wallpaper.extension.image
On modern Apple systems, the only situation in which the wallpaper should be generated at 1X is when the physical resolution and set resolution are 1:1. In any situation where the physical resolution is larger than the set resolution, the image should be generated at 2X the set resolution.
As far as we can tell, this issue impacts any format, and any resolution of image, and occurs independent of the set image resolution.