The YouTube app on Quest 3 now supports 8K videos, including immersive 360° & 180°.
At 8K, 2D 360° videos have enough pixels to almost match the Quest 3's display angular resolution, and 2D 180° videos will far exceed it. For 3D content, 8K 180° videos almost match Quest 3's display but 3D 360° videos are still be far below.
Around 16K would be required for 3D 360° videos that match Quest 3's display, and around 50K for 3D 360° videos that approach what the human eye can discern (assuming a capable headset such as Varjo XR-4).
The app on Quest 3 also now supports regular rectangular 8K videos, though this seems a tad pointless as the headset doesn't have sufficient resolution to do this justice at any reasonable size.
Strangely, I found when testing the update on my Quest 3 that most 360° videos labeled as 8K in the search results max out at 4K in the quality selector. It's unclear why exactly this is, and it wasn't a problem with any 180° videos I tried.
And even on the immersive YouTube videos that do play at 8K on Quest 3, the bitrate seems to be far below what Apple offers with its Apple Immersive Video, which is 8K 180° 3D. The compression artifacts resulting from low bitrate are much more apparent in an immersive video inside a headset than on a traditional screen, and Apple Immersive Video's high bitrate was a key feature I praised in my Vision Pro review. Apple Vision Pro doesn't have a YouTube app though. You can access it in the Safari web browser, or through the third-party app Juno, but neither option allows playing 360° or 180° content.
