This question is about Apple's new ARKit framework introduced with iOS 11:
Is there a way to configure the captured video quality while using an ARSession? It seems to default to 1280x720 - which looks pretty bad, especially on an iPad. I'd love to change this to 1080p or 4k.
If this isn't possible, is there any way to use ARKit by providing a custom video stream?
Nope, and nope.
ARKit owns and entirely controls its underlying video capture session. It's hard to know why, but there are some likely guesses... to ensure that it gets video samples in a format and rate that works well for the computer vision work it does to provide world tracking. And/or to make sure said work is done efficiently enough to leave headroom for your app to do awesome things with SceneKit, Metal, etc. And/or to make world tracking performance/accuracy consistent across all supported hardware.
More capture session flexibility might be a good feature request to send to Apple, though.