Again, this is a connector from your mobile device to a display. Nobody has ever done any lossy compression on that pathway; both because you can drive simple circuits at frequencies high enough to have plenty of bandwidth and because it would actively destroy information (hence lossy). This is such a big problem because the video you are playing has already been lossy compressed (H.265) and it will most certainly not get better by a second pass that only has the raw pixel data available.

I could imagine people want to play 4k movies from their mobile device on their TV, but you wouldn't install a 4k screen as the display on the mobile device.

Edit: Yes, computer is inaccurate. I wanted to get the idea across that we are talking about connectors (DVI, HDMI) that you would normally use to hook up a computer or laptop to a TV or LCD. They have only recently appeared on mobile devices, but serve the same purpose here: video (and audio, for HDMI) out.

I think that if you have a 1080p source on your phone, it goes to tv as 1080 and without another compression step, it's only mirroring that reduces the resolution (which, for ipad mini, is less than 1080p btw) and introduces compression pass

rescaling does not mean you need to do compression.

> Again, this is a connector from your computer to a display.

Huh? Its the plug on the bottom of a phone (and ipads now i guess). Lightning isn't on any computers.

a) This is a connector from a mobile device to a TV. That is the primary use case here. I doubt anybody is hooking up an iPhone to any other type of display.

b) Why is there a second pass compression/decompression stage ? Isn't iOS outputting the compressed H.264 stream and the adapter decoding it i.e. one stage ?

c) Given that Retina displays by definition are the best resolution we will need and it is far less than 4K it is questionable whether there will be a use for 4K on mobile devices. Other than using your iOS device as a media player for your TV (very small use case).