It seems that it is currently impossible to view photos at 4k resolution using Chromecast Ultra. There is not much 4k video content available, but viewing photos at 4k would be a great feature. In fact, that was the single reason to buy Ultra for me.
I have checked that Chromecast Ultra receiver reports:
I guess the first two are for backwards compatibility with the original Chromecast, but devicePixelRatio of 1.5 means that the native resolution is only 1920x1080, where Chromecast Ultra should report devicePixelRatio of 3 to lead to 3840x2160 native 4k resolution.
The TV still reports that HDMI signal is UHD 2160p60, so that Chromecast really is providing the 4k signal.
I have tried creating a canvas of 4k resolution and drawing images there, but it was again downscaled to 1920x1080 on the TV screen :-(
What is the reason for this?
Are there are plans to fix devicePixelRatio for Chromecast Ultra anytime soon?