Video playback on retina iMac: I'm confused

[FredT2]

Running the retina iMac at "Best for display", as we know it generally acts like a 2560x1440 display. For apps that display photos, images are shown at actual pixel dimensions within the 5120x2880 display, so a 1920x1080 image, for example, takes up a relatively small part of the screen. The same doesn't seem to hold for video, however. A 1080p video takes up the same space that it does on a normal 27-inch iMac. When shown at full screen size, Quicktime Player reports "Current Size" as 2560x1440. Same when running VLC or iTunes. When I view a 4K 3840x2160 video, Quicktime Player says it's 1920x1080 half-size, and in full screen it's again 2560x1440. Anyone have an explanation for this?

 

[Sciuriware]

Each and every application has to request for high density.
Some have such in their parameter files.
;JOOP!

 

[redheeler]

Most UI elements such as windows are measured on a standard pixel grid. When the Quicktime window is sized to 1920x1080 on a standard pixel grid, the video is in fact being displayed in full HiDPI 3840x2160.

That's how HiDPI scaling normally works. It takes assets that are twice the resolution they appear on a standard pixel grid and sizes them down by half. If assets are not twice the resolution they are upscaled and end up looking blurry or pixelated.

Edit: However, when Quicktime is displaying a 1080p video it chooses to keep the window size the same as it would be on a standard display, meaning the video must be upscaled.