Is it possible to play purchased movies at 4K in the TV app on MacBook Pro 2017 (using the built-in retina display)?

Ultra HD 4K resolution is 3840x2160 pixels. The screen on a 15" 2017 MacBook Pro has a native resolution of 2880x1800 pixels. So if you played a 16:9 widescreen movie full screen, without distorting the aspect ratio or butchering the movie with "Pan and Scan",

• A 1080p movie source would need to be upscaled from 1920x1080 pixels to 2880x1620 pixels
• A UHD 4K movie source would need to be downscaled from 3840x2160 pixels to 2880x1620 pixels (although, based on the previous reply, it sounds like you wouldn't be offered the option to play 4K source material)

There would be thin horizontal "black bars" because of the difference in the shape of the display (16:10) and the shape of the movie (16:9).

Looking at current Mac notebooks, their native resolutions are

• 2560x1664 for the 13" M2 MacBook Air
• 2880x1864 for the 15" M2 MacBook Air
• 2560x1600 for the 13" M2 MacBook Pro
• 3024x1964 for the 14" M2 {Pro/Max} MacBook Pro
• 3456x2234 for the 16" M2 {Pro/Max} MacBook Pro

So even the 16" MBP doesn't quite hit the Ultra HD TV resolution of 3840x2160 pixels. You might be able to play UHD 4K content on it, but you'd have to downscale the content to fit a 3456x1944 pixel area on the screen.

The controls in Displays Settings don't have an effect on the native resolution of the LCD panel. Your 15" Retina screen has a native resolution of 2880x1800 pixels.

Older, non-Retina, 15" MBPs came with a choice of two screens:

• A standard 1440x900 pixel screen
• A "hi-res" 1680x1050 pixel one

See the "1680x1050 (Default)"? That's a Retina mode. Select it, and your Mac creates a virtual 3360x2100 pixel canvas. It tells applications that the screen has a resolution of 1680x1050 pixels – for sizing purposes. A Retina-aware application will use special drawing calls to fill in photo areas and the like at the 3360x2100 pixel resolution.

When the Mac goes to display that 3360x2100 pixel screen on the actual screen, it will downsample the contents to fit the actual 2800x1800 pixels of the display.

The whole thing is an elaborate way to provide backwards compatibility with applications that think that the only purpose of more pixels is to cram more stuff onto the screen, while keeping text and objects at a readable size, and letting Retina-aware applications draw their text, icons, and photo areas in greater detail.

When you select 3840x2400, applications that see that high "resolution" think that it means that you have a really, really large screen onto which they ought to cram a lot of stuff. So they do, and everything gets really tiny.

But it doesn't magically turn your 2880x1800 pixel screen into a 3840x2400 pixel screen. All that would happen here is that you would be changing where downsampling took place.

If you were running the screen in Retina 1440x900 mode or native 2880x1800 mode, then, presumably the movie player application would be doing the downscaling, to match the (virtual) 2880x1800 pixel screen. The 2880x1880 pixel canvas would exactly match the resolution of the 2880x1880 pixel screen, so copying from the canvas to the screen would not involve further downscaling.

If you were running the screen in Retina 3400x2400 mode, then the movie player application might see no need to downscale the image to fit the (virtual) screen, but the operating system would have to do downscaling to get the contents of the (virtual) screen onto the LCD panel.

Not in 4K. You can play it of course.

_nVn_ wrote:

Or, are you saying that the screen can't display 4K?

Short answer – yes.

If so, then why is that option (stream quality - 4K) present?

That Displays option has nothing directly to do with streaming quality. It's about simulating a huge workspace, that will then be crammed onto a small screen. Some people might want to cram a lot of stuff onto the screen, with little or no regard as to whether it is readable, and that setting is for them.