Help understanding Retina scaling (for a FCP editor)

The "Retina" term can be a bit confusing as it is both used in a hardware and software context.

In a hardware context, it means: "LCD display with IPS technology with a pixel density so high you can't see individual pixels at average viewing distance".

In a software context, it means: HiDPI.

Now what is HiDPI? It basically means your computer will render to a frame buffer twice as large (in pixels) compared to a non-HiDPI computer that displays elements with the same physical size (in millimeters/inches). I won't go into details as to what a frame buffer is, but just consider it "the area on which your computer draws visual elements before they are displayed".

For example, the iPhone 4's 640×960 display is the HiDPI equivalent of the 3GS' 320×480 display, as elements are rendered to it with twice the number of pixels for each axis, but are displayed with exactly the same physical size. If the iPhone 4 did not have HiDPI enabled, each element it displays would be half the size. So you could have fitted 10 rows of tiny icons including the dock. Instead, the iPhone 4 enabled HiDPI to display the same number of rows of icons (5), but made them display with 114x144px size rather than the old 52x52px size instead, which means you don't have more usable space than before, but visual elements are more detailed.

In 1920×1200 scaling mode, HiDPI is still enabled. That means your Mac will render to a 3840×2400 frame buffer then scale it down to 2880×1800 to display it on your screen. Any loss in quality is due to the downscaling interpolation. As you're rendering 3840×2400 pixels rather than 2880×1800, it does have a slight negative effect on performance/battery life.

Achieving 2880×1800 with a third-party utility such as SwitchResX means you're putting your Mac in 2880×1800 (no HiDPI) rather than the default 1440×900 (with HiDPI). That means the frame buffer will have the exact same resolution.

The performance of 2880×1800 (no HiDPI) might actually slightly better than 1440×900 (with HiDPI), since it has lower-res assets (no @2x bitmaps) to decompress.

So performance would be in that order (or close):

• 2880×1800 (no HiDPI)
• 1440×900 (HiDPI) - also known as "Best for display"
• 3360×2100 (no HiDPI)
• 1680×1050 (HiDPI)
• 3840×2400 (no HiDPI)
• 1920×1200 (HiDPI)

Note that any performance difference is not likely to have any significant impact on your FCP renders/exports time. It has more to do with how responsive the app's UI is, especially during animations like scrolling. I have not experimented with battery life, but I'd expect battery life to follow this performance list as well, with differences being pretty minimal.

Visual quality would be in that order:

• 2880×1800 (no HiDPI) or 1440×900 (HiDPI) - also known as "Best for display" TIE
• All scaled resolutions and their non-HiDPI equivalent (3360×2100 (no HiDPI), 1680×1050 (HiDPI), 3840×2400 (no HiDPI)...) TIE
• All resolutions below 2880×1800 with HiDPI off (those are not achievable without a third-party utility)

Amount of screen estate would be in that order:

• 3840×2400 (no HiDPI)
• 3360×2100 (no HiDPI)
• 2880×1800 (no HiDPI)
• 1920×1200 (HiDPI) or 1920×1200 (no HiDPI) TIE
• 1680×1050 (HiDPI) or 1680×1050 (no HiDPI) TIE
• 1440×900 (HiDPI) - also known as "Best for display" or 1440×900 (no HiDPI) TIE
  [...]

Also, how exactly does video display? I mean, 1080p video is 1920x1080, but I can see a 1080p video at 100% just fine in FCP X... Even in quicktime player 1080p videos don't seem to take up the whole screen so I'm assuming there's some scaling or something going on here.

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That's really app-specific. Apple software (Finder's quick view, Preview, Quicktime...) seem to always display elements smaller than on non-HiDPI equivalent computers (an image/video on a 15" rMBP would be half the size as on a 15" cMBP). This is basically an exception to general HiDPI rule I mentioned earlier. Third-party software varies.

ehmjay said:

Amazing! Thanks for breaking it down for me. That really does help a lot.

So now I know that I can run this thing at 1680x1050 and still be seeing lovely retina quality images but actually have a useable amount of screen real-estate.

Cheers!

Click to expand...

If you need the desktop space, don't hesitate from choosing the still HiDPI 1920*1200. The only downside I can see after using this desktop resolution is the text size but if your eyesight is still good, why not.

ehmjay said:

It wasn't until I installed a third party app and discovered that I could actually go up to a whopping 2880x1800 that things got confusing.

Click to expand...

What 3rd party app?

JustinePaula said:

What 3rd party app?

Click to expand...

I use Pupil, it has a nice clean UI that is easy to use even in high resolutions.

There are also other alternatives, like the simple QuickRes, and the feature-rich SwitchResX.
None of these three are free, but are not that expensive either. All have trial versions available.

Retina DisplayMenu is also a very good option, and free.