What is the max. resolution for native smooth scaling and custom resolutions on Apple Silicon?

[cprcrack]

This is my setup:

• Philips PHL325E1 monitor with a native resolution of 2560x1440
• Macbook Air M1
• macOS Monterey 12.4
• BetterDisplay 1.2.7

I've correctly configured the new native smooth scaling feature for hardware displays without using any dummy, which is a feature I was really waiting for months. However when I was changing the Low Resolution "2560x1440, 75Hz, Default, Native" to the HiDPI equivalent, the max I can get is "2544x1431, 75Hz", which is strange.

If I go to Settings > App Menu > Show additional resolution options, I do get the expected HiDPI 2560x1440 option, but it is flagged as Invalid, and when I click it, nothing happens, and the resolutions remains the previous selected one.

Is this a bug or is it expected? I fear that missing those horizontal and vertical pixels may have an effect on image quality although it's definitely better than the low resolution one.

[waydabber]

This is expected. The native resolution is only available as LoDPI, this is a macOS limitation. But using HiDPI for the native resolution would only result in a supersampling/antialiasing effect (some do like this). If you really want this effect, you still need to use dummy mirroring. I think most of the benefits of HiDPI come only if you use a resolution that is lower than the native resolution (the more you lower the HiDPI resolution, the more details emerge as more physical pixel belongs to a logical pixel (which contains 4 framebuffer pixels).

[waydabber]

Great, thanks for the feedback! :)

[Kagami]

Sorry for bothering, I have one more question.
I was experimenting with different resolutions a bit and lower HiDPI resolutions (e.g. 1920x1080) actually look better on my 27" 2560x1440 screen (Dell U2713H).

But I don't have explanation for that. Technically, when macOS renders 5120x2878 virtual screen and scales it down to 2560x1440 physical screen, it shouldn't look worse than 3840x2160 scaled down to 2560x1440. But I can clearly see that fonts are much better in the latter case.

Is it because the particular font macOS uses doesn't look very good being small? With 1080p HiDPI everything is (obviously) bigger.

2560x1439 HiDPI
1920x1080 HiDPI

[waydabber]

The desktop is rendered at 2x with HiDPI. The lower the HiDPI resolution, more physical pixel fits into a logical pixel (which contains 2x2 framebuffer pixels), so the rendering is more detailed. For full crispness with a 2560x1440 panel you need to set 1280x720 HiDPI. 1920x1080 is an in-between resolution and should give you a more detailed view than using 2560x1440 HiDPI and then scale it back to 2560x1440 (which is basically a 2x supersampling/antialiasing).

[Kagami]

Ah, I get it, thanks.
So to get quality similar to ClearType/RGB subpixel rendering, on macOS we have to sacrifice normal physical pixels of the panel. (Which is fine if you have e.g. 24" 4K, but not in case of 27" 2K.)
A bit strange, but Apple probably had its own reasons to use bitmap scaling...

[waydabber]

Subpixel rendering requires 1:1 pixel mapping and an accurate knowledge of the subpixel layout. With scaling you don't have 1:1 mapping, rotation messes up subpixel arrangement (that's why this technique does not work on iPhones, iPads for example), also most OLED panels or LCD panels with non-RGB layout won't work properly with it (or a different algorithm needs to be used for each subpixel layout/rotation mode which requires some kind of user-accessible "tuner" - ClearType in Windows actually has but in practice it is rarely used, most users can't set it up properly so it does not really work). Because of all these issues with subpixel rendering and since it is not really needed for mid-to-high PPI displays anyway (which Apple exclusively uses) they just abandoned it altogether. In light of all the above-mentioned issues, I actually don't think this was a bad decision on their behalf. Windows has ClearType but it is actually a hit-and-miss, modern and legacy apps use different rendering, you cannot properly disable it system-wide - or last time I tried I had trouble with it (modern apps will still do it) - even when it does not work properly with your display etc.

[3DCoded]

Yes. If you use a "HiDPI" resolution at the same resolution as your monitor, then it isn't even HiDPI anymore. The definition of HiDPI requires it to be less then the amount of pixels in the screen to bring out the details.

[cprcrack]

I don't think this is the case. Using a HiDPI resolution at the same resolution as my monitor should make everything look much better on screen because 4 times more framebuffer pixels are being used. Otherwise with the default LoDPI it looks like the screen has much poorer resolution, with big visible pixels, something that doesn't happen on Windows using the same screen.

But as @waydabber mentioned this is a MacOS limitation where you simply cannot set the exact same resolution.

[waydabber]

Yes, some like the HiDPI rendered supersampled desktop + it really helps with zooming or screenshots as well. But yes, this is an OS limitation (except for the internal MBP display seemingly). You can still use a resolution one notch below the native or you can use a dummy.

[3DCoded]

Use the new native HiDPI scaling and set it to something like 1920x1200 60hz and HiDPI

[cprcrack]

And would it be possible to choose HiDPI resolutions above 2560x1440 like it was possible with the dummies just to see how it looks? Currently the max valid setting is 2544x1431, is it also a MacOS limitation that you cannot go above 2560x1440?

[3DCoded]

What I am saying is that if your screen has 2560x1440 resolution, it is impossible to go above that and show every pixel, let alone HiDPI.

[3DCoded]

If you wanted 2560x1440 HiDPI resolution, get a monitor with more than 2560x1440 resolution.

[cprcrack]

@smattar2020, I completely understand you, but you do not seem to understand me. This is related to how MacOS manages those pixels internally which has an effect on perceived image quality, not actual hardware pixels. My question is directed to @waydabber.

[waydabber]

With your supposed permission I renamed the discussion and gave answer to the question of what is the max resolution. :) I hope it's all right!

[cprcrack]

Of course, thanks for tidying it up.

[waydabber]

On Monterey 12.4+ the max. framebuffer resolution for native displays is limited by the following factors:

• 6K (6144px) horizontal pixel resolution on base M1.
• sub-8K (7680px) horizontal pixel resolution on M1 pro and higher.
• twice the native resolution (not including the native resolution itself on external displays - for the internal display it is included, at least on MBPs with the notch).
• using the HDMI port further limits the maximum resolution to 2560px horizontally. Use USB-C if possible.

Update: on Ventura dev beta4 the native panel resolution seems to be available as HiDPI. See #834 for more info.
Update 2: on Ventura dev beta5 the native panel resolution is not available as HiDPI anymore... :(

For HiDPI the framebuffer resolution is 2x the normal resolution (4x as many pixels).

With dummies you can go as high as 16Kx16K even on the entry level M1.