8 ms·

New Apple Silicon M4 and M5 HiDPI Limitation on 4K External Displays

313 points by smcleod 7mo ago

imrozim 7mo ago
[flagged]
wronglebowski 7mo ago
Props to the author for putting in what looks like ton of work trying to navigate this issue, shame they have to go to these lengths to even have their case considered.
- MarcelOlsz 7mo ago
  I went to hell and back trying to get PIP/PBP monitors on my 57" g9 ultrawide to work with my M2 pro. ended up having to use a powered hdmi dongle, displaylink cable, and displayport, with 3 virtual monitors via betterdisplay. Allowing resolutions outside of macs limitations setting in BD is what did the trick. I don't envy OP. Having 5120x1440 @ lodpi was the worst, just ever so slightly too fuzzy but perfect UI size but eventually got a steady 10240x2880 @ 120hz with HDR. I literally laughed out loud when I read the title of the thread. Poor guy.
  - moeadham 7mo ago
    betterdisplay is a life saver
  - jakehilborn 7mo ago
    You may be able to get this working using PBP and 2 cables without virtual displays. This is my write up for using HiDPI@120hz for two 57” G9s on my M2 MacBook. https://www.reddit.com/r/ultrawidemasterrace/s/VrBLFDxYzg https://www.reddit.com/r/ultrawidemasterrace/s/VrBLFDxYzg
    - MarcelOlsz 7mo ago
      Ah but you see, the challenge is to get a 3-split PBP on an M2 pro on a monitor with a native res of 7680x2160, each one scaled down 33%, working at 120hz with HDR, all hidpi like so: ┌─┐┌────┐┌─┐ │ ││ ││ │ └─┘└────┘└─┘ It creates some wonky math and requires plenty of dock and cable shenanigans and unlocking resolutions above 8k via BD. It's the third "monitor" where it gets tricky with the M2 pro especially at these resolutions.
      - MarcelOlsz 7mo ago
        ...And then there is the near-infinite trickle down of apps that rely on apps that rely on arcane configs and so on. This is truly the OS from hell. At least with Windows you know it's going to be garbage so when anything works on any level you are maximally impressed. But I have to spend my weekends isolating window shadow disabling functionality from yabai into it's own binary because I switched to aerospace which requires 'displays have separate spaces' to off, which just so happens to be exactly what yabai requires to be on, to remove window shadows, which is the only use I have left for it. Just like the excel world championship I would find a macos ricing/window tiling competition equally enthralling. You read articles like the OP and at some point all you can do is laugh because lord (Cook) knows you've cried.
      - fragmede 7mo ago
        Fascinating. What's that gain you over using the monitor's native resolution full screen vs PBP mode?
        MarcelOlsz 6mo ago
        I hate spending any unnecessary clicks or keyboard shortcuts on getting whats out of my head into the computer. I used yabai before primarily, now using aerospace. Since the monitor is super ultra-wide (57 inches with a very high DPI) the native resolution makes everything ultra small to my eyes. It's the same height as my 34-inch Samsung G5s which are 1440 pixels tall natively, but since this one is 2160, it would have to be 1.5 times larger physically to look decent at native res especially on macOS. The only other option is to scale the UI 1.5x which is where all the problems begin. I like the three-column separate monitor layout because I have hotkeys, primarily driven by my mouse but also usable keyboard-only where I can easily switch between monitors with `⌘+`` which moves my cursor between them. I can select whichever monitor I want and put my mouse to it, and I can switch to any workspace on any monitor quickly. I also have hotkeys that sync three workspace numbers across monitors, so switching between them switches all AeroSpace workspaces on all three monitors simultaneously. If I have five projects going, I'd have the terminal on the left, Linear and other communication tools on the right in accordion mode with AeroSpace, and I can use my mouse or keyboard exclusively to find exactly what I'm looking for almost as fast as I think of it. I spend zero time on window management or organization now so it makes it thoughtless to use. If I'm just using the monitor's native resolution there's no real way to do portals — having two apps open as sticky and only switching a portion of the monitor space to a different app while keeping the other sticky. There are hacks you can do with AeroSpace, especially since AeroSpace doesn't use native macOS Spaces, but the three-monitor layout is a much more robust approach in my opinion just a bit of a nightmare to setup. Theres a million little mac annoyances you have to fix.
- smcleod 7mo ago
  Thanks, it was a good portion of my weekend bashing my head against the keyboard trying to figure out what was going on and if there was a workaround I could use (there isn't that I've found).
- dostick 7mo ago
  The post reminded me how I investigated a similar issue having no idea. Using Claude or GPT to investigate this kind of hardware issue is fast and easy. It gives you next command to try and then next one and you end up with similar summary. I wouldn’t be surprised that author didn’t know anything about displays before this.
skullone 7mo ago
I thought I was going crazy when my new m4 seemed "fuzzier" on my external 4ks. I tried replicating settings from my old MacBook to no avail. I wonder if Apple is doing this on purpose except for their own displays.
- NBJack 7mo ago
  It's a bit nit-picky on my part, but this bizarre world of MacOS resolution/scaling handling vs. other operating systems (including Windows 11 for crying out loud) is one of my biggest gripes with using Apple hardware. I remember having to work hard to make my non-Apple display look 'right' years ago on an Intel-based mac due to weirdness with scaling and resolutions that a Windows laptop didn't even flinch handling. It was a mix of hardware limitations and the lack of options I had to address the resolution and refresh rates I had available over a Thunderbolt doc that I shouldn't have to think about. I honestly hope they finally fix this. I would love it if they allowed sub-pixel text rendering options again too.
  - Sir_Twist 7mo ago
    https://news.ycombinator.com/item?id=17477526 https://news.ycombinator.com/item?id=17477526 This reminds me of this comment, which I feel is a somewhat unsatisfying explanation, given that despite these difficulties, Windows somehow makes it work.
- paozac 7mo ago
  Same here, I tried all combinations with BetterDisplay but kept getting fuzzy fonts. It was very frustrating on a large 5k2k display
- semi-extrinsic 7mo ago
  I don't understand what is fuzzy about "lodpi". I've been using it for 8 years on a 4k 43" screen with 1x scaling. Can't say I noticed any difference when switching several times per day between Linux and an M1 MBP, nor any difference when upgrading to an M4 MBP.
  - 7mo ago
bsimpson 7mo ago
Wouldn't HiDPI be 1080p@2x? Is that still available?
- TheCoreh 7mo ago
  Yeah if I understand it correctly, this is more like 2160p@2x which is... unusual?
  - TheTon 7mo ago
    Yes, I would actually be surprised to learn that mode is available on any system. I’ve never seen that anywhere, though I only have a M1 Pro and an M4 Pro (and various Intel Macs). You’re rendering to a framebuffer exactly 2x the size of your display and then scaling it down by exactly half to the physical display? Why not just use a 1x mode then!? The 1.75x limit of framebuffer to physical screen size makes perfect sense. Any more than that and you should just use the 1x mode, it will look better and perform way better!
    - wpm 7mo ago
      Because 1x mode has no subpixel antialiasing and thus looks absolutely terrible. I have a 32:9 Ultrawide I would love to use on macOS but the text looks awful on it.
      - TheTon 7mo ago
        Then complain about that. That would make a much more sensible blog post and discussion. Asking for a crazy workaround to a sane problem isn't a great way to get good results, especially with Apple. Beyond the obvious performance pitfall, this scale up to scale down approach will also destroy the appearance of some controls. There is some UI that aims for 1px lines on hidpi modes that will get lost if you do this. It's hardly a perfect mode.
        wpm 7mo ago
        The crazy workaround only needs to be done because of what Apple did probably around a decade ago and probably already heard a bunch of crying about and didn't care. No one removed subpixel antialiasing on their own, we do this bullshit because Apple forced us to to make text look halfway decent.
wmf 7mo ago
This is not a normal retina configuration. This is a highly unusual configuration where the framebuffer is much larger than the screen resolution and gets scaled down. Obviously it sucks if it used to work and now it doesn't but almost no one wants this which probably explains why Apple doesn't care.
- sgerenser 7mo ago
  I don’t know why this was downvoted, I agree that this is a highly unusual configuration. Why render to a frame buffer with 2x the pixels in each direction va the actual display, only to then just scale the whole thing down by 2x in each direction?
  - eptcyka 7mo ago
    Because Apple no longer implements subpixel rendering for fonts?
    - Rohansi 7mo ago
      Supersampling the entire framebuffer is a bad way to anti-alias fonts. Especially since your font rendering is almost certainly doing grayscale anti-aliasing already, which is going to look better than 2x supersampling alone. And supersampling will not do subpixel rendering.
  - mlyle 7mo ago
    Because it's a decent way to get oversampling.
- phonon 7mo ago
  Isn't that just 2x supersampling? If you want "perfect" antialiasing that's the minimum you need, no?
  - wmf 7mo ago
    Yes, it is supersampling but historically almost no one runs that way.
nuker 7mo ago
Send it to Tim Cook email. It worked for me fixing DisplayPort DSC bug. After Catalina, later MacOSes lost ability to drive monitors at higher than 60Hz refresh. Apple support tortured me with all kinds of diagnostics, with WontFix few weeks later. Wrote email and it got fixed in Sonoma :) https://egpu.io/forums/mac-setup/4k144hz-no-longer-available-after-upgrade-to-big-sur/paged/10/ https://egpu.io/forums/mac-setup/4k144hz-no-longer-available...
- nerdsniper 7mo ago
  Thank you
- FireBeyond 7mo ago
  No, it didn’t get fully fixed. Fucking with DP 1.4 was how they managed to drive the ProDisplay XDR. If your monitor could downgrade to DP 1.2 you got better refresh rates than 1.4 (mine could do 95Hz SDR, 60Hz HDR, but if my monitor said it could only do 1.2, that went to 120/95 on Big Sur and above, when they could do 144Hz HDR with Catalina). I would be absolutely unsurprised if their fix was to lie to the monitor in negotiation if it was non-Apple and say that the GPU only supported 1.2, and further, I would be also unsurprised to learn that this is related to the current issue.
  - nuker 7mo ago
    Ahh, true, I now have 120Hz top, but it's fine, why I said fixed :) I now recall in Catalina I had full 144Hz and VRR options! Monitor is Dell G3223Q via Caldigit TS4 DP.
    - FireBeyond 7mo ago
      I was using 2 27" LG 27GM950-Bs (IIRC), that could do up to 165Hz and VRR on a 2019 cheesegrater Mac Pro, wasn't the cables, or the monitors, or the card. People at the time were trying to figure out the math of "How did Apple manage to make 6K HDR work over that bandwidth?" and the answer was simply "by completely fucking the DP 1.4 DSC spec" (it was broken in Big Sur, which was released at the same time). The ProDisplay XDR worked great (for added irony, I ended up with one about a year later), but at the cost of Apple saying "we don't care how much money you've spent on your display hardware if you didn't spend it with us" (which tracks perfectly with, I think, Craig Federighi spending so much time and effort shooting down iMessage on Android and RCS for a long time saying, quote, "It would remove obstacles towards iPhone families being able to give their kids Android phones").
PedroBatista 7mo ago
Now I know I was not crazy and the "cheap" 4K screen I bought a couple months ago doesn't actually suck. Tim Apple's Apple has been fu#$%& me again..
whatever1 7mo ago
How did none of the Apple devs notice this? 4k 32" inch is the industry standard for HiDPI monitors.
- MBCook 7mo ago
  Apple doesn’t make an 4k external monitor. They’re likely all on Studio Displays.
  - cosmic_cheese 7mo ago
    And prior to Apple’s re-entry into the display market, everybody internally was likely on 2x HiDPI LG UltraFine displays or integrated displays on iMacs and MacBooks. Fractional scaling (and lately, even 1x scaling “normal”) displays really are not much of a consideration for them, even if they’re popular. 2x+ integer scaling HiDPI is the main target.
- robertoandred 7mo ago
  Don’t think I’d call 4K at 32” high dpi.
  - whatever1 7mo ago
    I agree with you. It's my IT who disagrees. But to be fair, until last year there were no retina monitors in the market except the Apple ones. In 2025, the tides turned, there are now way more options both for 5k and 6k retina displays.
- Gigachad 7mo ago
  Tbh I'm not even sure what the issue is here. I have a personal M1 macbook and a work M4 and a 4k display. I don't see any issues or differences between them on my display. The M4 seems to be outputting a 4k image just fine. The article could just be AI slop since it just contains hyper in depth debugging without articulating what the problem is.
  - whatever1 7mo ago
comex 7mo ago
Well, it sounds like a real issue, but the diagnosis is AI slop. You can see, for example, how it takes the paragraph quoted from waydabber (attributing the issue to dynamic resource allocation) and expands it into a whole section without really understanding it. The section is in fact self-contradictory: it first claims that the DCP firmware implements framebuffer allocation, then almost immediately goes on to say it's actually the GPU driver and "the DCP itself is not the bottleneck". Similar confusion throughout the rest of the post.
- xbar 7mo ago
  I think you are probably right--it's a real problem. As an article, it is not 100% coherent, but there is a valid data and a real problem that is clear.
- Aurornis 7mo ago
  Agree. I started reading the article until I realized it wasn’t even self-coherent. Then I got to the classic two-column table setup and realized I was just reading straight LLM output. There might be a problem but it’s hard to know what to trust with these LLM generated reports. I might be jaded from reading one too many Claude-generated GitHub issues that look exactly like this that turned out to be something else.
  - smcleod 7mo ago
    Parts of it were pasted from my Claude Code Logs, Parts were written by me - and the table - that was me!
deleted 7mo ago
[deleted]
mil22 7mo ago
This would be even more compelling if you included screenshots with magnified detail insets showing the text blur.
- smcleod 7mo ago
  Thanks for the feedback, I'll try to take some photos, it's not an easy thing to do accurately without a good camera setup, but I'll reply here after work if I get something setup and added to the post.
pier25 7mo ago
Is this for specific verisons of macOS? The article doesn't mention it.
- smcleod 7mo ago
  It's in the environment and test setup (26.4 at the moment, but it is the same across all 26.x releases I've tried).
  - pier25 7mo ago
    I'm still on Sequoia and haven't noticed these issues on my M4 Pro.
    - smcleod 7mo ago
      Out of interest - whats the output of `system_profiler SPDisplaysDataType`?
      - pier25 7mo ago
        You can see the result here: https://gist.github.com/PierBover/99f44e5cba1657eb7e4ce8e2aabbcdcf https://gist.github.com/PierBover/99f44e5cba1657eb7e4ce8e2aa... I'm on macOS 15.7.4. It's so weird that it says 5120 x 2880 when the ASUS PA329CV is 3840 x 2160.
        smcleod 7mo ago
        That's a 404, but if it only says 5120x2880 then you're not running full 4K HiDPI which would show 7680x as per the blog post.
arjie 7mo ago
I'm sure you've already given this a crack via some other technique (I just Cmd-F for it and didn't find) but I have had monitors with confusing EDIDs before that MacOS didn't handle well and the "screenresolution" CLI app https://github.com/jhford/screenresolution https://github.com/jhford/screenresolution always let me set an arbitrary one. It was the only way to get some monitors to display at 100 Hz for me and worked very well for that since the resolution is mostly sticky.
- smcleod 7mo ago
  Hey, thanks I hadn't tried screenresolution but that seems to simply set the resolution and refresh rate without controlling the scaling which is what is needed for configuring the HiDPI mode scaling.
LuxBennu 7mo ago
Sadly I have the issue on a new m5 air. I have a 60hz 4k work monitor and two high refresh 4k gaming displays. The 60hz pairs fine with either gaming monitor, but the two gaming ones together and one just doesn't get recognized. Spent way too long trying new cables before realizing it's a bandwidth limitation.
chaostheory 7mo ago
What are they doing with MacOS? Is this due to VisionOS?
keyle 7mo ago
This is the sort of Apple gotchas that really upset me. They've got a good thing going, but they keep finding ways to alienate people.
- compounding_it 7mo ago
  Their suggestion : get an Apple monitor that we just launched.
  - saagarjha 7mo ago
    I don't think that one supports this use.
pier25 7mo ago
I use a 4K 32'' Asus ProArt monitor and didn't notice any difference between my M2 Pro and my M4 Pro (on Sequoia). I will admit my eyesight is not the best anymore but I think I would notice given I'm a bit allergic to blurry monitors. Anyway I will run the diagnostic commands and see what I get.
- ProllyInfamous 6mo ago
  I use a 4K 45" Aorus (Gigabyte) on M4 and M2 Pro — also eyesight/age... Both will run at 144Hz, but the M4 will occassionally flicker if you're close to limits on dual-screens. I set to 120Hz and don't notice any difference. ---- My M4 complaint is that I can no longer update my OS with a USB Installer offline. My M3 (and lower) will, using the same Sequoia media.
compounding_it 7mo ago
The ideal work/coding resolutions and sizes for macOS that I would suggest if you are going down this rabbit hole. 24 inch 1080p 24 inch 4k (2x scaling) 27 inch 1440p 27 inch 5k (2x scaling) 32 inch 6k (2x scaling) Other sizes are going to either look bizarre or you’ll have to deal with fractional scaling. Given that 4k is common in 27/32 inches and those are cheap displays these kinds of problems are expected. I have personally refused to accept in the past that 27 inch 4k isn’t as bad as people say and got one myself only to regret buying it. Get the correct size and scaling and your life will be peaceful. I would recommend the same for Linux and Windows too tbh but people who game might be fine with other sizes and resolutions.
- jbellis 7mo ago
  So MacOS supports only a handful of low dpi resolutions and high dpi must be an integer multiple of one of those?
  - wmf 7mo ago
    It doesn't have to be but it's really designed to run at exactly 2x scale.
    - stefanfisk 7mo ago
      What makes you say that? Unless I am mistaken, it’s only the Pro models who run at 2x by default.
- danny8000 7mo ago
  32" 4k display at fraction scaling of 1.5 (150%) is fine for my day-to-day work (Excel, VS Code, Word, Web browsing, Teams etc.). It delivers sharp enough text at an effective resolution of 2560x1440 px. There are many 32" 4k displays that are affordable and good enough for office workers. I work in a brightly lit room, so I find that monitor brightness (over 350 nits) is the most important monitor feature for me, over text sharpness, color accuracy, or refresh rate.
- smcleod 7mo ago
7e 7mo ago
[flagged]
- faangguyindia 7mo ago
  what's a codeslave? I couldn't get it from the username.
jiveturkey 7mo ago
TFA doesn't say -- does anyone know if this applies to 5k and 6k monitors? On my 5k display on a M4 Max, I see the default resolution in system settings is 2560x1440. Which is what I'd expect. If the theory about framebuffer pre-allocation strategy is to hold any water, I would think that 5k and 6k devices would suffer too, maybe even more. Given that you can attach 2x 5k monitors, the pre-allocation strategy as described would need to account for that.
- smcleod 7mo ago
  I believe it will, it won't be until you push up to an 8k display that you'll get the old level of scaling back (could be wrong though as I don't have a way to test this).
lovegrenoble 7mo ago
They do this on purpose ...
spoaceman7777 7mo ago
Yep. Apple sells 5k displays, which work fine. Just another case of Apple intentionally going against established open standards to price gouge their users. I wouldn't mind it as much if I didn't have to hear said users constantly moaning in ecstasy about just how much better "Apple's way" is. High quality desktop Linux has been made real by KDE, and the AI-fueled FOSS development boom is accelerating this eclipse of proprietary nonsense like this. If you're a developer, you should be using a system that isn't maintained by a company that intentionally stabs developers in the back at every turn. (Unless you're into that. U do u.)
jval43 7mo ago
Not again! Had these issues with 2016 Macbook Pro (the touchbar one). That one also wasn't a hardware limitation as it ran my displays just fine in bootcamp, but macOS would just produce fuzzy output all the way. It's infuriating.
tgma 7mo ago
This might be a dumb question: Is the author looking to run 4k display at HiDPI 8k framebuffer and then downscale? What's the advantage of doing so versus direct 4k low-DPI? Some sort of "free" antialiasing?
- mono442 7mo ago
  That's exactly what he is trying to do. It's really just because MacOS has bad text rendering when graphics aren't rendered at 2x scale.
- LarsAlereon 7mo ago
  From what I understand, the main goal is to fix the problem that non-native (1:1 pixel mapping) resolutions and scaling look worse than native. This is a problem when you ship high-dpi displays that need UI scaling in order for things to be readable. Apple's solution was to render everything at a higher, non-native resolution so that images were always downscaled to fit the display. So to oversimplify, Windows can have a problem where if you are running 1.5X scaling so text is big enough, you can't fit 4K of native pixels on a 4K display so videos are blurry. If instead you were rendering a scaled image to a 6K framebuffer and then downscaling to 4K, there would be minimal loss of resolution.
  - tgma 7mo ago
    > From what I understand, the main goal is to fix the problem that non-native (1:1 pixel mapping) resolutions and scaling look worse than native. That would be my instinct as well, but the author seems to be delibarately doing the exact opposite. Trying to force a 2x HiDPI and then downscaling to native display resolution whereas he could have just done a 1:1 LoDPI rendering. What you get in the end is some equivalent of hack/brute-force smoothing/antialiasing of what was rendered in the downsample.
    - adrian_b 7mo ago
      The author said that the problem is that Apple has introduced a size limit for the display (3360x1890) that is lower than the size of the actual display, which is a standard 4k display (3840x2160). So 1:1 rendering can cover only a part of the screen, while the remainder remains unused. If the maximum size limit is used but applied to the entire screen, it does not match the native resolution so interpolation is used to convert between images with different resolutions, blurring the on-screen image. All the attempts were done with the hope that there is some way to convince the system to somehow use the greater native image size instead of the smaller size forced by the limits.
deleted 7mo ago
[deleted]
Someone 7mo ago
> This aligns with our findings. The M4/M5 DCP firmware implements a conservative framebuffer pre-allocation strategy that: > Caps the HiDPI backing store to approximately 1.75x the native resolution (6720x3780 for 3840x2160 native), rather than the 2.0x needed for full HiDPI (7680x4320) So, that could be an off by one bug? That might be testable by tweaking the system to think the display supports an even higher resolution. Also, instead of messing with the Display Override Plist, patching drivers, etc, did they try using the “Advanced…” button in the “Displays” UI? They don’t mention they did. For me (with a 27 inch 4K monitor not on M4 or M5) that replaces the 5-way choice by one with a list of 11 choices. With the then appearing “Show all resolutions” toggle, that becomes 18.
- smcleod 7mo ago
  I tried quite a few things in the "Advanced" section of BetterDisplay - is there something specifically you're suggesting?
  - Someone 7mo ago
    I’m not talking of BetterDisplay. I’m talking about Apple’s UI. Scroll down the content of the “Displays” pane, and you find a row of buttons “Advanced…”, “Night Shift…” and “?”
    - mmcnl 7mo ago
      Ofcourse this has been tried, the highest possible resolution with HiDPI is limited to what the article says.
tmsh 7mo ago
You might need a higher quality usb cable. I run ok with LG 5k display and MacBook Pro m4 max.
- tonyedgecombe 7mo ago
  My LG 5K needs a Thunderbolt cable. With USB-C it will only run at 4K.
  - tmsh 6mo ago
    Good to know. Thanks!
- smcleod 7mo ago
  5k might be a different story, are you actually running it at 5120p with 2.0 scale? You can get that from running `system_profiler SPDisplaysDataType`, if you are running HiDPI with 5k that'd show something like Resolution: "10240 x 5760", UI Looks like "5120 x 2880"
  - tmsh 6mo ago
    Interesting yeah. Thanks for posting I see: system_profiler SPDisplaysDataType Graphics/Displays: Apple M4 Max: Chipset Model: Apple M4 Max Type: GPU Bus: Built-In Total Number of Cores: 40 Vendor: Apple (0x106b) Metal Support: Metal 4 Displays: LG UltraFine: Resolution: 4096 x 2304 UI Looks like: 2048 x 1152 @ 60.00Hz Main Display: Yes Mirror: Off Online: Yes Rotation: Supported Automatically Adjust Brightness: Yes To be honest it feels crisp. But good to know maybe I need to upgrade more.
    - tmsh 6mo ago
      [dead]
    - smcleod 6mo ago
      It might be crisp but everything in the UI will be massive leaving you little space to work with.
deleted 7mo ago
[deleted]
ralferoo 7mo ago
Unlike the article, I'd assume it's hardware related rather than software. Assuming the article is correct and the hardware can do 7680x4320 @60, which requires 8GB/s memory bandwidth, in theory it should be able to do the same to read the same memory and interleave every other line for the down-sampling. However, it's possible that the new memory controller can't support 2 simultaneous burst streams (because the 2 lines are 30KB apart in memory), or if it's doing a single burst and buffering the first line until the second line is available, then maybe the cache is smaller than 30KB. Another possibility is that previously the scale averaged pairs of pixel horizontally and cached them until the next line was available to average with that, and for some reason it was changed to average all 4 at the same time and so the cache isn't sufficient (although it'd be weird as 25.25KB is a fairly weird size to limit the cache to) Alternatively, looking at clock rates needed for the sampler, 3360x1890 @60 is 381MHz, 3840x2160 @60 is 497MHz. It's quite possible that they've lowered the base clock on some hardware and not considered that it'd impact the maximum effect on the scaler. But whatever IMHO, it's unlikely to be a software bug with an easy fix.
- smcleod 7mo ago
  The 4 values in MaxSrcRectWidthForPipe are sub-pipes within each display controller, not separate display outputs. Every external display controller on the M5 Max has the same pattern: sub-pipe 0 = 6720, sub-pipes 1-3 = 7680. A single-stream 4K display only uses sub-pipe 0. Sub-pipes 1-3 are for multi-pipe configurations (8K displays use 2 sub-pipes, which is why an 8K EDID causes the DCP to assign PipeIDs=(0,2) with MaxPipes=2). Your scaler clock theory could be right: the single-stream scaler path may genuinely have a lower throughput limit than the multi-pipe path. Interestingly, the M2 Max uses a completely different IOMFBMaxSrcPixels structure. Instead of per-sub-pipe arrays, it has a flat MaxSrcRectWidth=7680 and MaxSrcRectTotal=33177600 (exactly 7680x4320) per controller. Every external display controller gets the full 7680 budget. Apple seems to have restructured the display controller architecture between M2 and M4/M5 to per-sub-pipe budgets, and the single-stream sub-pipe got a reduced allocation (6720 vs 7680) in the process. Whether that's a hardware change in the scaler or a firmware allocation policy is hard to say without Apple's documentation.
neurostimulant 7mo ago
Maybe they shouldn't get rid of subpixel antialiasing. At least this problem wouldn't be as painful if we still have that feature.
sda2 7mo ago
Could one emulate an 8K display with software and mirror the result to the external display? I feel like some remote desktop software is already doing that sort of thing.
- mmcnl 7mo ago
  Yes, BetterDisplay offers this functionality and it kind of works, with some drawbacks.
rdtsc 7mo ago
This is why I hesitate updating my laptop. It's a 7 years old intel but my 2 external 4K displays work nicely and I like my setup. Upgrading looks like could mess them up. Doesn't matter how much faster compiling and testing will be, if I have to buy new monitors it's not worth it.
rlam2x51 6mo ago
I recently bought my first MacBook Pro (M5 Pro) and I kinda regret it. It is a nice notebook but I would never bought it if I knew that it does not work properly with my LG 38WN95C-W Monitor (3840x1600@144Hz via USB-C). BetterDisplay allows me to use 3360x1400 with HiDPI but I still lose screen estate that I got use to. The worst part is that I did my research if my monitor works nicely with Apple Silicon and got confirmation that this is the case. I would never expected that M5 would perform worse compared to previous generations. Apple really does a lot of things right but then they mess up the basic. Guess I will ask Apple Intelligence for advise how to explain to my wife that I need a new Monitor…
- smcleod 6mo ago
  If you're feeling brave, I've hacked up a way to make proper 4K scaling work, I only have a 60hz screen so it will need some modifications for 120hz but - https://github.com/sammcj/force-hidpi https://github.com/sammcj/force-hidpi
  - rlam2x51 6mo ago
    Impressive, that's quite a feat. It is great that you found a workaround - it gives me hope that Apple will eventually fix this issue. I will hold out for a bit longer an wait for apple to get their act together. I am still in disbelieve about the issue. I thought I have bought the most advanced notebook on this planet just to find out that it does not support external displays?

313 points by smcleod 7mo ago

[flagged]

Props to the author for putting in what looks like ton of work trying to navigate this issue, shame they have to go to these lengths to even have their case considered.

MarcelOlsz 7mo ago
I went to hell and back trying to get PIP/PBP monitors on my 57" g9 ultrawide to work with my M2 pro. ended up having to use a powered hdmi dongle, displaylink cable, and displayport, with 3 virtual monitors via betterdisplay. Allowing resolutions outside of macs limitations setting in BD is what did the trick. I don't envy OP. Having 5120x1440 @ lodpi was the worst, just ever so slightly too fuzzy but perfect UI size but eventually got a steady 10240x2880 @ 120hz with HDR. I literally laughed out loud when I read the title of the thread. Poor guy.
- moeadham 7mo ago
  betterdisplay is a life saver
- jakehilborn 7mo ago
  You may be able to get this working using PBP and 2 cables without virtual displays. This is my write up for using HiDPI@120hz for two 57” G9s on my M2 MacBook. https://www.reddit.com/r/ultrawidemasterrace/s/VrBLFDxYzg https://www.reddit.com/r/ultrawidemasterrace/s/VrBLFDxYzg
  - MarcelOlsz 7mo ago
    Ah but you see, the challenge is to get a 3-split PBP on an M2 pro on a monitor with a native res of 7680x2160, each one scaled down 33%, working at 120hz with HDR, all hidpi like so: ┌─┐┌────┐┌─┐ │ ││ ││ │ └─┘└────┘└─┘ It creates some wonky math and requires plenty of dock and cable shenanigans and unlocking resolutions above 8k via BD. It's the third "monitor" where it gets tricky with the M2 pro especially at these resolutions.
    - MarcelOlsz 7mo ago
      ...And then there is the near-infinite trickle down of apps that rely on apps that rely on arcane configs and so on. This is truly the OS from hell. At least with Windows you know it's going to be garbage so when anything works on any level you are maximally impressed. But I have to spend my weekends isolating window shadow disabling functionality from yabai into it's own binary because I switched to aerospace which requires 'displays have separate spaces' to off, which just so happens to be exactly what yabai requires to be on, to remove window shadows, which is the only use I have left for it. Just like the excel world championship I would find a macos ricing/window tiling competition equally enthralling. You read articles like the OP and at some point all you can do is laugh because lord (Cook) knows you've cried.
    - fragmede 7mo ago
      Fascinating. What's that gain you over using the monitor's native resolution full screen vs PBP mode?
      - MarcelOlsz 6mo ago
        I hate spending any unnecessary clicks or keyboard shortcuts on getting whats out of my head into the computer. I used yabai before primarily, now using aerospace. Since the monitor is super ultra-wide (57 inches with a very high DPI) the native resolution makes everything ultra small to my eyes. It's the same height as my 34-inch Samsung G5s which are 1440 pixels tall natively, but since this one is 2160, it would have to be 1.5 times larger physically to look decent at native res especially on macOS. The only other option is to scale the UI 1.5x which is where all the problems begin. I like the three-column separate monitor layout because I have hotkeys, primarily driven by my mouse but also usable keyboard-only where I can easily switch between monitors with `⌘+`` which moves my cursor between them. I can select whichever monitor I want and put my mouse to it, and I can switch to any workspace on any monitor quickly. I also have hotkeys that sync three workspace numbers across monitors, so switching between them switches all AeroSpace workspaces on all three monitors simultaneously. If I have five projects going, I'd have the terminal on the left, Linear and other communication tools on the right in accordion mode with AeroSpace, and I can use my mouse or keyboard exclusively to find exactly what I'm looking for almost as fast as I think of it. I spend zero time on window management or organization now so it makes it thoughtless to use. If I'm just using the monitor's native resolution there's no real way to do portals — having two apps open as sticky and only switching a portion of the monitor space to a different app while keeping the other sticky. There are hacks you can do with AeroSpace, especially since AeroSpace doesn't use native macOS Spaces, but the three-monitor layout is a much more robust approach in my opinion just a bit of a nightmare to setup. Theres a million little mac annoyances you have to fix.
smcleod 7mo ago
Thanks, it was a good portion of my weekend bashing my head against the keyboard trying to figure out what was going on and if there was a workaround I could use (there isn't that I've found).
dostick 7mo ago
The post reminded me how I investigated a similar issue having no idea. Using Claude or GPT to investigate this kind of hardware issue is fast and easy. It gives you next command to try and then next one and you end up with similar summary. I wouldn’t be surprised that author didn’t know anything about displays before this.

skullone 7mo ago

I thought I was going crazy when my new m4 seemed "fuzzier" on my external 4ks. I tried replicating settings from my old MacBook to no avail. I wonder if Apple is doing this on purpose except for their own displays.

NBJack 7mo ago
It's a bit nit-picky on my part, but this bizarre world of MacOS resolution/scaling handling vs. other operating systems (including Windows 11 for crying out loud) is one of my biggest gripes with using Apple hardware. I remember having to work hard to make my non-Apple display look 'right' years ago on an Intel-based mac due to weirdness with scaling and resolutions that a Windows laptop didn't even flinch handling. It was a mix of hardware limitations and the lack of options I had to address the resolution and refresh rates I had available over a Thunderbolt doc that I shouldn't have to think about. I honestly hope they finally fix this. I would love it if they allowed sub-pixel text rendering options again too.
- Sir_Twist 7mo ago
  https://news.ycombinator.com/item?id=17477526 https://news.ycombinator.com/item?id=17477526 This reminds me of this comment, which I feel is a somewhat unsatisfying explanation, given that despite these difficulties, Windows somehow makes it work.
paozac 7mo ago
Same here, I tried all combinations with BetterDisplay but kept getting fuzzy fonts. It was very frustrating on a large 5k2k display
semi-extrinsic 7mo ago
I don't understand what is fuzzy about "lodpi". I've been using it for 8 years on a 4k 43" screen with 1x scaling. Can't say I noticed any difference when switching several times per day between Linux and an M1 MBP, nor any difference when upgrading to an M4 MBP.
- 7mo ago

bsimpson 7mo ago

Wouldn't HiDPI be 1080p@2x? Is that still available?

TheCoreh 7mo ago
Yeah if I understand it correctly, this is more like 2160p@2x which is... unusual?
- TheTon 7mo ago
  Yes, I would actually be surprised to learn that mode is available on any system. I’ve never seen that anywhere, though I only have a M1 Pro and an M4 Pro (and various Intel Macs). You’re rendering to a framebuffer exactly 2x the size of your display and then scaling it down by exactly half to the physical display? Why not just use a 1x mode then!? The 1.75x limit of framebuffer to physical screen size makes perfect sense. Any more than that and you should just use the 1x mode, it will look better and perform way better!
  - wpm 7mo ago
    Because 1x mode has no subpixel antialiasing and thus looks absolutely terrible. I have a 32:9 Ultrawide I would love to use on macOS but the text looks awful on it.
    - TheTon 7mo ago
      Then complain about that. That would make a much more sensible blog post and discussion. Asking for a crazy workaround to a sane problem isn't a great way to get good results, especially with Apple. Beyond the obvious performance pitfall, this scale up to scale down approach will also destroy the appearance of some controls. There is some UI that aims for 1px lines on hidpi modes that will get lost if you do this. It's hardly a perfect mode.
      - wpm 7mo ago
        The crazy workaround only needs to be done because of what Apple did probably around a decade ago and probably already heard a bunch of crying about and didn't care. No one removed subpixel antialiasing on their own, we do this bullshit because Apple forced us to to make text look halfway decent.

wmf 7mo ago

This is not a normal retina configuration. This is a highly unusual configuration where the framebuffer is much larger than the screen resolution and gets scaled down. Obviously it sucks if it used to work and now it doesn't but almost no one wants this which probably explains why Apple doesn't care.

sgerenser 7mo ago
I don’t know why this was downvoted, I agree that this is a highly unusual configuration. Why render to a frame buffer with 2x the pixels in each direction va the actual display, only to then just scale the whole thing down by 2x in each direction?
- eptcyka 7mo ago
  Because Apple no longer implements subpixel rendering for fonts?
  - Rohansi 7mo ago
    Supersampling the entire framebuffer is a bad way to anti-alias fonts. Especially since your font rendering is almost certainly doing grayscale anti-aliasing already, which is going to look better than 2x supersampling alone. And supersampling will not do subpixel rendering.
- mlyle 7mo ago
  Because it's a decent way to get oversampling.
phonon 7mo ago
Isn't that just 2x supersampling? If you want "perfect" antialiasing that's the minimum you need, no?
- wmf 7mo ago
  Yes, it is supersampling but historically almost no one runs that way.

nuker 7mo ago

Send it to Tim Cook email. It worked for me fixing DisplayPort DSC bug. After Catalina, later MacOSes lost ability to drive monitors at higher than 60Hz refresh. Apple support tortured me with all kinds of diagnostics, with WontFix few weeks later. Wrote email and it got fixed in Sonoma :) https://egpu.io/forums/mac-setup/4k144hz-no-longer-available-after-upgrade-to-big-sur/paged/10/ https://egpu.io/forums/mac-setup/4k144hz-no-longer-available...

nerdsniper 7mo ago
Thank you
FireBeyond 7mo ago
No, it didn’t get fully fixed. Fucking with DP 1.4 was how they managed to drive the ProDisplay XDR. If your monitor could downgrade to DP 1.2 you got better refresh rates than 1.4 (mine could do 95Hz SDR, 60Hz HDR, but if my monitor said it could only do 1.2, that went to 120/95 on Big Sur and above, when they could do 144Hz HDR with Catalina). I would be absolutely unsurprised if their fix was to lie to the monitor in negotiation if it was non-Apple and say that the GPU only supported 1.2, and further, I would be also unsurprised to learn that this is related to the current issue.
- nuker 7mo ago
  Ahh, true, I now have 120Hz top, but it's fine, why I said fixed :) I now recall in Catalina I had full 144Hz and VRR options! Monitor is Dell G3223Q via Caldigit TS4 DP.
  - FireBeyond 7mo ago
    I was using 2 27" LG 27GM950-Bs (IIRC), that could do up to 165Hz and VRR on a 2019 cheesegrater Mac Pro, wasn't the cables, or the monitors, or the card. People at the time were trying to figure out the math of "How did Apple manage to make 6K HDR work over that bandwidth?" and the answer was simply "by completely fucking the DP 1.4 DSC spec" (it was broken in Big Sur, which was released at the same time). The ProDisplay XDR worked great (for added irony, I ended up with one about a year later), but at the cost of Apple saying "we don't care how much money you've spent on your display hardware if you didn't spend it with us" (which tracks perfectly with, I think, Craig Federighi spending so much time and effort shooting down iMessage on Android and RCS for a long time saying, quote, "It would remove obstacles towards iPhone families being able to give their kids Android phones").

PedroBatista 7mo ago

Now I know I was not crazy and the "cheap" 4K screen I bought a couple months ago doesn't actually suck. Tim Apple's Apple has been fu#$%& me again..

whatever1 7mo ago

How did none of the Apple devs notice this? 4k 32" inch is the industry standard for HiDPI monitors.

MBCook 7mo ago
Apple doesn’t make an 4k external monitor. They’re likely all on Studio Displays.
- cosmic_cheese 7mo ago
  And prior to Apple’s re-entry into the display market, everybody internally was likely on 2x HiDPI LG UltraFine displays or integrated displays on iMacs and MacBooks. Fractional scaling (and lately, even 1x scaling “normal”) displays really are not much of a consideration for them, even if they’re popular. 2x+ integer scaling HiDPI is the main target.
robertoandred 7mo ago
Don’t think I’d call 4K at 32” high dpi.
- whatever1 7mo ago
  I agree with you. It's my IT who disagrees. But to be fair, until last year there were no retina monitors in the market except the Apple ones. In 2025, the tides turned, there are now way more options both for 5k and 6k retina displays.
Gigachad 7mo ago
Tbh I'm not even sure what the issue is here. I have a personal M1 macbook and a work M4 and a 4k display. I don't see any issues or differences between them on my display. The M4 seems to be outputting a 4k image just fine. The article could just be AI slop since it just contains hyper in depth debugging without articulating what the problem is.
- whatever1 7mo ago

comex 7mo ago

Well, it sounds like a real issue, but the diagnosis is AI slop. You can see, for example, how it takes the paragraph quoted from waydabber (attributing the issue to dynamic resource allocation) and expands it into a whole section without really understanding it. The section is in fact self-contradictory: it first claims that the DCP firmware implements framebuffer allocation, then almost immediately goes on to say it's actually the GPU driver and "the DCP itself is not the bottleneck". Similar confusion throughout the rest of the post.

xbar 7mo ago
I think you are probably right--it's a real problem. As an article, it is not 100% coherent, but there is a valid data and a real problem that is clear.
Aurornis 7mo ago
Agree. I started reading the article until I realized it wasn’t even self-coherent. Then I got to the classic two-column table setup and realized I was just reading straight LLM output. There might be a problem but it’s hard to know what to trust with these LLM generated reports. I might be jaded from reading one too many Claude-generated GitHub issues that look exactly like this that turned out to be something else.
- smcleod 7mo ago
  Parts of it were pasted from my Claude Code Logs, Parts were written by me - and the table - that was me!

deleted 7mo ago

[deleted]

mil22 7mo ago

This would be even more compelling if you included screenshots with magnified detail insets showing the text blur.

smcleod 7mo ago
Thanks for the feedback, I'll try to take some photos, it's not an easy thing to do accurately without a good camera setup, but I'll reply here after work if I get something setup and added to the post.

pier25 7mo ago

Is this for specific verisons of macOS? The article doesn't mention it.

smcleod 7mo ago
It's in the environment and test setup (26.4 at the moment, but it is the same across all 26.x releases I've tried).
- pier25 7mo ago
  I'm still on Sequoia and haven't noticed these issues on my M4 Pro.
  - smcleod 7mo ago
    Out of interest - whats the output of `system_profiler SPDisplaysDataType`?
    - pier25 7mo ago
      You can see the result here: https://gist.github.com/PierBover/99f44e5cba1657eb7e4ce8e2aabbcdcf https://gist.github.com/PierBover/99f44e5cba1657eb7e4ce8e2aa... I'm on macOS 15.7.4. It's so weird that it says 5120 x 2880 when the ASUS PA329CV is 3840 x 2160.
      - smcleod 7mo ago
        That's a 404, but if it only says 5120x2880 then you're not running full 4K HiDPI which would show 7680x as per the blog post.

arjie 7mo ago

I'm sure you've already given this a crack via some other technique (I just Cmd-F for it and didn't find) but I have had monitors with confusing EDIDs before that MacOS didn't handle well and the "screenresolution" CLI app https://github.com/jhford/screenresolution https://github.com/jhford/screenresolution always let me set an arbitrary one. It was the only way to get some monitors to display at 100 Hz for me and worked very well for that since the resolution is mostly sticky.

smcleod 7mo ago
Hey, thanks I hadn't tried screenresolution but that seems to simply set the resolution and refresh rate without controlling the scaling which is what is needed for configuring the HiDPI mode scaling.

LuxBennu 7mo ago

Sadly I have the issue on a new m5 air. I have a 60hz 4k work monitor and two high refresh 4k gaming displays. The 60hz pairs fine with either gaming monitor, but the two gaming ones together and one just doesn't get recognized. Spent way too long trying new cables before realizing it's a bandwidth limitation.

chaostheory 7mo ago

What are they doing with MacOS? Is this due to VisionOS?

keyle 7mo ago

This is the sort of Apple gotchas that really upset me. They've got a good thing going, but they keep finding ways to alienate people.

compounding_it 7mo ago
Their suggestion : get an Apple monitor that we just launched.
- saagarjha 7mo ago
  I don't think that one supports this use.

pier25 7mo ago

I use a 4K 32'' Asus ProArt monitor and didn't notice any difference between my M2 Pro and my M4 Pro (on Sequoia). I will admit my eyesight is not the best anymore but I think I would notice given I'm a bit allergic to blurry monitors. Anyway I will run the diagnostic commands and see what I get.

ProllyInfamous 6mo ago
I use a 4K 45" Aorus (Gigabyte) on M4 and M2 Pro — also eyesight/age... Both will run at 144Hz, but the M4 will occassionally flicker if you're close to limits on dual-screens. I set to 120Hz and don't notice any difference. ---- My M4 complaint is that I can no longer update my OS with a USB Installer offline. My M3 (and lower) will, using the same Sequoia media.

compounding_it 7mo ago

The ideal work/coding resolutions and sizes for macOS that I would suggest if you are going down this rabbit hole. 24 inch 1080p 24 inch 4k (2x scaling) 27 inch 1440p 27 inch 5k (2x scaling) 32 inch 6k (2x scaling) Other sizes are going to either look bizarre or you’ll have to deal with fractional scaling. Given that 4k is common in 27/32 inches and those are cheap displays these kinds of problems are expected. I have personally refused to accept in the past that 27 inch 4k isn’t as bad as people say and got one myself only to regret buying it. Get the correct size and scaling and your life will be peaceful. I would recommend the same for Linux and Windows too tbh but people who game might be fine with other sizes and resolutions.

jbellis 7mo ago
So MacOS supports only a handful of low dpi resolutions and high dpi must be an integer multiple of one of those?
- wmf 7mo ago
  It doesn't have to be but it's really designed to run at exactly 2x scale.
  - stefanfisk 7mo ago
    What makes you say that? Unless I am mistaken, it’s only the Pro models who run at 2x by default.
danny8000 7mo ago
32" 4k display at fraction scaling of 1.5 (150%) is fine for my day-to-day work (Excel, VS Code, Word, Web browsing, Teams etc.). It delivers sharp enough text at an effective resolution of 2560x1440 px. There are many 32" 4k displays that are affordable and good enough for office workers. I work in a brightly lit room, so I find that monitor brightness (over 350 nits) is the most important monitor feature for me, over text sharpness, color accuracy, or refresh rate.
smcleod 7mo ago

7e 7mo ago

[flagged]

faangguyindia 7mo ago
what's a codeslave? I couldn't get it from the username.

jiveturkey 7mo ago

TFA doesn't say -- does anyone know if this applies to 5k and 6k monitors? On my 5k display on a M4 Max, I see the default resolution in system settings is 2560x1440. Which is what I'd expect. If the theory about framebuffer pre-allocation strategy is to hold any water, I would think that 5k and 6k devices would suffer too, maybe even more. Given that you can attach 2x 5k monitors, the pre-allocation strategy as described would need to account for that.

smcleod 7mo ago
I believe it will, it won't be until you push up to an 8k display that you'll get the old level of scaling back (could be wrong though as I don't have a way to test this).

lovegrenoble 7mo ago

They do this on purpose ...

spoaceman7777 7mo ago

Yep. Apple sells 5k displays, which work fine. Just another case of Apple intentionally going against established open standards to price gouge their users. I wouldn't mind it as much if I didn't have to hear said users constantly moaning in ecstasy about just how much better "Apple's way" is. High quality desktop Linux has been made real by KDE, and the AI-fueled FOSS development boom is accelerating this eclipse of proprietary nonsense like this. If you're a developer, you should be using a system that isn't maintained by a company that intentionally stabs developers in the back at every turn. (Unless you're into that. U do u.)

jval43 7mo ago

Not again! Had these issues with 2016 Macbook Pro (the touchbar one). That one also wasn't a hardware limitation as it ran my displays just fine in bootcamp, but macOS would just produce fuzzy output all the way. It's infuriating.

tgma 7mo ago

This might be a dumb question: Is the author looking to run 4k display at HiDPI 8k framebuffer and then downscale? What's the advantage of doing so versus direct 4k low-DPI? Some sort of "free" antialiasing?

mono442 7mo ago
That's exactly what he is trying to do. It's really just because MacOS has bad text rendering when graphics aren't rendered at 2x scale.
LarsAlereon 7mo ago
From what I understand, the main goal is to fix the problem that non-native (1:1 pixel mapping) resolutions and scaling look worse than native. This is a problem when you ship high-dpi displays that need UI scaling in order for things to be readable. Apple's solution was to render everything at a higher, non-native resolution so that images were always downscaled to fit the display. So to oversimplify, Windows can have a problem where if you are running 1.5X scaling so text is big enough, you can't fit 4K of native pixels on a 4K display so videos are blurry. If instead you were rendering a scaled image to a 6K framebuffer and then downscaling to 4K, there would be minimal loss of resolution.
- tgma 7mo ago
  > From what I understand, the main goal is to fix the problem that non-native (1:1 pixel mapping) resolutions and scaling look worse than native. That would be my instinct as well, but the author seems to be delibarately doing the exact opposite. Trying to force a 2x HiDPI and then downscaling to native display resolution whereas he could have just done a 1:1 LoDPI rendering. What you get in the end is some equivalent of hack/brute-force smoothing/antialiasing of what was rendered in the downsample.
  - adrian_b 7mo ago
    The author said that the problem is that Apple has introduced a size limit for the display (3360x1890) that is lower than the size of the actual display, which is a standard 4k display (3840x2160). So 1:1 rendering can cover only a part of the screen, while the remainder remains unused. If the maximum size limit is used but applied to the entire screen, it does not match the native resolution so interpolation is used to convert between images with different resolutions, blurring the on-screen image. All the attempts were done with the hope that there is some way to convince the system to somehow use the greater native image size instead of the smaller size forced by the limits.

deleted 7mo ago

[deleted]

Someone 7mo ago

> This aligns with our findings. The M4/M5 DCP firmware implements a conservative framebuffer pre-allocation strategy that: > Caps the HiDPI backing store to approximately 1.75x the native resolution (6720x3780 for 3840x2160 native), rather than the 2.0x needed for full HiDPI (7680x4320) So, that could be an off by one bug? That might be testable by tweaking the system to think the display supports an even higher resolution. Also, instead of messing with the Display Override Plist, patching drivers, etc, did they try using the “Advanced…” button in the “Displays” UI? They don’t mention they did. For me (with a 27 inch 4K monitor not on M4 or M5) that replaces the 5-way choice by one with a list of 11 choices. With the then appearing “Show all resolutions” toggle, that becomes 18.

smcleod 7mo ago
I tried quite a few things in the "Advanced" section of BetterDisplay - is there something specifically you're suggesting?
- Someone 7mo ago
  I’m not talking of BetterDisplay. I’m talking about Apple’s UI. Scroll down the content of the “Displays” pane, and you find a row of buttons “Advanced…”, “Night Shift…” and “?”
  - mmcnl 7mo ago
    Ofcourse this has been tried, the highest possible resolution with HiDPI is limited to what the article says.

tmsh 7mo ago

You might need a higher quality usb cable. I run ok with LG 5k display and MacBook Pro m4 max.

tonyedgecombe 7mo ago
My LG 5K needs a Thunderbolt cable. With USB-C it will only run at 4K.
- tmsh 6mo ago
  Good to know. Thanks!
smcleod 7mo ago
5k might be a different story, are you actually running it at 5120p with 2.0 scale? You can get that from running `system_profiler SPDisplaysDataType`, if you are running HiDPI with 5k that'd show something like Resolution: "10240 x 5760", UI Looks like "5120 x 2880"
- tmsh 6mo ago
  Interesting yeah. Thanks for posting I see: system_profiler SPDisplaysDataType Graphics/Displays: Apple M4 Max: Chipset Model: Apple M4 Max Type: GPU Bus: Built-In Total Number of Cores: 40 Vendor: Apple (0x106b) Metal Support: Metal 4 Displays: LG UltraFine: Resolution: 4096 x 2304 UI Looks like: 2048 x 1152 @ 60.00Hz Main Display: Yes Mirror: Off Online: Yes Rotation: Supported Automatically Adjust Brightness: Yes To be honest it feels crisp. But good to know maybe I need to upgrade more.
  - tmsh 6mo ago
    [dead]
  - smcleod 6mo ago
    It might be crisp but everything in the UI will be massive leaving you little space to work with.

deleted 7mo ago

[deleted]

ralferoo 7mo ago

Unlike the article, I'd assume it's hardware related rather than software. Assuming the article is correct and the hardware can do 7680x4320 @60, which requires 8GB/s memory bandwidth, in theory it should be able to do the same to read the same memory and interleave every other line for the down-sampling. However, it's possible that the new memory controller can't support 2 simultaneous burst streams (because the 2 lines are 30KB apart in memory), or if it's doing a single burst and buffering the first line until the second line is available, then maybe the cache is smaller than 30KB. Another possibility is that previously the scale averaged pairs of pixel horizontally and cached them until the next line was available to average with that, and for some reason it was changed to average all 4 at the same time and so the cache isn't sufficient (although it'd be weird as 25.25KB is a fairly weird size to limit the cache to) Alternatively, looking at clock rates needed for the sampler, 3360x1890 @60 is 381MHz, 3840x2160 @60 is 497MHz. It's quite possible that they've lowered the base clock on some hardware and not considered that it'd impact the maximum effect on the scaler. But whatever IMHO, it's unlikely to be a software bug with an easy fix.

smcleod 7mo ago
The 4 values in MaxSrcRectWidthForPipe are sub-pipes within each display controller, not separate display outputs. Every external display controller on the M5 Max has the same pattern: sub-pipe 0 = 6720, sub-pipes 1-3 = 7680. A single-stream 4K display only uses sub-pipe 0. Sub-pipes 1-3 are for multi-pipe configurations (8K displays use 2 sub-pipes, which is why an 8K EDID causes the DCP to assign PipeIDs=(0,2) with MaxPipes=2). Your scaler clock theory could be right: the single-stream scaler path may genuinely have a lower throughput limit than the multi-pipe path. Interestingly, the M2 Max uses a completely different IOMFBMaxSrcPixels structure. Instead of per-sub-pipe arrays, it has a flat MaxSrcRectWidth=7680 and MaxSrcRectTotal=33177600 (exactly 7680x4320) per controller. Every external display controller gets the full 7680 budget. Apple seems to have restructured the display controller architecture between M2 and M4/M5 to per-sub-pipe budgets, and the single-stream sub-pipe got a reduced allocation (6720 vs 7680) in the process. Whether that's a hardware change in the scaler or a firmware allocation policy is hard to say without Apple's documentation.

neurostimulant 7mo ago

Maybe they shouldn't get rid of subpixel antialiasing. At least this problem wouldn't be as painful if we still have that feature.

sda2 7mo ago

Could one emulate an 8K display with software and mirror the result to the external display? I feel like some remote desktop software is already doing that sort of thing.

mmcnl 7mo ago
Yes, BetterDisplay offers this functionality and it kind of works, with some drawbacks.

rdtsc 7mo ago

This is why I hesitate updating my laptop. It's a 7 years old intel but my 2 external 4K displays work nicely and I like my setup. Upgrading looks like could mess them up. Doesn't matter how much faster compiling and testing will be, if I have to buy new monitors it's not worth it.

rlam2x51 6mo ago

I recently bought my first MacBook Pro (M5 Pro) and I kinda regret it. It is a nice notebook but I would never bought it if I knew that it does not work properly with my LG 38WN95C-W Monitor (3840x1600@144Hz via USB-C). BetterDisplay allows me to use 3360x1400 with HiDPI but I still lose screen estate that I got use to. The worst part is that I did my research if my monitor works nicely with Apple Silicon and got confirmation that this is the case. I would never expected that M5 would perform worse compared to previous generations. Apple really does a lot of things right but then they mess up the basic. Guess I will ask Apple Intelligence for advise how to explain to my wife that I need a new Monitor…

smcleod 6mo ago
If you're feeling brave, I've hacked up a way to make proper 4K scaling work, I only have a 60hz screen so it will need some modifications for 120hz but - https://github.com/sammcj/force-hidpi https://github.com/sammcj/force-hidpi
- rlam2x51 6mo ago
  Impressive, that's quite a feat. It is great that you found a workaround - it gives me hope that Apple will eventually fix this issue. I will hold out for a bit longer an wait for apple to get their act together. I am still in disbelieve about the issue. I thought I have bought the most advanced notebook on this planet just to find out that it does not support external displays?