macOS menu bar app that forces 3840x2160 HiDPI (scale 2.0) on 4K external displays connected to Apple Silicon M4/M5 Macs. This is a workaround - not the solution which lies with Apple fixing their external monitor support and buggy UI scaling.
Apple Silicon M4/M5 DCP firmware has a hardcoded pipe 0 width budget of 6720 pixels. HiDPI at 3840x2160 requires a 7680x4320 backing store, which exceeds this budget. The maximum HiDPI mode offered is 3360x1890.
This is a firmware regression from M1/M2/M3, where the same display gets 3840x2160 HiDPI without issue.
For the full technical analysis including DCP firmware disassembly, see the blog post.
- Creates a virtual display via CoreGraphics private API (
CGVirtualDisplay) at 3840x2160 points with a 7680x4320 pixel backing store (HiDPI scale 2.0) - Configures the physical 4K display as a hardware mirror of the virtual display
- The DCP's hardware scaler outputs the composited framebuffer at native 3840x2160
The hardware mirror path bypasses the verify_downscaling budget check that blocks direct 7680-wide backing stores on pipe 0.
The default 2x creates a 7680x4320 render buffer for the 3840x2160 logical resolution. Higher scale factors increase the render buffer further for super-sampled output with more screen real estate:
| Scale | Logical | Render buffer | Notes |
|---|---|---|---|
| 2x | 3840x2160 | 7680x4320 | Standard HiDPI (default) |
| 2.25x | 4320x2430 | 8640x4860 | |
| 2.5x | 4800x2700 | 9600x5400 | |
| 3x | 5760x3240 | 11520x6480 | |
| 3.5x | 6720x3780 | 13440x7560 | |
| 4x | 7680x4320 | 15360x8640 |
- 16-bit compositing (default): Uses PQ (ST 2084) EOTF for 16-bit/64bpp compositing with a PQ-to-SDR gamma correction table applied via
CGSetDisplayTransferByTable. Toggleable from the menu bar. - Colour profile matching: Compares ICC profiles between virtual and physical displays. If they differ, copies the physical display's profile to the virtual display via SkyLight API.
- Consistent display identity: Uses the physical panel's vendor/product IDs and a fixed serial number so macOS preserves display arrangement between sessions.
- Hardware scaling: The DCP's hardware scaler handles the downscale, not software.
- Apple Silicon M4/M5 Mac
- External 4K (3840x2160) display
- macOS 26+
- Swift 5.9+ toolchain
make build # release build
make installInstalls the binary to a user-writable directory on PATH if one exists (~/.local/bin, ~/bin), otherwise falls back to /usr/local/bin (prompts for sudo). Registers a LaunchAgent and starts the app.
If a previous version is running, make install stops it and starts the new version automatically.
make uninstall # stop and remove everything
make stop # stop without uninstalling
make start # start after a stopYou can also toggle "Start at Login" from the menu bar dropdown.
make build-debug
.build/debug/ForceHiDPIWhen run from a terminal, diagnostic output (quality info, ICC profiles, PPI) is printed to stdout.
- The process must remain running (it owns the virtual display lifecycle)
- An extra "display" appears in System Settings
- Uses CoreGraphics private API (
CGVirtualDisplay) which could break between macOS versions, though this API is also used by DisplayLink and other hardware vendors - Text is sharper than without HiDPI but may not be identical to native HiDPI on M1/M2/M3
- This is a workaround, not a fix. If you want well-designed, actively maintained display management software, check out BetterDisplay
The M4/M5 DCP firmware contains a hardcoded constant 0x1A40 (6720) for the external display pipe 0 width budget. The runtime check in IOMFB::UPPipe::verify_downscaling() compares the backing store width against MaxVideoSrcDownscalingWidth (6720 for external displays).
The internal display gets MaxVideoSrcDownscalingWidth = 10744, and external sub-pipes 1-3 get 7680. The hardware supports the resolution; the limitation is a single firmware constant on pipe 0.
The hardware mirror path bypasses this check because mirrored displays receive pre-composited frames rather than direct backing store swaps through verify_downscaling.
MIT