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

Original Article ↗ Hacker News Discussion ↗

Apple’s latest M4 and M5 Macs appear to impose new limits on HiDPI rendering for 4K external monitors, breaking previously possible “supersampled” modes that rendered at 8K and downscaled to 4K for sharper text. Users report noticeably fuzzier fonts and reduced scaling options compared to earlier Apple Silicon, especially when relying on tools like BetterDisplay to force high-quality HiDPI on non-Apple displays. Opinions are split on whether this is an intentional hardware/firmware tradeoff, a regression Apple doesn’t prioritize because it mainly affects third‑party monitors, or a bug that should be escalated via formal feedback to Apple.

Nature of the issue

  • Discussion centers on new M4/M5 Macs limiting HiDPI framebuffer size on 4K external displays.
  • Previously, some users rendered at 8K (e.g., 7680×4320) and downscaled to 4K to improve text quality; on M4/M5 this appears capped to ~1.75× rather than 2×, breaking that setup.
  • The regression mainly affects users forcing “HiDPI at native 4K” via tools, not default macOS scaling.

Why people were doing this

  • macOS text at 1× (“LoDPI”) is widely described as fuzzy/harsh, especially after Apple removed subpixel antialiasing.
  • Supersampling (rendering at 2× and downscaling) produces noticeably better text, especially on 4K 27–32" and ultrawide monitors.
  • Some consider this a necessary workaround on non-Apple displays; others call it an odd, power-hungry edge case.

User experiences and disagreement

  • Some M4/M5 owners strongly notice blur or fuzziness on 4K/5K2K/dual-4K setups despite experimenting with BetterDisplay.
  • Others report no visible difference between M2 and M4/M5 on 4K screens, or say they can still use 1080p@2× fine.
  • A number of commenters argue 4K at 27–32" is not true HiDPI for macOS; they recommend 5K@27" or 6K@32". Others say 4K@27"/32" is fine on Windows or Linux with 150%/fractional scaling.

Tools, workarounds, and limitations

  • BetterDisplay is widely praised, but several confirm it can no longer force full 4K HiDPI on M5.
  • Some mention alternative tools (e.g., screenresolution) but note they don’t control scaling/HiDPI behavior.
  • Complex setups (PBP/PIP ultrawides, multiple cables, virtual displays) are used to regain acceptable text rendering, often with significant effort.

Technical hypotheses

  • Thread cites parsed driver/firmware structures showing per-“sub-pipe” max source widths (e.g., 6720 vs 7680) and architectural changes from M2 to M4/M5.
  • Speculation includes scaler clock limits, memory bandwidth/cache constraints, and conservative DCP firmware allocation.
  • Some think it’s a hardware/architecture choice; others suspect firmware/software policy that could be revised.

Meta: article quality and Apple response

  • Multiple commenters say the blog post appears partially LLM-generated and internally inconsistent, making diagnosis harder.
  • Suggested effective path: file concise, evidence-based bugs about “poor text on LoDPI displays,” include photographic comparisons, and/or escalate via Apple CEO email.
  • Broader frustration surfaces about Apple’s neglect of third-party displays, past DisplayPort/refresh-rate regressions, and removal of subpixel antialiasing.