Decide HOW LOW to go for a bare-metal 2D/text rendering pipeline on Apple GPUs,
and pay the right costs. Covers the layered choice — pure objc2-metal +
hand-written MSL (own command queue, glyph atlas, frame pacing) vs wgpu (Metal
under the hood, cross-platform) vs Vello-on-wgpu (compute vector renderer you
don't have to write) — plus CAMetalLayer/CADisplayLink frame pacing, ProMotion
120Hz, the glyph-atlas / signed-distance-field text problem, CPU-GPU sync, and
honest cost accounting of "pure Metal" vs standing on Linebender. Activate on:
"objc2-metal", "objc2", "CAMetalLayer", "CADisplayLink", "Metal command queue",
"glyph atlas", "SDF text", "ProMotion frame pacing", "bare-metal Rust
rendering on macOS", "pure Metal vs wgpu", "MTLDrawable". NOT for: writing the
MSL shaders themselves (use metal-shader-expert), the Vello/Parley high-level
API (use vello-parley-rendering), 3D engines (use wgpu/bevy), or iOS UIKit
drawing.
Decide HOW LOW to go for a bare-metal 2D/text rendering pipeline on Apple GPUs,
and pay the right costs. Covers the layered choice — pure objc2-metal +
hand-written MSL (own command queue, glyph atlas, frame pacing) vs wgpu (Metal
under the hood, cross-platform) vs Vello-on-wgpu (compute vector renderer you
don't have to write) — plus CAMetalLayer/CADisplayLink frame pacing, ProMotion
120Hz, the glyph-atlas / signed-distance-field text problem, CPU-GPU sync, and
honest cost accounting of "pure Metal" vs standing on Linebender. Activate on:
"objc2-metal", "objc2", "CAMetalLayer", "CADisplayLink", "Metal command queue",
"glyph atlas", "SDF text", "ProMotion frame pacing", "bare-metal Rust
rendering on macOS", "pure Metal vs wgpu", "MTLDrawable". NOT for: writing the
MSL shaders themselves (use metal-shader-expert), the Vello/Parley high-level
API (use vello-parley-rendering), 3D engines (use wgpu/bevy), or iOS UIKit
drawing.
allowed-tools
Read,Write,Edit,Bash,Glob,Grep,WebSearch,WebFetch
metadata
{"category":"Frontend & UI","tags":["metal","objc2","objc2-metal","rust","gpu","text-rendering","glyph-atlas","frame-pacing","promotion","apple"],"pairs-with":[{"skill":"vello-parley-rendering","reason":"The high-level path this skill decides whether to drop below."},{"skill":"metal-shader-expert","reason":"When you do go bare-metal, this is who writes the MSL."},{"skill":"rust-app-distribution","reason":"Signing/notarizing the resulting macOS binary."}],"provenance":{"kind":"first-party","owners":["port-daddy"]},"authorship":{"maintainers":["port-daddy"]}}
The Metal Text / Render Pipeline (how low to go on Apple GPUs)
There are three rungs on the ladder for a custom 2D + text surface on macOS, and
the expensive mistake is picking the wrong rung. This skill is the decision and
the cost accounting — not the MSL itself (metal-shader-expert) and not
the Vello API (vello-parley-rendering).
Rung 3 (lowest): objc2 + objc2-metal, hand-written MSL.
You own: CAMetalLayer, command queue, render passes, a glyph
atlas (rasterize glyphs -> texture -> quads), AA, frame pacing.
Rung 2 (middle): wgpu. Metal under the hood on macOS; portable. You still write
pipelines/shaders, but no Obj-C, no CAMetalLayer plumbing.
Rung 1 (highest): Vello on wgpu. The compute vector renderer + Parley text are
written FOR you. You build a Scene; the GPU does the rest.
The honest headline, measured on this repo's core/pd-timeline-proto (Apple
M4 Max, Metal): a full timeline frame — beziers, dozens of markers, shaped text —
costs ~0.5–2 ms of GPU build+submit, vsync-bound otherwise. That performance
came from Rung 1. You do not need Rung 3 to hit 120fps ProMotion for a 2D
operator UI. Rung 3 is justified only by specific constraints below.
When To Use (this decision skill)
You're choosing between pure objc2-metal and a higher-level renderer for a
Rust macOS app and want the costs spelled out before you commit.
You already went bare-metal and need the frame-pacing / glyph-atlas / CPU-GPU
sync patterns done right.
Someone proposed "let's write it in pure Metal for performance" and you need to
pressure-test that against the measured ceiling.
NOT For
Writing the actual MSL vertex/fragment/compute shaders → metal-shader-expert.
The Vello + Parley high-level API and its gotchas → vello-parley-rendering.
3D engines / scene graphs → wgpu directly, bevy.
iOS UIKit/CoreGraphics 2D drawing, or SwiftUI Canvas → those are different
stacks (swiftui-data-flow-expert, etc.).
Decision Points
1. Which rung?
Do you need a CUSTOM 2D vector + text surface on macOS in Rust?
├─ Yes, and you want it shipped this quarter
│ └─ Rung 1: Vello + Parley on wgpu. (default — proven 120fps for 2D UI)
├─ Yes, AND you have a HARD reason to control the metal directly:
│ ├─ Need a specific Metal feature wgpu doesn't expose
│ │ (e.g. tile shaders / imageblocks, MTLHeap aliasing tricks,
│ │ raster order groups, custom MSL compute you must hand-tune)
│ ├─ Must share one MTLCommandQueue with existing Obj-C/Swift Metal code
│ ├─ Sub-100µs frame budgets where even wgpu's abstraction is measurable
│ └─ then → Rung 3: objc2 + objc2-metal, and bring metal-shader-expert
└─ You also want portability (Vulkan/DX12/GL) or 3D
└─ Rung 2: wgpu directly.
Default to Rung 1. Drop a rung only when you can NAME the constraint that forces
it. "It'll be faster" is not a constraint until you've measured Rung 1 missing
the budget — and for 2D UI it won't.
2. Frame pacing on ProMotion (all rungs)
Target 120Hz on ProMotion displays?
├─ Rung 3 (CAMetalLayer): drive with CADisplayLink; set
│ layer.maximumDrawableCount = 3 (triple buffer); set
│ CAMetalLayer.displaySyncEnabled = true; request the 120Hz cadence via
│ the display link's preferredFrameRateRange.
├─ Rung 1/2 (wgpu): PresentMode::AutoVsync caps to the display automatically;
│ you only need to request a redraw every frame to actually hit 120.
└─ Common trap: animation looks "60fps" because you only redraw on input.
The renderer isn't the limit — your event loop is. Request redraw each frame.
3. Text: glyph atlas vs Parley
How are you drawing text?
├─ Rung 1: Parley shapes+lays out; Vello rasterizes glyphs on GPU. Done.
├─ Rung 3: you own the whole thing:
│ 1. Shape with a HarfBuzz/HarfRust/CoreText pass (don't reinvent shaping).
│ 2. Rasterize each needed glyph once into a texture ATLAS (CPU raster via
│ Swash/CoreText, upload to MTLTexture). Cache by (glyph_id, size, subpixel).
│ 3. Emit a textured quad per glyph; sample the atlas in the fragment shader.
│ 4. For crisp scaling, consider SDF/MSDF atlases (one raster, many sizes) —
│ trades a fancier shader for not re-rasterizing per size.
└─ NEVER: a fixed bitmap font. It throws away shaping, hinting, variable fonts,
RTL/complex scripts, and HiDPI crispness.
4. CPU-GPU synchronization (Rung 3)
Avoiding stalls and races on a hand-rolled queue?
├─ Triple-buffer your per-frame uniform/vertex buffers (3 in flight).
├─ Gate frame N+3 on a dispatch_semaphore signaled in the command buffer's
│ completion handler (the canonical Metal pattern).
├─ Never write a buffer the GPU might still be reading. The semaphore is what
│ makes that safe without a full stall.
└─ Rung 1/2 (wgpu) handles this for you; this whole box is a cost of Rung 3.
Failure Modes
1. "Let's go pure Metal for speed" with no measurement
Symptom: weeks spent on CAMetalLayer/atlas plumbing to render a 2D UI that
Vello did in an afternoon at 0.6ms/frame.
Root cause: assuming the abstraction is the bottleneck without profiling.
Fix: build the Rung-1 version FIRST, measure with the GPU frame capture
(Xcode Metal debugger) or a simple submit-time log. Only drop rungs against a
named, measured miss.
2. Reinventing the vector rasterizer
Symptom: hand-writing anti-aliased bezier fill/stroke in MSL.
Root cause: not realizing that's exactly what Vello's compute pipeline is
(coarse rasterization + fine per-tile AA). It's a research-grade renderer.
Fix: unless your constraint is "I can't link Vello," don't. This is the
single biggest cost of Rung 3 and the main reason this prototype chose Rung 1.
3. ProMotion locked to 60
Symptom: 120Hz display, app renders 60.
Root cause: not requesting the high frame-rate range (Rung 3:
preferredFrameRateRange; all rungs: Info.plistCADisableMinimumFrameDurationOnPhone / display-link cadence) OR only
redrawing on input.
Fix: request the next frame unconditionally while animating; set the
display link / present mode to the display cadence.
4. Glyph atlas thrash
Symptom: frame time spikes whenever new text appears.
Root cause: rasterizing+uploading glyphs mid-frame, or an atlas too small so
it evicts and re-rasters.
Fix: size the atlas generously, cache by (glyph, size, subpixel offset),
rasterize off the hot path, and consider an SDF atlas so one raster serves many
sizes.
5. objc2 memory-management surprises
Symptom: crashes / leaks at the Rust↔Obj-C boundary.
Root cause: mishandling retain/release, autorelease pools, or & vs
Retained<…> in objc2.
Fix: use objc2's typed Retained<T> and the framework crates
(objc2-metal, objc2-quartz-core, objc2-foundation) rather than raw
message sends; wrap frame work in an autoreleasepool. This boilerplate is a
Rung-3-only tax.
Worked Example: the rung decision for this repo
Context: a Track-B "Voyage Timeline" operator surface — horizontal tracks,
timestamped markers, smooth causal bezier threads, shaped text labels, a
scrubable playhead, target 120fps ProMotion.
Considered Rung 3 (pure objc2-metal): would have meant owning a CAMetalLayer,
a command queue, a glyph atlas + shaping pass, an anti-aliased bezier
rasterizer in MSL, and triple-buffered CPU-GPU sync. Weeks of work, most of it
re-implementing Vello.
Chose Rung 1 (winit + wgpu + Vello + Parley): days, not weeks. wgpu lowers to
Metal on macOS (confirmed at runtime: backend: Metal, Apple M4 Max), so we
keep "bespoke GPU vector rendering" — the beziers and text are genuinely GPU
vector, fully under our control via the Scene API — without writing the
rasterizer.
Measured result: ~0.5–2 ms GPU build+submit per frame; vsync-bound otherwise;
uncapped benchmark logged 750–900 FPS. Conclusion: for a 2D operator UI, Rung 1
clears 120fps ProMotion with ~99% of the frame budget to spare. Bare-metal
(Rung 3) is not worth it here; reserve it for a future surface that needs a
specific Metal feature wgpu/Vello don't expose.
See references/objc2-metal-skeleton.md for the Rung-3 skeleton (what you'd own
if you did drop down) and references/cost-ledger.md for the side-by-side cost
accounting.
Anti-Patterns
Anti-Pattern 1: Equating "lower level" with "faster" for 2D UI
The GPU work for a 2D vector+text frame is tiny at every rung. The win from Rung 3
is control of specific Metal features, not raw 2D throughput. If you can't name
the feature, you're paying for nothing.
Anti-Pattern 2: Hand-rolling shaping
CoreText / HarfRust exist. Writing your own shaper (kerning, ligatures, cluster
mapping, bidi) is a multi-year project you will get wrong. Even at Rung 3, shape
with a real shaper and only own the atlas+raster.
Anti-Pattern 3: Single-buffering a hand-rolled Metal frame
One per-frame buffer the GPU is still reading = tearing/races or a full stall.
Triple-buffer + a completion-handler semaphore. (Free at Rung 1/2.)
Anti-Pattern 4: Skipping the Rung-1 measurement
"We'll go straight to Metal" with no baseline means you can never prove the lower
rung was warranted. Always have the Rung-1 number in hand.
References
references/objc2-metal-skeleton.md — the minimal Rung-3 pipeline you'd own:
CAMetalLayer + command queue + glyph-atlas sketch + the triple-buffer
semaphore, in objc2/objc2-metal terms.
references/cost-ledger.md — side-by-side cost accounting (Rung 1 vs Rung 3):
what you write, what you maintain, what you measure.
Provenance
Decision and measurements are from core/pd-timeline-proto/ in this repo (Rung 1:
winit+wgpu+Vello+Parley), run on Apple M4 Max with the Metal backend.
Skill Bundle Index
Every file in this skill, and when to open it. Auto-generated; run scripts/index_references.py --fix.
references/
references/cost-ledger.md — Cost ledger: Rung 1 (Vello/Parley) vs Rung 3 (pure objc2-metal) — Side-by-side of what you actually write, maintain, and measure.
references/objc2-metal-skeleton.md — Rung-3 skeleton: what you own with pure objc2-metal — This is the pipeline you'd hand-write if you dropped below wgpu/Vello.
