gpui Shaders

Run your own WGSL fragment shaders on Metal (via wgpu) and get their pixels onto a native gpui window — for the per-pixel effects the element tree can't do (water, noise fields, dither, glints). The aesthetic house style is pixelated + dithered retro-futurism on the maritime palette; the engineering rule is earn the GPU pass — most "shader" ideas are better done with a gpui primitive or a Vello vector pass.

When to Use

• A surface needs genuine per-pixel work: ocean/water, noise/fbm fields, raymarching, full-res dither, glints, a living-harbor backdrop.
• You want a shader-toy effect (SDF shapes, palettes, domain-warped noise) inside or behind a gpui pane.
• Building the "living harbor" viz, dithered chrome borders, a sonar/radar sweep, signal-flag shimmer, or a CRT/scanline post pass.

NOT for

• Web shaders (GLSL/three.js/WebGL/<canvas>) — different runtime entirely.
• Non-shader motion/transitions in gpui — use rust-gpui-motion.
• Plain vector/2D drawing that Vello or gpui paint/canvas already does — don't reach for a raw fragment pass.
• General layout, color systems, typography — beautiful-gui-design. CLI/TUI — beautiful-cli-design.

Decision Points

flowchart TD
  A[Want a visual effect on a gpui surface] --> B{Does it need TRUE per-pixel work?\n(noise, SDF field, water, raymarch, full-res dither)}
  B -->|No, it's shapes/quads/text| C[gpui element tree or paint/canvas]
  B -->|No, but vector + crisp text| D[Vello vector pass]
  B -->|Yes| E{Must it sit INSIDE the pane tree,\nreflowing with splits?}
  E -->|No — focused/modal/full-window| F[Companion window: own winit+wgpu (ADR-0086 path 3) — ship-now]
  E -->|Yes — ambient, in a pane| G[Render-to-texture, sample back as a gpui image (ADR-0086 path 2)]
  F --> H[Push gpui state as uniforms: time, res, mouse, accent token]
  G --> H

Route: gpui primitive → Vello vector → wgpu fragment pass, in that order of preference. A raw WGSL pass is justified only by real per-pixel cost. Companion-window is the ship-now path (it's the timeline-proto skeleton); in-window embed (render-to-texture) is the target for ambient surfaces — budget the round-trip honestly.

Core Rules

• Earn the GPU pass. Don't shader a button. Per-pixel work or nothing.
• Sample theme tokens. Color comes from u.accent (the gpui theme token pushed as a uniform) + a cosine palette() — never hardcode brand hex in WGSL, so light/dark + brand flow through.
• time is the only animation input — so reduced-motion = freeze time to a static, still-beautiful frame (never a blank).
• Pause offscreen/unfocused. A shader pane that isn't visible must stop rendering; an ambient shader must cap its frame rate.
• Dither + pixelate are the style, applied throughout (Bayer threshold on luminance/palette-t, snap-to-grid UV) — not a post-filter bolted on.
• Pixelate first for budget. Fewer effective fragments; cap fbm octaves; render ambient surfaces at reduced res and upscale (the dither hides it).
• Keep wgpu out of the Linux CI workspace (companion crate, like pd-timeline-proto) so the rust-console gate never compiles it.

Failure Modes

Anti-Pattern: "Shadering a widget"

Symptom: a fragment pass drawing a rectangle, a border, or text that the element tree draws for free. Detection: the shader has no noise/sdf/loop — it's just a fill or gradient. Fix: use a gpui div/paint_quad/Vello. Reserve WGSL for true per-pixel work.

Anti-Pattern: "The ambient shader that never sleeps"

Symptom: CPU/GPU pegged while the shader pane is offscreen, backgrounded, or the window's unfocused. Detection: frame counter keeps climbing with the window hidden; fans spin on an idle app. Fix: gate rendering on visibility/focus; cap ambient shaders to 30fps; stop the loop when occluded.

Anti-Pattern: "Hardcoded #FFDB33 in WGSL"

Symptom: the shader looks right in dark mode, wrong (or off-brand) in light; theme changes don't reach it. Detection: literal colors in the .wgsl; no accent uniform. Fix: push the theme token as u.accent; derive everything via palette()/mix from it.

Anti-Pattern: "fbm to the moon"

Symptom: gorgeous in isolation, drops frames at full window size or on an iGPU. Detection: profiler shows the fragment stage dominating; octave count ≥ 6 at full res. Fix: pixelate first, cap octaves (4–5), reduce render resolution + upscale, precompute constants.

Anti-Pattern: "Per-RGB dither rainbow"

Symptom: dithering produces colored speckle instead of a clean two-tone dissolve. Detection: Bayer threshold applied to r,g,b independently. Fix: dither luminance or the palette parameter t, then map through the palette.

Worked Example: a living-harbor water shader behind the Quay

Goal: a calm, dithered, pixelated ocean drifting behind the agent dock — ambient, on-brand, reduced-motion-safe, cheap.

1. Earn it: water = domain-warped fbm + a sun glint = true per-pixel work → a fragment pass is justified (not a Vello vector).
2. Path: it lives in the pane behind the Quay → render-to-texture, sample back as a gpui image (ADR-0086 path 2). Ambient → cap at 30fps, pause when the window's unfocused.
3. Shader (from 03-the-stockpile.md + 02): aspect-correct p; warped = fbm(p + 4*q) for swells; horizon gradient via palette(uv.y, …, u.accent); pixelate UV to fat pixels; Bayer-dither the luminance to ~4 levels for the retro water; a soft sd_circle sun with additive glint.
4. Theme: u.accent = the mustard token → warm sun on navy water in dark, muted in light.
5. Reduced-motion: if the OS prefers reduced motion, freeze u.time at a hand-picked still — the water becomes a static dithered seascape, never blank.
6. Compose: rust-gpui-motion fades the shader pane in (opacity, 200ms); beautiful-gui-design keeps the Quay text contrast ≥4.5:1 over the busiest water region (test it).

Quality Gates

• The effect genuinely needs per-pixel work (else use a primitive/Vello).
• Companion-vs-embed chosen deliberately; wgpu kept out of the Linux CI workspace.
• Color derives from the accent theme uniform + palette(); no hardcoded brand hex.
• Reduced-motion freezes time to a still, non-blank frame.
• Shader pauses/caps when offscreen, occluded, or unfocused.
• AA uses fwidth; dither targets luminance/palette-t; pixelation applied before heavy noise.
• fbm octaves tuned to the frame budget; ambient surfaces render at reduced res if needed.
• Text/controls over the shader stay ≥4.5:1 contrast on the busiest region.

Fork Guidance

Fork by lane: integration (01 — surfaces, companion vs embed, uniform plumbing) · technique (02 — SDF/noise/dither WGSL) · content (03 — the stockpile of finished effects) · shipping (04 — performance, reduced-motion, packaging). Keep the "earn the pass" decision in the parent.

Reference Map

• references/01-shader-surfaces-in-gpui.md — getting a custom fragment pass onto a gpui window: companion window vs render-to-texture embed (ADR-0086), wgpu setup, uniform plumbing from gpui state.
• references/02-wgsl-fundamentals-for-ui.md — the per-pixel toolkit: UV spaces, SDFs + fwidth AA, hash/value-noise/fbm, domain warping, cosine palettes, Bayer dithering + pixelation, cost.
• references/03-the-stockpile.md — copy-pasteable WGSL examples: pixelated ocean waves, boats, the living harbor, signal-flag shimmer, aurora/starfield, CRT post, dithered chrome border, sonar sweep.
• references/04-integration-performance-a11y.md — when a shader earns its place, frame budget, pausing offscreen, reduced-motion, theme-token sampling, packaging/hot-reload.

Sibling skills: pairs with rust-gpui-motion (animate/fade shader surfaces), beautiful-gui-design (contrast/hierarchy over shaders), and the Metal/Vello skills vello-parley-rendering + metal-text-pipeline (the vector + bare-metal neighbors).

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Skill Bundle Index

Every file in this skill, and when to open it. Auto-generated; run scripts/index_references.py --fix.

references/

• references/01-shader-surfaces-in-gpui.md — Shader Surfaces in gpui — Getting a Custom GPU Fragment-Shader Pass Into a Native Rust Window — > Scope. The sibling doc 05-bespoke-graphics-vello-wgpu.md maps the three tiers of bespoke drawing (T1 element tree → T2 gpui paint/
• references/02-wgsl-fundamentals-for-ui.md — WGSL Shader Fundamentals for UI Surfaces — > The Shadertoy mental model, ported to a WGSL fragment shader feeding a gpui pane (Metal via wgpu).
• references/03-the-stockpile.md — The Stockpile — Copy-Pasteable WGSL Shader-Toy Examples for the Harbor Console — > Scope. This is the showpiece.
• references/04-integration-performance-a11y.md — Integration, Performance & Accessibility — Earning a Shader in a gpui App — > Scope. A shader is the most expensive pixel in your app.

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