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skills/ck-shader/SKILL.md

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+---
+name: ck-shader
+description: Write GLSL fragment shaders for procedural graphics. Use for generative art, procedural textures, visual effects, WebGL shaders, Three.js materials, shapes (SDF), noise, patterns, and animations.
+---
+
+# GLSL Fragment Shaders
+
+GPU-accelerated fragment shaders for procedural graphics, textures, and visual effects.
+
+## When to Use
+
+- Creating procedural textures (wood, marble, clouds, terrain)
+- Drawing shapes with signed distance fields (SDF)
+- Generating patterns, noise, gradients, animations
+- Writing custom shaders for Three.js, WebGL, Processing, ShaderToy
+
+## Don't Use When
+
+- Simple CSS animations suffice
+- Image manipulation that doesn't need GPU parallelism
+
+## Core Concepts
+
+Each thread receives pixel position via `gl_FragCoord`, returns color via `gl_FragColor` (vec4 RGBA 0.0–1.0). Threads cannot communicate — fully stateless.
+
+## Standard Uniforms
+
+```glsl
+uniform float u_time;       // Elapsed seconds
+uniform vec2 u_resolution;  // Canvas size (width, height)
+uniform vec2 u_mouse;       // Mouse position in pixels
+```
+
+Normalize coordinates: `vec2 st = gl_FragCoord.xy / u_resolution;`
+
+## Essential Functions
+
+| Function | Purpose |
+|----------|---------|
+| `mix(a,b,t)` | Linear interpolate |
+| `step(edge,x)` | Hard threshold |
+| `smoothstep(e0,e1,x)` | Smooth threshold |
+| `fract(x)` | Fractional part (tiling) |
+| `length(v)` | Vector magnitude |
+| `distance(a,b)` | Euclidean distance |
+| `atan(y,x)` | Angle in radians |
+| `sin/cos/pow/abs` | Math (hardware-accelerated) |
+
+## Quick Patterns
+
+```glsl
+// Circle
+float d = distance(st, vec2(0.5));
+float circle = 1.0 - smoothstep(0.2, 0.21, d);
+
+// Rectangle
+vec2 bl = step(vec2(0.1), st);
+vec2 tr = step(vec2(0.1), 1.0 - st);
+float rect = bl.x * bl.y * tr.x * tr.y;
+
+// Tiling
+st = fract(st * 4.0);  // 4x4 grid
+
+// Animation
+float wave = sin(st.x * 10.0 + u_time) * 0.5 + 0.5;
+```
+
+## Reference Files (Progressive Disclosure)
+
+**Fundamentals:**
+- `references/glsl-fundamentals-data-types-vectors-precision-coordinates.md`
+- `references/glsl-shaping-functions-step-smoothstep-curves-interpolation.md`
+
+**Drawing:**
+- `references/glsl-colors-rgb-hsb-gradients-mixing-color-spaces.md`
+- `references/glsl-shapes-sdf-circles-rectangles-polar-distance-fields.md`
+- `references/glsl-shapes-polygon-star-polar-sdf-combinations.md`
+
+**Procedural:**
+- `references/glsl-patterns-tiling-fract-matrices-transformations.md`
+- `references/glsl-noise-random-perlin-simplex-cellular-voronoi.md`
+- `references/glsl-fbm-fractional-brownian-motion-turbulence-octaves.md`
+- `references/glsl-procedural-textures-clouds-marble-wood-terrain.md`
+
+**API Reference:**
+- `references/glsl-shader-builtin-functions-complete-api-reference.md`
+
+## Tools
+
+- **Online editor:** editor.thebookofshaders.com
+- **glslViewer:** CLI tool for running `.frag` files
+- **ShaderToy:** iTime, iResolution, iMouse uniforms
+- **LYGIA Library:** https://lygia.xyz (reusable shader functions)
+
+## Resources
+
+- The Book of Shaders: https://thebookofshaders.com
+- Inigo Quilez Articles: https://iquilezles.org/articles/
+- ShaderToy: https://shadertoy.com