---
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