Shadertoy Shader Development

Overview

Shadertoy is a platform for creating and sharing GLSL fragment shaders that run in the browser using WebGL. This skill provides comprehensive guidance for writing shaders including GLSL ES syntax, common patterns, mathematical techniques, and best practices specific to real-time procedural graphics.

When to Use This Skill

Activate this skill when:

• Writing or editing .glsl shader files
• Creating procedural graphics, generative art, or visual effects
• Working with Shadertoy.com projects or WebGL fragment shaders
• Implementing ray marching, distance fields, or procedural textures
• Debugging shader code or optimizing shader performance
• Need GLSL ES syntax reference or Shadertoy input variables

Core Concepts

Shader Entry Point

Every Shadertoy shader implements the mainImage function:

void mainImage(out vec4 fragColor, in vec2 fragCoord)
{
    // fragCoord: pixel coordinates (0 to iResolution.xy)
    // fragColor: output color (RGBA, typically alpha = 1.0)

    vec2 uv = fragCoord / iResolution.xy;
    fragColor = vec4(uv, 0.0, 1.0);
}

Shadertoy Built-in Inputs

Always available in shaders:

Coordinate System Setup

Standard patterns for normalizing coordinates:

// Aspect-corrected UV centered at origin (-1 to 1, aspect-preserved)
vec2 uv = (fragCoord.xy - 0.5 * iResolution.xy) / min(iResolution.y, iResolution.x);

// Alternative compact form:
vec2 uv = (fragCoord * 2.0 - iResolution.xy) / min(iResolution.x, iResolution.y);

// Simple normalized (0 to 1)
vec2 uv = fragCoord / iResolution.xy;

Common Shader Patterns

1. Procedural Color Palettes

Use Inigo Quilez's cosine palette for smooth color gradients:

vec3 palette(float t, vec3 a, vec3 b, vec3 c, vec3 d) {
    return a + b * cos(6.28318 * (c * t + d));
}

// Example usage:
vec3 col = palette(
    t,
    vec3(0.5, 0.5, 0.5),    // base
    vec3(0.5, 0.5, 0.5),    // amplitude
    vec3(1.0, 1.0, 0.5),    // frequency
    vec3(0.8, 0.90, 0.30)   // phase
);

2. Hash Functions (Pseudo-Random)

Simple 2D hash for noise and randomness:

float hash21(vec2 p) {
    p = fract(p * vec2(234.34, 435.345));
    p += dot(p, p + 34.23);
    return fract(p.x * p.y);
}

3. Ray Marching

Standard pattern for 3D rendering via sphere tracing:

// Distance field function
float map(vec3 p) {
    return length(p) - 1.0;  // Sphere at origin, radius 1
}

// Normal calculation
vec3 calcNormal(vec3 p) {
    vec2 e = vec2(0.001, 0.0);
    return normalize(vec3(
        map(p + e.xyy) - map(p - e.xyy),
        map(p + e.yxy) - map(p - e.yxy),
        map(p + e.yyx) - map(p - e.yyx)
    ));
}

// Ray marching loop
vec3 render(vec3 ro, vec3 rd) {
    float t = 0.0;
    for (int i = 0; i < 100; i++) {
        vec3 p = ro + rd * t;
        float d = map(p);
        if (d < 0.001) {
            // Hit - calculate lighting
            vec3 n = calcNormal(p);
            return n * 0.5 + 0.5;  // Normal visualization
        }
        if (t > 10.0) break;
        t += d * 0.5;  // Step (0.5 factor for safety)
    }
    return vec3(0.0);  // Miss
}

4. Rotations

2D rotation:

mat2 rot2d(float a) {
    float c = cos(a), s = sin(a);
    return mat2(c, -s, s, c);
}
// Usage: p.xy *= rot2d(iTime);

3D axis-angle rotation (modifies in-place):

void rot(inout vec3 p, vec3 axis, float angle) {
    axis = normalize(axis);
    float s = sin(angle), c = cos(angle), oc = 1.0 - c;
    mat3 m = mat3(
        oc * axis.x * axis.x + c,           oc * axis.x * axis.y - axis.z * s,  oc * axis.z * axis.x + axis.y * s,
        oc * axis.x * axis.y + axis.z * s,  oc * axis.y * axis.y + c,           oc * axis.y * axis.z