Shader Fundamentals
GLSL (OpenGL Shading Language) runs on the GPU. Vertex shaders transform geometry; fragment shaders color pixels.
Quick Start
// Vertex Shader
uniform mat4 projectionMatrix;
uniform mat4 modelViewMatrix;
attribute vec3 position;
attribute vec2 uv;
varying vec2 vUv;
void main() {
vUv = uv;
gl_Position = projectionMatrix * modelViewMatrix * vec4(position, 1.0);
}
// Fragment Shader
uniform float uTime;
varying vec2 vUv;
void main() {
vec3 color = vec3(vUv, sin(uTime) * 0.5 + 0.5);
gl_FragColor = vec4(color, 1.0);
}
Graphics Pipeline
Vertex Data → [Vertex Shader] → Primitives → Rasterization → [Fragment Shader] → Pixels
↑ ↑ ↑
attributes transforms per-pixel color
| Stage |
Runs Per |
Purpose |
| Vertex Shader |
Vertex |
Transform positions, pass data to fragment |
| Fragment Shader |
Pixel |
Calculate final color |
Data Types
Scalars
bool b = true;
int i = 42;
float f = 3.14;
Vectors
vec2 v2 = vec2(1.0, 2.0);
vec3 v3 = vec3(1.0, 2.0, 3.0);
vec4 v4 = vec4(1.0, 2.0, 3.0, 4.0);
// Integer vectors
ivec2 iv2 = ivec2(1, 2);
ivec3 iv3 = ivec3(1, 2, 3);
// Boolean vectors
bvec2 bv2 = bvec2(true, false);
Swizzling
vec4 color = vec4(1.0, 0.5, 0.2, 1.0);
vec3 rgb = color.rgb; // (1.0, 0.5, 0.2)
vec2 rg = color.rg; // (1.0, 0.5)
float r = color.r; // 1.0
// Reorder
vec3 bgr = color.bgr; // (0.2, 0.5, 1.0)
// Duplicate
vec3 rrr = color.rrr; // (1.0, 1.0, 1.0)
// Position aliases (xyzw = rgba = stpq)
vec3 pos = v4.xyz;
vec2 uv = v4.st;
Matrices
mat2 m2; // 2x2
mat3 m3; // 3x3
mat4 m4; // 4x4
// Access columns
vec4 col0 = m4[0];
// Access element
float val = m4[1][2]; // column 1, row 2
Samplers
uniform sampler2D uTexture; // 2D texture
uniform samplerCube uCubemap; // Cube map
// Sample texture
vec4 texColor = texture2D(uTexture, vUv);
vec4 cubeColor = textureCube(uCubemap, direction);
Variable Qualifiers
Uniforms (CPU → GPU, constant per draw)
// Set from JavaScript, same for all vertices/fragments
uniform float uTime;
uniform vec3 uColor;
uniform mat4 uModelMatrix;
uniform sampler2D uTexture;
Attributes (Per-vertex data)
// Only in vertex shader
attribute vec3 position; // Built-in: vertex position
attribute vec3 normal; // Built-in: vertex normal
attribute vec2 uv; // Built-in: texture coordinates
attribute vec3 color; // Built-in: vertex color
// Custom attributes
attribute float aScale;
attribute vec3 aOffset;
Varyings (Vertex → Fragment, interpolated)
// Vertex shader: write
varying vec2 vUv;
varying vec3 vNormal;
void main() {
vUv = uv;
vNormal = normal;
}
// Fragment shader: read (interpolated across triangle)
varying vec2 vUv;
varying vec3 vNormal;
void main() {
// vUv is interpolated between triangle vertices
}
Built-in Variables
Vertex Shader
// Output (must write)
vec4 gl_Position; // Clip-space position
// Output (optional)
float gl_PointSize; // Point sprite size (for gl.POINTS)
Fragment Shader
// Input
vec4 gl_FragCoord; // Window-space position (pixel coordinates)
bool gl_FrontFacing; // True if front face
vec2 gl_PointCoord; // Point sprite coordinates [0,1]
// Output
vec4 gl_FragColor; // Final pixel color
Coordinate Spaces
Local/Object Space
↓ modelMatrix
World Space
↓ viewMatrix
View/Eye/Camera Space
↓ projectionMatrix
Clip Space (-1 to 1)
↓ perspective divide
NDC (Normalized Device Coordinates)
↓ viewport transform
Screen Space (pixels)
Common Matrices (Three.js/R3F)
uniform mat4 modelMatrix; // Local → World
uniform mat4 viewMatrix; // World → View
uniform mat4 projectionMatrix; // View → Clip
uniform mat4 modelViewMatrix; // Local → View (modelMatrix * viewMatrix)
uniform mat3 normalMatrix; // For transforming normals
uniform vec3 cameraPosition; // Camera world position
Standard Vertex Transform
void main() {
// Full transform chain
vec4 worldPosition = modelMatrix * vec4(position, 1.0);
vec4 viewPosition = viewMatrix * worldPosition;
vec4 clipPosition = projectionMatrix * viewPosition;
gl_Position = clipPosition;
// Or combined (more efficient)
gl_Position = projectionMatrix * modelViewMatrix * vec4(position, 1.0);
}
Built-in Functions
Math
// Trigonometry
sin(x), cos(x), tan(x)
asin(x), acos(x), atan(x)
atan(y, x) // atan2
// Exponential
pow(x, y) // x^y
exp(x) // e^x
log(x) // ln(x)
sqrt(x) // √x
inversesqrt(x) // 1/√x
// Common
abs(x)
sign(x) // -1, 0, or 1
floor(x)
ceil(x)
fract(x) // x - floor(x)
mod(x, y) // x % y (floating point)
min(x, y)
max(x, y)
clamp(x, min, max)
mix(a, b, t) // Linear interpolation: a*(1-t) + b*t
step(edge, x) // 0 if x < edge, else 1
smoothstep(e0, e1, x) // Smooth Hermite interpolation
Vector
length(v) // Vector magnitude
distance(a, b) // length(a - b)
dot(a, b) // Dot product
cross(a, b) // Cross product (vec3 only)
normalize(v) // Unit vector
reflect(I, N) // Reflection vector
refract(I, N, eta) // Refraction vector
faceforward(N, I, Nref) // Flip normal if needed
Common Patterns
UV Coordinates
// vUv ranges from (0,0) at bottom-left to (1,1) at top-right
varying vec2 vUv;
void main() {
// Center UVs: -0.5 to 0.5
vec2 centered = vUv - 0.5;
// Aspect-corrected (assuming you pass uResolution)
vec2 uv = vUv;
uv.x *= uResolution.x / uResolution.y;
// Tiling
vec2 tiled = fract(vUv * 4.0); // 4x4 tiles
// Polar coordinates
float angle = atan(centered.y, centered.x);
float radius = length(centered);
}
Color Operations
// Grayscale (perceptual weights)
float gray = dot(color.rgb, vec3(0.299, 0.587, 0.114));
// Contrast
color = (color - 0.5) * contrast + 0.5;
// Brightness
color += brightness;
// Saturation
float gray = dot(color, vec3(0.299, 0.587, 0.114));
color = mix(vec3(gray), color, saturation);
// Gamma correction
color = pow(color, vec3(1.0 / 2.2)); // Linear to sRGB
color = pow(color, vec3(2.2)); // sRGB to linear
Smooth Transitions
// Hard edge
float mask = step(0.5, value);
// Soft edge
float mask = smoothstep(0.4, 0.6, value);
// Anti-aliased edge (screen-space)
float mask = smoothstep(-fwidth(value), fwidth(value), value);
Debugging
Visualize Values
// Show UVs as color
gl_FragColor = vec4(vUv, 0.0, 1.0);
// Show normals
gl_FragColor = vec4(vNormal * 0.5 + 0.5, 1.0);
// Show depth
float depth = gl_FragCoord.z;
gl_FragColor = vec4(vec3(depth), 1.0);
// Show value range (red=negative, green=positive)
gl_FragColor = vec4(max(0.0, value), max(0.0, -value), 0.0, 1.0);
Common Errors
| Issue |
Likely Cause |
| Black screen |
gl_Position not set, or NaN values |
| Uniform not updating |
Wrong name or type mismatch |
| Texture black |
Texture not loaded, wrong UV |
| Flickering |
Z-fighting, precision issues |
| Faceted look |
Normals not interpolated |
Precision
// Declare precision (required in fragment shader for WebGL 1)
precision highp float;
precision mediump float;
precision lowp float;
| Precision |
Range |
Use Case |
| highp |
~10^38 |
Positions, matrices |
| mediump |
~10^14 |
UVs, colors |
| lowp |
~2 |
Simple flags |
File Structure
shader-fundamentals/
├── SKILL.md
├── references/
│ ├── glsl-types.md # Complete type reference
│ ├── builtin-functions.md # All built-in functions
│ └── coordinate-spaces.md # Transform pipeline
└── scripts/
└── templates/
├── basic.glsl # Starter template
└── fullscreen.glsl # Fullscreen quad shader
Reference
references/glsl-types.md — Complete data type referencereferences/builtin-functions.md — All GLSL built-in functionsreferences/coordinate-spaces.md — Transform pipeline deep-dive
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