How to write a shader background

One full-screen triangle and a fragment shader that drifts like ink, stays behind the text, and freezes for reduced motion.

TL;DR

Layer noise (fbm), then warp it with more noise.
Turn the field into shapes with smoothstep, multiply the colours, add grain.
Keep text readable, cap DPR, pause off screen, freeze for reduced motion.

Goal: a living texture that sets the mood without fighting the words on top.

You need

OGL, a tiny WebGL library. No three.js needed.
The full example: components/ink-field.tsx.

DensityClear zoneInk AInk B

An ogl ink shader with its uniforms on sliders. Deliberately in colour: two finale inks multiply where they overlap. Every change is a uniform update, not a recompile.

 noise ─► warp ─► threshold ─► multiply ─► grain
 smooth   noise    field to     inks over    print
 random   bends    ink shapes   paper        texture
          noise

Steps

1. Draw one full-screen triangle

A single triangle bigger than the screen covers every pixel with no seam. OGL's Triangle geometry does it for you. All the work happens in the fragment shader.

2. Layer noise

noise(p) gives smooth random values. Add several layers at doubling frequency and halving strength. That is fbm (fractal Brownian motion), and it looks like clouds.

3. Warp it

Feed noise into the coordinates of more noise. Straight bands turn into ink swirling in water.

vec2 q = vec2(fbm(p + vec2(0.0, t)), fbm(p + vec2(5.2, 1.3) - t));
vec2 r = vec2(fbm(p + 3.0 * q + vec2(1.7, 9.2) + t * 1.4),
              fbm(p + 3.0 * q + vec2(8.3, 2.8) - t * 1.1));
float f = fbm(p + 3.2 * r);

4. Threshold, multiply, grain

float inkA = smoothstep(0.56 - uDensity, 0.66 - uDensity, f + 0.12 * r.x);
vec3 col = mix(uPaper, uPaper * uInkA, inkA * 0.92);          // multiply, like ink
col += (hash(uv * uRes + fract(uTime) * 91.0) - 0.5) * 0.045;  // grain hides banding

You want	Use
A living gradient, tuned by eye	ShaderGradient
A premium background, fast	GetLayers
Animated backgrounds as React components	React Bits
Something that is yours	Write it. Start with The Book of Shaders