thebookofshaders/11/1d-noise.frag

#ifdef GL_ES
precision mediump float;
#endif

uniform vec2 u_resolution;
uniform vec2 u_mouse;
uniform float u_time;

float plot(vec2 _st, float _pct){
  return  smoothstep( _pct-0.01, _pct, _st.y) - 
          smoothstep( _pct, _pct+0.01, _st.y);
}

float random (in float _x) {
    return fract(sin(_x)*1e4);
}

// Based on Morgan McGuire @morgan3d
// https://www.shadertoy.com/view/4dS3Wd
float noise (in float _x) {
    float i = floor(_x);
    float f = fract(_x);
    float u = f * f * (3.0 - 2.0 * f);
    return mix(random(i), random(i + 1.0), u);
}

void main() {
    vec2 st = gl_FragCoord.xy/u_resolution.xy;
    st.x *= u_resolution.x/u_resolution.y;
    vec3 color = vec3(0.0);

    float y = 0.0;
    // y = random(st.x*0.001+u_time);
    y = noise(st.x*3.+u_time);

    // color = vec3(y);
    float pct = plot(st,y);
    color = (1.0-pct)*color+pct*vec3(0.0,1.0,0.0);

    gl_FragColor = vec4(color,1.0);
}