thebookofshaders/examples/06/dothh489-sunset.frag

// By Hang Do Thi Duc ( http://22-8miles.com)
// For Shader Studio Course https://github.com/patriciogonzalezvivo/ss2015

// Inspired on this picture http://www.tate.org.uk/art/artworks/turner-red-sunset-on-lake-d36200 
// by William Turner Sunset

#ifdef GL_ES
precision mediump float;
#endif

#define PI 3.14159265359

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);
}
//Inigo Quiles
float cubicPulse( float highPos, float width, float x ){
    // e.g. 0.4__0.2__0.0__0.2__0.4
    x = abs(x - highPos);
    // make flat before and after highPos value
    if( x>width ) return 0.0;
    x /= width;
    // reverse Quadratfunction * smooth edges
    return 1.0 - pow(x, 2.) * (3.0 - 2.0*x);
}

void main() {
    vec3 colorA = vec3(0.909,0.961,0.982);
    vec3 colorB = vec3(0.760,0.533,0.404);
    vec2 st = gl_FragCoord.xy/u_resolution.xy;
    vec3 color = vec3(1.0);
    vec3 pct = vec3(st.y);

    pct.r = 1.2 * cubicPulse(0.44, 0.09, st.y) - .2;
    pct.g = 1.2 * (smoothstep(.28,.5, st.y) - smoothstep(.5,.8, st.y));
    pct.b = 1.1 * (smoothstep(.2,.6, st.y) - smoothstep(.6,.9, st.y));

    color = mix(colorA, colorB, pct);

    gl_FragColor = vec4(color,1.0);
}