thebookofshaders/09/zigzag.frag

// Author @patriciogv - 2015
// Title: Zigzag

#ifdef GL_ES
precision mediump float;
#endif

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


vec2 mirrorTile(vec2 _st, float _zoom){
    _st *= _zoom;
    if (fract(_st.y * 0.5) > 0.5){
        _st.x = _st.x+0.5;
        _st.y = 1.0-_st.y;
    }
    return fract(_st);
}

float fillY(vec2 _st, float _pct,float _antia){
  return  smoothstep( _pct-_antia, _pct, _st.y);
}

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

  st = mirrorTile(st*vec2(1.,2.),5.);
  float x = st.x*2.;
  float a = floor(1.+sin(x*3.14));
  float b = floor(1.+sin((x+1.)*3.14));
  float f = fract(x);

  color = vec3( fillY(st,mix(a,b,f),0.01) );

  gl_FragColor = vec4( color, 1.0 );
}