thebookofshaders/09/rotatedtiles.frag

// Author @patriciogv ( patriciogonzalezvivo.com ) - 2015

#ifdef GL_ES
precision mediump float;
#endif

#define PI 3.14159265358979323846
#define TWO_PI 6.28318530717958647693

uniform vec2 u_resolution;
uniform float u_time;

vec2 rotate2D (vec2 _st, float _angle) {
  _st -= 0.5;
  _st =  mat2(cos(_angle),-sin(_angle),
              sin(_angle),cos(_angle)) * _st;
  _st += 0.5;
  return _st;
}

vec2 tile (vec2 _st, float _zoom) {
  _st *= _zoom;
  return fract(_st);
}

vec2 rotateTile(vec2 _st){
    _st *= 2.0;

    float index = 0.0;
    if (fract(_st.x * 0.5) > 0.5){
        index += 1.0;
    }
    if (fract(_st.y * 0.5) > 0.5){
        index += 2.0;
    }

    _st = fract(_st);

    if(index == 1.0){
        _st = rotate2D(_st,PI*0.5);
    } else if(index == 2.0){
        _st = rotate2D(_st,PI*-0.5);
    } else if(index == 3.0){
        _st = rotate2D(_st,PI);
    }

    return _st;
}

// Based on https://www.shadertoy.com/view/4sSSzG
float triangle (vec2 _st,
                vec2 _p0, vec2 _p1, vec2 _p2,
                float _smoothness) {
  vec3 e0, e1, e2;

  e0.xy = normalize(_p1 - _p0).yx * vec2(+1.0, -1.0);
  e1.xy = normalize(_p2 - _p1).yx * vec2(+1.0, -1.0);
  e2.xy = normalize(_p0 - _p2).yx * vec2(+1.0, -1.0);

  e0.z = dot(e0.xy, _p0) - _smoothness;
  e1.z = dot(e1.xy, _p1) - _smoothness;
  e2.z = dot(e2.xy, _p2) - _smoothness;

  float a = max(0.0, dot(e0.xy, _st) - e0.z);
  float b = max(0.0, dot(e1.xy, _st) - e1.z);
  float c = max(0.0, dot(e2.xy, _st) - e2.z);

  return smoothstep(_smoothness * 2.0,
                    1e-7,
                    length(vec3(a, b, c)));
}

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

    st = tile(st,3.0);
    st = rotateTile(st);

    float pattern = 0.0;

    st = rotate2D(st,-PI*u_time*0.25);
    pattern =   triangle(st,
                         vec2(0.30,-0.5),
                         vec2(0.70,0.-0.5),
                         vec2(0.5,1.0),
                         0.01);

    vec3 color = vec3(pattern);

    gl_FragColor = vec4(color,1.0);
}