#define TAU 6.2831853072

uniform vec2 origin = vec2(0.0, 0.0);
uniform float direction = 0.0;
uniform float amplitude = 10.0;
uniform float length = 10.0;
uniform float evolution = 0.0;

vec2 distanceDirection(vec2 pos) {
  float angle = direction * TAU;
  vec2 normal = vec2(cos(angle), sin(angle));
  vec2 relative = pos - origin;
  return vec2(dot(normal, relative), angle + TAU / 4);
}

vec2 generateShift(vec2 pos) {
  vec2 dd = distanceDirection(pos);
  float shift = amplitude * sin(dd.x / length - evolution * TAU);
  return vec2(shift * cos(dd.y), shift * sin(dd.y));
}

void mainImage( out vec4 fragColor, in vec2 fragCoord ) {
  vec2 shift = generateShift(fragCoord.xy / iRenderScale) * iRenderScale;
  fragColor = texture2D( iChannel0, (fragCoord.xy + shift) / iResolution.xy);
}