precision mediump float;
varying vec2 v_texCoord;
uniform sampler2D u_texture;
uniform vec2 u_resolution;
uniform float u_amplitude; // px
uniform float u_frequency; // waves across image
uniform int   u_direction; // 0=horizontal, 1=vertical, 2=radial

void main() {
  vec2 uv = v_texCoord;
  float amp = u_amplitude / u_resolution.x;
  float freq = u_frequency * 6.28318;
  float offset = 0.0;