vec2 Cover(vec2 uv, vec2 screenSize, vec2 imageSize) {
  vec2 s = screenSize;
  vec2 i = imageSize;

  float rs = s.x / s.y;
  float ri = i.x / i.y;

  vec2 new = rs < ri ? vec2(i.x * s.y / i.y, s.y) : vec2(s.x, i.y * s.x / i.x);
  vec2 offset = (rs < ri ? vec2((new.x - s.x) / 2.0, 0.0) : vec2(0.0, (new.y - s.y) / 2.0)) / new;
  vec2 st = uv * s / new + offset;

  return st;
}

  // Use this new set of UVs to create the texture.
  // 
  // Notice that we're using the original vUv variable here.
  // That's because the 'Cover' function expect a set of UVs that
  // go from 0 to 1.
 //  vec2 coverUV = Cover(vUv, uResolution, uTexture0Size);
 // vec4 tex0 = texture2D(uTexture0, coverUV);