// http://byteblacksmith.com/improvements-to-the-canonical-one-liner-glsl-rand-for-opengl-es-2-0/
highp float rand(mediump vec2 co)
{
    highp float a = 12.9898;
    highp float b = 78.233;
    highp float c = 43758.5453;
    highp float dt= dot(co.xy, vec2(a,b));
    highp float sn= mod(dt, 3.14);
    return fract(sin(sn) * c);
}