precision mediump float;

#endif

​

// Coordinates of the pixel we are computing

varying vec2 vTexCoord;

​

// Variables we set with "setUniform" from our sketch

uniform sampler2D image; // image we are applying grain to

uniform float grainVary; // how much variance in the grain texture

uniform float seed; // Seed for rand() function

uniform float pixD;

​

// The GPU has no random() function, so instead

// do some maths on the position of the pixel to approximate a

// random number. This will be the same each time though

// so you could add another uniform variable that is set

// randomly from JavaScript to change it each time.

float rand(vec2 co, float salt){

    return fract(sin(dot(co, vec2(12.9898, 78.233) + salt + seed)) * 43758.5453);

}

​

void main() {

  // Retreive the colour from the image at

  // the pixel we are computing

  vec2 uv = vTexCoord;

  uv = uv;//pixD;

  uv.y = 1.0 - uv.y;  

  vec4 col = texture2D(image, uv);

  // Use the rand function to create a random number

  // with a different "salt" value for each colour channel

  // pass that into sin() to get something between -1, 1

  // multiply by the grain variance  

  col.r += sin(rand(uv, 0.0)) * grainVary;

  col.g += sin(rand(uv, 1.0)) * grainVary;

  col.b += sin(rand(uv, 2.0)) * grainVary;

  // Modified colour is our output

  gl_FragColor = col;

}