/*
*   Fragment shader for a "film grain" function. Applies a random amount of
*       noise to every pixel; intended to be used on a quad drawn over the
*       screen (but could be used, say, for the surface of a TV in a scene).
*       Very simple and stupid; uses a popular pseudorandom number generator.
*       Much room for improvement.
*
*   Alexandri Zavodny, Fall 2010 
*       for CSE 40166/60166 Computer Graphics, University of Notre Dame
*/

uniform float time;
varying vec4 vertexInImageSpace;

float rand(vec2 co)
{
    return fract(sin(dot(co.xy ,vec2(12.9898,78.233))) * 43758.5453);
}

void main()
{
    //just get a random number, using the vertex's screen-space position as a seed along with the time.
    float r = rand(vec2(vertexInImageSpace.x/2.0 + sin(time), vertexInImageSpace.y/2.0 + cos(time)));

    //play with the noise a bit; clamp it based on its value and scale the alpha.
    if(r < 0.75) r = 0.0;
    vec4 finalColor = vec4(r,r,r,r*0.3);

    gl_FragColor = clamp(finalColor, 0.0, 1.0);
}