Light Rays 0.6 - Fabien Sanglard's light ray shader

rays.c

 uniform float exposure;
 uniform float decay;
 uniform float density;
 uniform float weight;
 uniform vec2 lightPositionOnScreen;
 uniform sampler2D firstPass;
 const int NUM_SAMPLES = 100 ;

 void main() {	
 	vec2 deltaTextCoord = vec2( gl_TexCoord[0].st - lightPositionOnScreen.xy );
 	vec2 textCoo = gl_TexCoord[0].st;
 	deltaTextCoord *= 1.0 /  float(NUM_SAMPLES) * density;
 	float illuminationDecay = 1.0;

	for(int i=0; i < NUM_SAMPLES ; i++) {
  			textCoo -= deltaTextCoord;
    			vec4 sample = texture2D(firstPass, textCoo );
    			sample *= illuminationDecay * weight;
     			gl_FragColor += sample;
   			illuminationDecay *= decay;
 	}
 	gl_FragColor *= exposure;
}