Minecraft Ray Scattering

// Based on Fabien Sanglard's Light Scattering shader
// http://www.fabiensanglard.net/lightScattering/index.php
// Edited (unsuccessfully) for Minecraft by Uhyve

uniform sampler2D sampler0;
uniform sampler2D sampler1;
uniform sampler2D sampler2;

uniform float displayWidth;
uniform float displayHeight;

uniform vec3 sunVector;

const float exposure = 0.0034;
const float decay = 1.0;
const float density = 0.84;
const float weight = 5.65;
const int NUM_SAMPLES = 100;

void main()
{
    vec4 sun = gl_ProjectionMatrix * vec4(sunVector, 0.0);
    sun.xyz /= sun.w;
    vec2 lightPositionOnScreen = (1.0 + sun.xy)*vec2(displayWidth/2.0, displayHeight/2.0);

    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;
    vec4 crepRen = vec4(0.0, 0.0, 0.0, 1.0);

    for(int i=0; i < NUM_SAMPLES ; i++)
    {
        textCoo -= deltaTextCoord;
            vec4 sample = texture2D(sampler1, textCoo );
        sample *= illuminationDecay * weight;
        crepRen += sample;
        illuminationDecay *= decay;
    }

    crepRen*= exposure;
    crepRen.a = 1.0;

    vec4 sampler = texture2D(sampler0, gl_TexCoord[0].st);
    sampler.a = 1.0;
    gl_FragColor = clamp(sampler + crepRen, 0.0, 1.0);
}