#version 330 core
in vec2 vTex;
out vec4 FragColor;

uniform sampler2D occlusionTex;
uniform vec2 lightScreenPos; // in [0,1]
uniform vec3 sunColor;
uniform float sunIntensity;
uniform float globalIntensity;
uniform int samples;
uniform float density;
uniform float weight;
uniform float decay;

void main()
{
    vec2 texCoord = vTex;
    // vector from current pixel to light
    vec2 delta = lightScreenPos - texCoord;
    float dist = length(delta);
    vec2 stepv = delta * (1.0 / float(samples)) * density;

    vec3 illumination = vec3(0.0);
    float illumDecay = 1.0;

    vec2 coord = texCoord;
    for (int i = 0; i < samples; ++i)
    {
        coord += stepv;
        vec3 sample = texture(occlusionTex, coord).rgb;
        // occlusion tex stores white where geometry present
        float occ = sample.r;
        illumination += occ * illumDecay * weight;
        illumDecay *= decay;
    }

    vec3 result = illumination * sunColor * sunIntensity * globalIntensity;
    // tone-map / clamp to avoid extreme brightness
    result = clamp(result, vec3(0.0), vec3(1.0));
    // use luminance as alpha so we can composite less aggressively
    float lum = dot(result, vec3(0.299, 0.587, 0.114));
    float alpha = clamp(lum, 0.0, 1.0);
    FragColor = vec4(result, alpha);
}