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#ifdef GL_ES
        precision highp float;
#endif

uniform float exposure;
uniform float decay;
uniform float density;
uniform float weight;
uniform sampler2D depthTexture;
uniform mat4 u_invMVP;

varying vec2 v_sphereCenterScreenSpace;
varying vec4 v_position;
varying vec3 v_normal;

uniform vec3 u_sphereCenter;

uniform float u_sphereRadius;

const float NUM_SAMPLES = 80;

varying vec3 v_worldPos;
varying vec2 v_texCoord0;

// unpack float-packed depth
float unpack (vec4 colour)
{
    const vec4 bitShifts = vec4(1.0 / (256.0 * 256.0 * 256.0),
                                1.0 / (256.0 * 256.0),
                                1.0 / 256.0,
                                1);

    return dot(colour , bitShifts);
}

// calculate god ray from depth map
float getShadow(vec2 texCoord)
{
    float sample = unpack(texture2D(depthTexture, texCoord));

    return step(0.9999, sample);

    vec4 sampleWorldPos = u_invMVP * vec4(texCoord * 2 - 1, sample * 2 - 1, 1);

    float dist = distance(sampleWorldPos.xyz / sampleWorldPos.w, u_sphereCenter);

    return smoothstep(0, 50, dist);
}

void main()
{
    // get depth value at screen-space fragment position
    vec2 normalizedScreenCoords = (vec2(v_position.x, v_position.y) / v_position.w) / 2.0 + 0.5;

    vec2 textureSphereCenter = v_sphereCenterScreenSpace / 2.0 + 0.5;

    vec2 direction = normalizedScreenCoords - textureSphereCenter;

    float distance = length(direction);

    vec2 directionNormal = normalize(direction);

    float normalizedScreenDepth = v_position.z / v_position.w;

    // normalize interpolated normal
    vec3 N = normalize(v_normal);

    float accumulatedGodRay = 0;

    for (float i = 0; i < NUM_SAMPLES; i++)
    {
        accumulatedGodRay += 1.0 - getShadow(normalizedScreenCoords - (i / NUM_SAMPLES) * directionNormal
            * distance);
    }

    accumulatedGodRay /= NUM_SAMPLES;

    gl_FragColor = vec4(pow(accumulatedGodRay,1))  * pow(unpack(texture2D(depthTexture, normalizedScreenCoords)), 4);

}