float ShadowCalculation(vec3 fragPos, uint index, vec3 lightpos)
{
    // get vector between fragment position and light position
    vec3 fragToLight = vec3(inverse(view) * vec4(fragPos, 1.0)) - vec3(inverse(view) * vec4(lightpos, 1.0));
    // ise the fragment to light vector to sample from the depth map
    float closestDepth = texture(depthMap, vec4(fragToLight, index)).r;
    // it is currently in linear range between [0,1], let's re-transform it back to original depth value
    closestDepth *= far_plane;
    // now get current linear depth as the length between the fragment and light position
    float currentDepth = length(fragToLight);
    // test for shadows
    float bias = 0.15; // we use a much larger bias since depth is now in [near_plane, far_plane] range
    float shadow = 0.0;
    // display closestDepth as debug (to visualize depth cubemap)
    // FragColor = vec4(vec3(closestDepth / far_plane), 1.0);
    float samples = 20.0;
    float offset  = 0.1;

    vec3 sampleOffsetDirections[20] = vec3[]
    (
       vec3( 1,  1,  1), vec3( 1, -1,  1), vec3(-1, -1,  1), vec3(-1,  1,  1),
       vec3( 1,  1, -1), vec3( 1, -1, -1), vec3(-1, -1, -1), vec3(-1,  1, -1),
       vec3( 1,  1,  0), vec3( 1, -1,  0), vec3(-1, -1,  0), vec3(-1,  1,  0),
       vec3( 1,  0,  1), vec3(-1,  0,  1), vec3( 1,  0, -1), vec3(-1,  0, -1),
       vec3( 0,  1,  1), vec3( 0, -1,  1), vec3( 0, -1, -1), vec3( 0,  1, -1)
    );
    float diskRadius = 0.05;

    for(int i = 0; i < samples; ++i)
    {
        float closestDepth = texture(depthMap, vec4(fragToLight + sampleOffsetDirections[i] * diskRadius, index)).r;
        closestDepth *= far_plane;   // undo mapping [0;1]
        if(currentDepth - bias > closestDepth)
            shadow += 1.0;
    }
    shadow /= float(samples);
    return shadow;
}