#version 330 core

in vert_t {
    vec2 tex_coords;
} vert;

out float frag_color;

uniform sampler2D positions;
uniform sampler2D normals;
uniform sampler2D noise;

uniform vec3 samples[64];
uniform mat4 projection;

const vec2 noise_scale = vec2(1600.0 / 4.0, 900.0 / 4.0);

void main()
{
    vec3 position = texture(positions, vert.tex_coords).rgb;
    vec3 normal = texture(normals, vert.tex_coords).rgb;
    vec3 random = texture(noise, vert.tex_coords * noise_scale).rgb;

    vec3 tangent = normalize(random - normal * dot(random, normal));
    vec3 bitangent = cross(normal, tangent);
    mat3 TBN = mat3(tangent, bitangent, normal);

    int nsamples = 64;
    float radius = 0.5;
    float bias = 0.025;
    float occlusion = 0.0;
    for (int i = 0; i < nsamples; i++) {
        vec3 sample = TBN * samples[i];
        sample = position + sample * radius;
        vec4 offset = vec4(sample, 1.0);
        offset = projection * offset;
        offset.xyz /= offset.w;
        offset.xyz = offset.xyz * 0.5 + 0.5;
        float depth = texture(positions, offset.xy).z;
        float range_check = smoothstep(0.0, 1.0, radius / abs(position.z - depth));
        occlusion += (depth >= sample.z + bias ? 1.0 : 0.0) * range_check;
    }
    occlusion = 1.0 - (occlusion / nsamples);
    frag_color = occlusion;
}