Screen-Space Ambient Occlusion

const float SCENE_FAR = 12.0;
const float CAMERA_FOCAL = 1.8;

float sphereHit(vec3 rayOrigin, vec3 rayDirection, vec3 center, float radius) {
    vec3 offset = rayOrigin - center;
    float b = dot(offset, rayDirection);
    float c = dot(offset, offset) - radius * radius;
    float h = b * b - c;
    if (h < 0.0) return SCENE_FAR;
    float root = sqrt(h);
    float nearHit = -b - root;
    float farHit = -b + root;
    return nearHit > 0.0 ? nearHit : (farHit > 0.0 ? farHit : SCENE_FAR);
}

void traceScene(vec3 rayOrigin, vec3 rayDirection, out float depth, out vec3 normal) {
    float nearest = SCENE_FAR;
    normal = vec3(0.0, 0.0, 1.0);
    if (rayDirection.y < -0.001) {
        float floorHit = (-1.0 - rayOrigin.y) / rayDirection.y;
        if (floorHit > 0.0 && floorHit < nearest) {
            nearest = floorHit;
            normal = vec3(0.0, 1.0, 0.0);
        }
    }
    vec3 centers[3] = vec3[3](
        vec3(-1.05, -0.28, -4.1),
        vec3(0.75, -0.48, -3.25),
        vec3(0.15, 0.35, -5.0)
    );
    float radii[3] = float[3](0.72, 0.52, 0.68);
    for (int i = 0; i < 3; ++i) {
        float hit = sphereHit(rayOrigin, rayDirection, centers[i], radii[i]);
        if (hit < nearest) {
            nearest = hit;
            normal = normalize(rayOrigin + rayDirection * hit - centers[i]);
        }
    }
    if (nearest >= SCENE_FAR) {
        depth = SCENE_FAR;
    } else {
        depth = -(rayOrigin + rayDirection * nearest).z;
    }
}

vec3 cameraRay(vec2 fragCoord) {
    vec2 screen = (2.0 * fragCoord - iResolution.xy) / iResolution.y;
    return normalize(vec3(screen, -CAMERA_FOCAL));
}

vec3 reconstructViewPosition(vec2 uv, float depth) {
    vec2 screen = (2.0 * uv - 1.0) * vec2(iResolution.x / iResolution.y, 1.0);
    return vec3(screen * depth / CAMERA_FOCAL, -depth);
}