Sphere Lambert

[WoodNeck]

#define FLT_MAX 3.402823466e+38
precision mediump float;
uniform vec4 uScreenSize;
uniform float uTime;

float gSeed = 0.;

struct ray {
    vec3 o, d;
};
struct record {
    float t;
    vec3 p, n;
};
struct hitable {
    vec3 c;
    float r;
};

float rand(vec2 co) {
    return fract(sin(dot(co.xy ,vec2(12.9898,78.233))) * 43758.5453) * 2. - 1.;
}

vec3 randomInUnitSphere(float sd) {
    return vec3(
        rand(vec2(sd += .1, sd += .1)),
        rand(vec2(sd += .1, sd += .1)),
        rand(vec2(sd += .1, sd += .1))
    );
}

bool sphere(hitable sp, ray r, float tMin, float tMax, out record hr) {
    vec3 oc = r.o - sp.c;
    float a = dot(r.d, r.d);
    float b = dot(oc, r.d);
    float c = dot(oc, oc) - sp.r * sp.r;

    float d = b * b - a * c;
    if (d < 0.) return false;

    float s = sqrt(d);

    float val = (-b - s) / a;
    if (val > tMin && val < tMax) {
        hr.p = r.o + val * r.d;
        hr.n = (hr.p - sp.c) / sp.r;
        hr.t = val;
        return true;
    }
    val = (-b + s) / a;
    if (val > tMin && val < tMax) {
        hr.p = r.o + val * r.d;
        hr.n = (hr.p - sp.c) / sp.r;
        hr.t = val;
        return true;
    }

    return false;
}

bool worldHit(ray r, float tMin, float tMax, hitable[2] sp, out record rec) {
    bool hit = false;

    hit = hit || sphere(sp[0], r, tMin, tMax, rec);
    hit = hit || sphere(sp[1], r, tMin, tMax, rec);

    return hit;
}

vec3 rayDirection(float fov, vec2 size, vec2 fragCoord) {
    vec2 xy = fragCoord - size / 2.0;
    float z = size.y / tan(radians(fov) / 2.0);

    return normalize(vec3(xy, -z));
}

vec3 color(ray r, hitable[2] sp, float sd) {
    const int MAX_RECURSION = 20;
    float tMax = FLT_MAX;
    vec3 col = vec3(1);

    record hr;

    for (int i = 0; i < MAX_RECURSION; i++) {
        if (worldHit(r, 0.001, FLT_MAX, sp, hr)) {
            col *= .5;

            vec3 rd = normalize(hr.p + hr.n + randomInUnitSphere(sd));
            r.o = hr.p;
            r.d = rd;
        } else {
            float t = .5 * r.d.y + .5;
            col *= mix(vec3(1), vec3(.5, .7, 1.), t);
            return col;
        }
    }
    return col;
}

void main() {
    vec3 eye = vec3(0., 0., 5.);
    const int ns = 100;

    hitable spheres[2];
    spheres[0] = hitable(vec3(0, 0, -1), 1.);
    spheres[1] = hitable(vec3(0, -101, -1.), 100.);

    float sd = rand(gl_FragCoord.xy);
    vec3 col = vec3(0, 0, 0);
    float ins = 1. / float(ns);
    for (int i = 0; i < ns; i++) {
        float rv = rand(gl_FragCoord.xy + float(i) * ins);
        vec3 rd = rayDirection(45., uScreenSize.xy, gl_FragCoord.xy + rv);
        ray r = ray(eye, rd);
        col += color(r, spheres, sd);
        sd += 0.1;
    }
    col = col * ins;
    col = vec3(sqrt(col[0]), sqrt(col[1]), sqrt(col[2]));
    gl_FragColor = vec4(col, 1);
}