呑み込む闇に抗う光

float hash(vec2 p) {
    p = fract(p * vec2(123.34, 456.21));
    p += dot(p, p + 45.32);
    return fract(p.x * p.y);
}

float noise(vec2 p) {
    vec2 i = floor(p);
    vec2 f = fract(p);
    f = f * f * (3.0 - 2.0 * f);
    float a = hash(i);
    float b = hash(i + vec2(1.0, 0.0));
    float c = hash(i + vec2(0.0, 1.0));
    float d = hash(i + vec2(1.0, 1.0));
    return mix(mix(a, b, f.x), mix(c, d, f.x), f.y);
}

float fbm(vec2 p) {
    float v = 0.0;
    float a = 0.5;
    for (int i = 0; i < 5; i++) {
        v += a * noise(p);
        p = p * 2.1 + vec2(13.7, 7.3);
        a *= 0.5;
    }
    return v;
}

void mainImage(out vec4 fragColor, in vec2 fragCoord) {
    vec2 uv = (fragCoord - 0.5 * iResolution.xy) / iResolution.y;
    float t = iTime;

    float r = length(uv);
    float ang = atan(uv.y, uv.x);

    // 重力レンズ: 中心に近いほど空間が歪む
    float lens = 0.18 / (r + 0.08);
    vec2 wuv = uv * (1.0 + lens * 0.35);
    float wr = length(wuv);
    float wang = atan(wuv.y, wuv.x);

    vec3 col = vec3(0.0);

    // レンズで渦巻く星空
    float swirl = ang + lens * 2.2 + t * 0.02;
    vec2 sp = vec2(cos(swirl), sin(swirl)) * r;
    vec2 grid = sp * 28.0;
    vec2 gid = floor(grid);
    float star = hash(gid);
    float sd = length(fract(grid) - 0.5);
    float twinkle = 0.5 + 0.5 * sin(t * 3.0 + star * 40.0);
    float stars = smoothstep(0.12, 0.0, sd) * step(0.93, star) * twinkle;
    col += vec3(0.7, 0.8, 1.0) * stars * smoothstep(0.16, 0.4, r);

    // 星雲
    float neb = fbm(uv * 3.0 + vec2(t * 0.03, -t * 0.02));
    col += mix(vec3(0.02, 0.01, 0.06), vec3(0.1, 0.03, 0.16), neb) * smoothstep(0.15, 0.6, r);

    // 降着円盤: 渦巻く高温ガス
    float spiral = sin(wang * 3.0 - t * 1.6 + 12.0 / (wr + 0.25));
    float diskBand = exp(-pow((wr - 0.34) * 7.0, 2.0));
    float diskNoise = fbm(vec2(wang * 2.0 + t * 0.4, wr * 9.0 - t * 0.9));
    float disk = diskBand * (0.6 + 0.5 * spiral) * (0.5 + 0.8 * diskNoise);
    vec3 hot = mix(vec3(1.0, 0.45, 0.1), vec3(0.4, 0.7, 1.0), smoothstep(-1.0, 1.0, sin(wang - t * 0.7)));
    col += hot * disk * 2.2;

    // 抗う光: 引力に逆らって外へ走る光条
    float rayPulse = 0.5 + 0.5 * sin(t * 2.4);
    float rays = pow(abs(sin(ang * 7.0 + t * 1.3 + fbm(vec2(ang * 3.0, t * 0.5)) * 4.0)), 18.0);
    float rayReach = smoothstep(0.95, 0.18, r) * smoothstep(0.14, 0.3, r);
    col += vec3(1.0, 0.9, 0.6) * rays * rayReach * (0.6 + 0.9 * rayPulse);

    // 光子リング
    float ring = exp(-pow((r - 0.17) * 60.0, 2.0));
    col += vec3(1.0, 0.85, 0.55) * ring * 2.5;

    // 事象の地平面: すべてを呑み込む
    float horizon = smoothstep(0.165, 0.13, r);
    col = mix(col, vec3(0.0), horizon);

    // 吸い込まれる青い微光
    float infall = exp(-r * 3.0) * (0.5 + 0.5 * sin(ang * 2.0 - t * 3.0));
    col += vec3(0.15, 0.25, 0.6) * infall * (1.0 - horizon);

    // 周辺減光
    col *= 1.0 - 0.4 * smoothstep(0.6, 1.2, r);

    // トーンマップ
    col = col / (1.0 + col);
    col = pow(col, vec3(0.85));

    fragColor = vec4(col, 1.0);
}