/// “标准答案”：对每个法线方向做余弦加权的半球积分，得到 URP 约定下的间接漫反射（E/π）。
/// 用来和 9 个系数的 SH 近似做对比。
public static Cubemap BakeIrradiance(Func<Vector3, Color> radiance, int size = 32, int samples = 1024)
{
    var cube = new Cubemap(size, TextureFormat.RGBAHalf, false) { name = "Ch08Irradiance", hideFlags = HideFlags.DontSave };
    var px = new Color[size * size];
    for (int f = 0; f < 6; f++)
    {
        for (int y = 0; y < size; y++)
            for (int x = 0; x < size; x++)
            {
                Vector3 n = FaceDirection((CubemapFace)f, (x + 0.5f) / size * 2f - 1f, (y + 0.5f) / size * 2f - 1f);
                Vector3 up = Mathf.Abs(n.y) < 0.999f ? Vector3.up : Vector3.right;
                Vector3 tx = Vector3.Normalize(Vector3.Cross(up, n)), ty = Vector3.Cross(n, tx);
                Color sum = Color.black;
                int sq = Mathf.RoundToInt(Mathf.Sqrt(samples));
                for (int i = 0; i < sq; i++)
                    for (int j = 0; j < sq; j++)
                    {
                        // 分层的余弦加权采样：pdf = cosθ/π，正好与积分里的 cosθ/π 抵消，结果就是简单平均
                        float u1 = (i + 0.5f) / sq, u2 = (j + 0.5f) / sq;
                        float r = Mathf.Sqrt(u1), phi = 2f * Mathf.PI * u2;
                        Vector3 l = tx * (r * Mathf.Cos(phi)) + ty * (r * Mathf.Sin(phi)) + n * Mathf.Sqrt(1f - u1);
                        sum += radiance(l);
                    }
                px[y * size + x] = sum / (sq * sq);
            }
        cube.SetPixels(px, (CubemapFace)f);
    }
    cube.Apply(false, false);
    return cube;
}