Perlin algorithm generates weird artifacts

Question

I'm writing a Perlin noise generator in C++ for terrain generation but the generator produces weird JPEG-like artifacts instead of proper noise.

I've tried doing it using C# in Unity and confirmed the results are the same. When I swap the generator for Unity's, it generates correctly. The FBM function is working properly with the Unity implementation. It must be the Perlin noise itself then. This is the code:

public class Perlin
{
    private static float Grad(int hash, float x, float y)
    {
        return ((hash & 1) == 0 ? x : -x) + ((hash & 2) == 0 ? y : -y);
    }
    private static float Fade(float v)
    {
        return v * v * v * (v * (v * 6.0f - 15.0f) + 10.0f);
    }
    public static float ImprovedPerlin(float x, float y)
    {
        int X = (int)Mathf.Floor(x) & 255;
        int Y = (int)Mathf.Floor(y) & 255;

        x -= Mathf.Floor(x);
        y -= Mathf.Floor(y);

        float U = Fade(x);
        float V = Fade(y);

        int A = (Perms[X] + Y) & 255;
        int B = (Perms[X + 1] + Y) & 255;

        return Mathf.Lerp(V,
                Mathf.Lerp(U,
                    Grad(Perms[A], x, y),
                    Grad(Perms[B], x - 1.0f, y)
                ),
                Mathf.Lerp(U,
                    Grad(Perms[A + 1], x, y - 1.0f),
                    Grad(Perms[B + 1], x - 1.0f, y - 1.0f)
                )
            );
    }

    private static int[] Perms = new int[257] {
        151, 160, 137,  91,  90,  15, 131,  13, 201,  95,  96,  53, 194, 233,   7, 225,
        140,  36, 103,  30,  69, 142,   8,  99,  37, 240,  21,  10,  23, 190,   6, 148,
        247, 120, 234,  75,   0,  26, 197,  62,  94, 252, 219, 203, 117,  35,  11,  32,
        57, 177,  33,  88, 237, 149,  56,  87, 174,  20, 125, 136, 171, 168,  68, 175,
        74, 165,  71, 134, 139,  48,  27, 166,  77, 146, 158, 231,  83, 111, 229, 122,
        60, 211, 133, 230, 220, 105,  92,  41,  55,  46, 245,  40, 244, 102, 143,  54,
        65,  25,  63, 161,   1, 216,  80,  73, 209,  76, 132, 187, 208,  89,  18, 169,
        200, 196, 135, 130, 116, 188, 159,  86, 164, 100, 109, 198, 173, 186,   3,  64,
        52, 217, 226, 250, 124, 123,   5, 202,  38, 147, 118, 126, 255,  82,  85, 212,
        207, 206,  59, 227,  47,  16,  58,  17, 182, 189,  28,  42, 223, 183, 170, 213,
        119, 248, 152,   2,  44, 154, 163,  70, 221, 153, 101, 155, 167,  43, 172,   9,
        129,  22,  39, 253,  19,  98, 108, 110,  79, 113, 224, 232, 178, 185, 112, 104,
        218, 246,  97, 228, 251,  34, 242, 193, 238, 210, 144,  12, 191, 179, 162, 241,
        81,  51, 145, 235, 249,  14, 239, 107,  49, 192, 214,  31, 181, 199, 106, 157,
        184,  84, 204, 176, 115, 121,  50,  45, 127,   4, 150, 254, 138, 236, 205,  93,
        222, 114,  67,  29,  24,  72, 243, 141, 128, 195,  78,  66, 215,  61, 156, 180,
        151
    };
}

Answer 1

The comment provided by DMGregory solved the issue here:

Did you mean to use U & V as the third parameter of your lerps (the blend factor), rather than as one of the endpoints of the interpolation?

The corrected blending of the ImprovedPerlin function would look like this:

// ...
return Mathf.Lerp(Mathf.Lerp(
                    Grad(Perms[A], x, y),
                    Grad(Perms[B], x - 1.0f, y)
                    U,
                ),
                Mathf.Lerp(
                    Grad(Perms[A + 1], x, y - 1.0f),
                    Grad(Perms[B + 1], x - 1.0f, y - 1.0f),
                    U,
                ),
                V
            );
// ...

Answer 2

The comment provided by DMGregory solved the issue here:

Did you mean to use U & V as the third parameter of your lerps (the blend factor), rather than as one of the endpoints of the interpolation?