Perlin noise algorithm artefacts

I am tryng to imlpement Perlin 2D noise algorithm. Generally it works well, but it have got some artefacts in it. Here is my code:

#include "lodepng/lodepng.h"
#include <vector>
#include <numeric>
#include <iostream>
#include <algorithm>
#include <random>
#include <ctime>
#include  <glm/glm.hpp>
float lerp(float a, float b, float x)
{
return a + x * (b - a);
}
float fade(float x)
{
return x * x * x * (x * (6 * x - 15) + 10);
}
float gradient(glm::vec2 point, unsigned char seed)
{
glm::vec2 randomVectors[] = {
{ 0.0f, 1.0f },
{ 0.0f, -1.0f },
{ 1.0f, 0.0f },
{ -1.0f, 0.0f }
};
return glm::dot(point, randomVectors[seed % 4]);
}
float perlinNoise(glm::vec2 point, int seed)
{
std::vector<unsigned int> permutations(256);
std::iota(permutations.begin(), permutations.end(), 0);
std::mt19937 randomEngine(seed);
std::shuffle(permutations.begin(), permutations.end(), randomEngine);
for (int i = 0; i < 256; i++)
{
permutations.push_back(permutations[i]);
}
glm::vec2 gridCoords = glm::ivec2(point);
glm::vec2 pointCoords = point - gridCoords;
glm::vec2 distanceCoords = { fade(pointCoords.x), fade(pointCoords.y) };
return (lerp(
lerp(
gradient(pointCoords, permutations[permutations[gridCoords.x] + gridCoords.y]),
gradient({ pointCoords.x - 1, pointCoords.y }, permutations[permutations[gridCoords.x + 1] + gridCoords.y]),
distanceCoords.x
),
lerp(
gradient({ pointCoords.x, pointCoords.y - 1 }, permutations[permutations[gridCoords.x] + gridCoords.y + 1]),
gradient({ pointCoords.x - 1, pointCoords.y - 1 }, permutations[permutations[gridCoords.x + 1] + gridCoords.y + 1]),
distanceCoords.x
),
distanceCoords.y
) + 1) / 2;
}
int main()
{
glm::ivec2 size = { 512, 512 };
float frequency = 16.0f;
std::vector<unsigned char> noiseImage{};
noiseImage.reserve(size.x * size.y);
for (int y = 0; y < size.y; y++)
{
for (int x = 0; x < size.x; x++)
{
noiseImage.push_back(std::round(255 * perlinNoise({ x / (size.x / frequency), y / (size.y / frequency) }, time(nullptr))));
}
}
lodepng::encode("noise.png", noiseImage, size.x, size.y, LodePNGColorType::LCT_GREY);
}

Here is the result. It has got 2 horisontal lines, which are making it not smooth, but I can not see any bugs in my implementation. How can I fix this behaviour? Am I writing the result into image incorrectly, or the mistake is directly in algorithm?

The answer was quite simple: I should generate permutation vector only once. Here is the correct code:

#include "lodepng/lodepng.h"
#include <vector>
#include <numeric>
#include <iostream>
#include <algorithm>
#include <random>
#include <ctime>
#include  <glm/glm.hpp>
float lerp(float a, float b, float x)
{
return a + x * (b - a);
}
float fade(float x)
{
return x * x * x * (x * (6 * x - 15) + 10);
}
float gradient(glm::vec2 point, unsigned char seed)
{
glm::vec2 randomVectors[] = {
{ 0.0f, 1.0f },
{ 0.0f, -1.0f },
{ 1.0f, 0.0f },
{ -1.0f, 0.0f }
};
return glm::dot(point, randomVectors[seed % 4]);
}
float perlinNoise(const std::vector<unsigned int>& permutations, glm::vec2 point)
{
glm::vec2 gridCoords = glm::ivec2(point);
glm::vec2 pointCoords = point - gridCoords;
glm::vec2 distanceCoords = { fade(pointCoords.x), fade(pointCoords.y) };
return (lerp(
lerp(
gradient(pointCoords, permutations[permutations[gridCoords.x] + gridCoords.y]),
gradient({ pointCoords.x - 1, pointCoords.y }, permutations[permutations[gridCoords.x + 1] + gridCoords.y]),
distanceCoords.x
),
lerp(
gradient({ pointCoords.x, pointCoords.y - 1 }, permutations[permutations[gridCoords.x] + gridCoords.y + 1]),
gradient({ pointCoords.x - 1, pointCoords.y - 1 }, permutations[permutations[gridCoords.x + 1] + gridCoords.y + 1]),
distanceCoords.x
),
distanceCoords.y
) + 1) / 2;
}
std::vector<unsigned int> generatePermutations(std::mt19937& randomEngine)
{
std::vector<unsigned int> permutations(256);
std::iota(permutations.begin(), permutations.end(), 0);
std::shuffle(permutations.begin(), permutations.end(), randomEngine);
permutations.resize(512);
std::copy_n(permutations.begin(), 256, permutations.begin() + 256);
return permutations;
}
int main()
{
glm::ivec2 size = { 512, 512 };
float frequency = 16.0f;
std::vector<unsigned char> noiseImage{};
noiseImage.reserve(size.x * size.y);
std::mt19937 randomEngine(time(nullptr));
std::vector<unsigned int> permutations = generatePermutations(randomEngine);
for (int y = 0; y < size.y; y++)
{
for (int x = 0; x < size.x; x++)
{
noiseImage.push_back(std::round(255 * perlinNoise(permutations, { x / (size.x / frequency), y / (size.y / frequency) })));
}
}
lodepng::encode("noise.png", noiseImage, size.x, size.y, LodePNGColorType::LCT_GREY);
}