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I've been using Unity3D to procedurally generate terrain with Perlin Noise and I've come across a problem where the mesh that I've constructed only renders one set of triangles.

enter image description here

The following is my MeshGeneration code:

using System.Collections;
using System.Collections.Generic;
using System.Runtime.CompilerServices;
using NUnit.Framework.Internal.Execution;
using UnityEngine;

public static class MeshGenerator
{
    public static MeshData GenerateMesh(float[,] heightMap)
    {
        int height = heightMap.GetLength(0);
        int width = heightMap.GetLength(1);
        int vertexIndex = 0;
        
        MeshData meshData = new MeshData(width, height);

        for (int y = 0; y < height; y++)
        {
            for (int x = 0; x < width; x++)
            {
                meshData.vertices[vertexIndex] = new Vector3(x, heightMap[y, x], y);
                meshData.uvs[vertexIndex] = new Vector2( x/(float)width, y/(float)height);
                 
                // If we are not on the edge, then add two triangles to the mesh
                if ((x != width - 1) && (y != height - 1))
                {
                    meshData.AddTriangle(
                        vertexIndex,
                        vertexIndex + width,
                        vertexIndex + width + 1
                    );
                    meshData.AddTriangle(
                        vertexIndex,
                        vertexIndex + 1,
                        vertexIndex + width + 1
                    );
                }
                
                vertexIndex++;
            }
        }

        return meshData;
    }
}

public class MeshData
{
    public Vector3[] vertices;
    public Vector2[] uvs;
    public int[] triangles;

    public int triangleIndex;
    public MeshData(int meshWidth, int meshHeight)
    {
        vertices = new Vector3[meshWidth * meshHeight];
        uvs = new Vector2[meshWidth * meshHeight];
        triangles = new int[(meshWidth - 1) * (meshHeight - 1) * 6];
    }

    public void AddTriangle(int a, int b, int c)
    {
        triangles[triangleIndex] = a;
        triangles[triangleIndex + 1] = b;
        triangles[triangleIndex + 2] = c;
        triangleIndex += 3;
    }

    public Mesh CreateMesh()
    {
        Mesh mesh = new Mesh();
        mesh.vertices = this.vertices;
        mesh.uv = this.uvs;
        mesh.triangles = this.triangles;
        
        mesh.RecalculateNormals();
        return mesh;
    }
}

I'm then passing in the mesh that I get from MeshData.CreateMesh() into the following function.

public void BuildMesh(MeshData meshData, Texture2D texture)
    {
        meshFilter.sharedMesh = meshData.CreateMesh();
        meshRenderer.sharedMaterial.mainTexture = texture;
    }

I'm following this tutorial: https://www.youtube.com/watch?v=4RpVBYW1r5M&list=PLFt_AvWsXl0eBW2EiBtl_sxmDtSgZBxB3&index=5

The Mesh generation code works by creating arrays of vertices, uvs, and triangles, and the populating them by iterating over a Vector3[] heightMap that I created with perlin noise.

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Solved the problem myself. The remaining triangles were being generated on the other side of the mesh. This was I didn't know that the order of the vertices had to be in clockwise order.

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