Drawing a terrain mesh by script

Question

I am trying to draw some terrain with scripting. However I get strange results and cannot seem to pinpoint the issue.

At the Start method I'm building my initial mesh.

    Vector3[] verts = new Vector3[mapWidth * mapHeight * 6];
    Vector2[] uvs = new Vector2[mapWidth * mapHeight * 6];
    int[] triangles = new int[mapWidth * mapHeight * 6];

    for (int y = 0; y < height; y++)
    {
        for (int x = 0; x < width; x++)
        {
            verts[x * y * 6 + 0] = new Vector3(x, 0, y);
            verts[x * y * 6 + 1] = new Vector3(x + 1, 0, y);
            verts[x * y * 6 + 2] = new Vector3(x + 1, 0, y + 1);

            verts[x * y * 6 + 3] = new Vector3(x, 0, y);
            verts[x * y * 6 + 4] = new Vector3(x + 1, 0, y + 1);
            verts[x * y * 6 + 5] = new Vector3(x, 0, y + 1);

            uvs[x * y * 6 + 0] = new Vector2(0, 0);
            uvs[x * y * 6 + 1] = new Vector2(1, 0);
            uvs[x * y * 6 + 2] = new Vector2(1, 1);

            uvs[x * y * 6 + 3] = new Vector2(0, 0);
            uvs[x * y * 6 + 4] = new Vector2(1, 1);
            uvs[x * y * 6 + 5] = new Vector2(0, 1);

            for (int t = 0; t < 6; t++)
            {
                triangles[x * y * 6 + t] = x * y * 6 + 5 - t;
            }
        }
    }

    terrain.vertices = verts;
    terrain.uv = uvs;
    terrain.triangles = triangles;
    terrain.RecalculateNormals();

All I do to draw my mesh is Graphics.DrawMesh(terrain, Vector3.zero, Quaternion.identity, mat, 1);. Material(mat) is just a simple grid texture and seems to be working correctly. However Unity is deciding to not draw triangle couples at seemingly random.

The gizmo is at 0, 0, 0 and each run it renders the same shape. I need to be drawing a 10 x 10 shape. I am creating duplicate vertices because I need hard edges eventually.

Answer 1

The problem with the way you do it are your indices. The formula for "flattening" a 2D array is x + y * width (or x * height + y), not x * y.

IMO the best way to approach such issues is to manually calculate values that work and dont work as expected and see if you find a pattern.

In your case it seems (0|0) does not work, while (0|9) does work, lets see why.

So if we use x = 0 and y = 0, we get verts[0 * 0 * 6 + num] = 0 + num. Now we do it for x = 0, y = 9 and alas verts[0 * 9 * 6 + num] still is 0 + num. Since (0|9) is calculated later, the problem is that the verts, uvs, and triangles of (0|0) are overwritten by those values - the same is the problem for everything inbetween, like for example, 3 * 5 is the same as 5 * 3.

So to fix your code, the following should work.

    Vector3[] verts = new Vector3[mapWidth * mapHeight * 6];
    Vector2[] uvs = new Vector2[mapWidth * mapHeight * 6];
    int[] triangles = new int[mapWidth * mapHeight * 6];

    for (int y = 0; y < height; y++) {
        for (int x = 0; x < width; x++) {

            int tileIndex = x + y * width;

            verts[tileIndex * 6 + 0] = new Vector3(x, 0, y);
            verts[tileIndex * 6 + 1] = new Vector3(x + 1, 0, y);
            verts[tileIndex * 6 + 2] = new Vector3(x + 1, 0, y + 1);

            verts[tileIndex * 6 + 3] = new Vector3(x, 0, y);
            verts[tileIndex * 6 + 4] = new Vector3(x + 1, 0, y + 1);
            verts[tileIndex * 6 + 5] = new Vector3(x, 0, y + 1);

            uvs[tileIndex * 6 + 0] = new Vector2(0, 0);
            uvs[tileIndex * 6 + 1] = new Vector2(1, 0);
            uvs[tileIndex * 6 + 2] = new Vector2(1, 1);

            uvs[tileIndex * 6 + 3] = new Vector2(0, 0);
            uvs[tileIndex * 6 + 4] = new Vector2(1, 1);
            uvs[tileIndex * 6 + 5] = new Vector2(0, 1);

            for (int t = 0; t < 6; t++) {
                triangles[tileIndex * 6 + t] = tileIndex * 6 + 5 - t;
            }
        }
    }

    terrain.vertices = verts;
    terrain.uv = uvs;
    terrain.triangles = triangles;
    terrain.RecalculateNormals();