How do I render triangles in a mesh in depth order?

Questions & Answers
1

I’m using Unity to create a 2D game and I’m having a problem with meshes. It seems that triangles in the same mesh are drawn in creation order instead of depth order.

I created a simple example:

public class MeshTest : MonoBehaviour
{
	Mesh mesh;

	void Start ()
	{
		mesh = GetComponent<MeshFilter> ().mesh;
		BuildMesh ();
	}
	
	List<Vector3> newVertices = new List<Vector3>();
	List<int> newTriangles = new List<int>();
	List<Color> newColor = new List<Color>();

	void BuildMesh()
	{
		Vector3 pos = Vector3.zero;

        // Create three quads in the mesh
		GenerateQuad (Vector3.zero, Color.red);
		GenerateQuad (new Vector3(1f, 1f, 1f), Color.yellow);

		GenerateQuad (new Vector3(0.5f, 0.5f, 0.5f), Color.green);
        // The green square has a z coordinate in-between the other two squares


		mesh.Clear ();
		mesh.vertices = newVertices.ToArray();
		mesh.triangles = newTriangles.ToArray();
		mesh.colors = newColor.ToArray ();
		mesh.Optimize ();
		mesh.RecalculateNormals ();
		
		newVertices.Clear ();
		newTriangles.Clear ();
		newColor.Clear ();
	}


	void GenerateQuad(Vector3 topLeft, Color color)
	{
		int triangleIndex = newVertices.Count;
		float x = topLeft.x;
		float y = topLeft.y;
		float z = topLeft.z;
		
		newVertices.Add( new Vector3 (x, y, z) );
		newVertices.Add( new Vector3 (x + 1f, y, z));
		newVertices.Add( new Vector3 (x, y - 1f, z));
		newVertices.Add( new Vector3 (x + 1f, y - 1f, z));
		
		newTriangles.Add(triangleIndex+0);
		newTriangles.Add(triangleIndex+1);
		newTriangles.Add(triangleIndex+2);
		newTriangles.Add(triangleIndex+1);
		newTriangles.Add(triangleIndex+2);
		newTriangles.Add(triangleIndex+3);

		newColor.Add (color);
		newColor.Add (color);
		newColor.Add (color);
		newColor.Add (color);		
	}
}

When I run a scene with just this game object in I see this:

[35900-screen+shot+2014-11-27+at+01.17.35.png|35900]

The green square is on top of the other two when it should be in between them.

Any ideas what I’m doing wrong? Is this the normal behaviour?

Thanks

2

You need to use a shader that writes to the depth buffer. (Also, unrelated, your bottom-right triangles in each quad have the winding order reversed.)