How to apply a texture to a submesh

Question

I created a cube mesh because I need to apply two materials to it : one on 5 faces (basic colored material) and one on the remaining face (textured with an image)

Here's my code to create the submesh and apply the materials.

 public class CubeMesh : MonoBehaviour
 {
     public float size;
 
     public Material[] materials;
     // Start is called before the first frame update
     void Start()
     {
         createCube();
     }
 
     // Update is called once per frame
     void createCube()
     {
         Vector3[] vertices = {
             new Vector3(0, 0, 0)*size,
             new Vector3(1, 0, 0)*size,
             new Vector3(1, 1, 0)*size,
             new Vector3(0, 1, 0)*size,
             new Vector3(0, 1, 1)*size,
             new Vector3(1, 1, 1)*size,
             new Vector3(1, 0, 1)*size,
             new Vector3(0, 0, 1)*size,
         };
 
         int[] triangles = {
             0, 2, 1, //face front
             0, 3, 2,
             2, 3, 4, //face top
             2, 4, 5,
             1, 2, 5, //face right
             1, 5, 6,
             0, 7, 4, //face left
             0, 4, 3,
             5, 4, 7, //face back
             5, 7, 6,
             0, 6, 7, //face bottom
             0, 1, 6
         };
         print(triangles.Length);
 
         Mesh mesh = new Mesh();
         GetComponent<MeshFilter>().mesh = mesh;
 
         mesh.vertices = vertices;
         mesh.triangles = triangles;
 
         mesh.subMeshCount = 2;

         int[] subTriangles = new int[6];    // Front face submesh
         Array.Copy(triangles, 0, subTriangles, 0, 6);
         mesh.SetTriangles(subTriangles, 0);
 
         subTriangles = new int[30];
         Array.Copy(triangles, 6, subTriangles, 0, 30);
         mesh.SetTriangles(subTriangles, 1);
 
 
         mesh.RecalculateNormals();
 
         Renderer r = GetComponent<Renderer>();
         r.materials = materials;
     }
 }

I made the textured material by dropping the image in the Albedo grey square. It's working on regular cubes. However, after assigning the script to a GameObject and setting the materials, I cannot see the image on the face of the cube. I only get a white texture (the image is a logo which is mostly white)

Is there something wrong in what I'm doing?

Answer 1

You haven't created any UV coordinates for your vertices. So there's no texture mapping taking place. By default all vertices would have a UV coordinate of 0,0 so all faces would be mapped to the lower left corner of your image.

Also your mesh doesn't represent a cube. It's more like a lowpoly sphere since you have shared vertices everywhere. The vertices at the corners need to be split into 3, one for each face that meets at that corner. That's why a cube mesh needs 24 vertices instead of 8. You have 6 faces and each face needs 4 seperate vertices. In order to have a cube look like a cube the 4 vertices of a face need to have all the same vertex normal. However if you only have 8 vertices in total, you can not assign 3 different normals to the same vertex. Likewise the UV coordinates. To map a single face to a certain portion of a texture you need to specify a specific UV coordinate at each of the 4 corners. However if you share vertices between two or more faces you can only specify one UV coordinate which would be shared between all faces. This won't work.

Creating a UV mapped cube mesh procedurally is of course possible, but it's a bit more work as what you've done here.

The code example I've posted over here doesn't really create a cube but the frustom of a camera which is just a deformed cube. Though the concept would be similar. I also started with the initial 8 vertices but have created that "m_VertOrder" array which creates the 24 required vertices out of the initial 8. Though I do not generate any UVs in this example since there is not only one valid way how to unwrap a cube.

If you want to create an unwrapped cube (with UV coordinates) it might be simpler to just create each face one by one.

How to apply a texture to a submesh

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