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what's the difference between mesh and submesh


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I'm reading the source code, and I'm a bit confused about the difference between mesh and submesh.

So when I call `Mesh.createGround` I created a Mesh(with transform matrix, geometry and material), right? But what is submesh for?

Is that when I set the parent mesh (`groundMesh.parent = sphereMesh`, eg), the groundMesh became a submesh? I'm not sure.

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3 hours ago, guanglei said:

Is that when I set the parent mesh (`groundMesh.parent = sphereMesh`, eg), the groundMesh became a submesh?

I would say no - by setting the sphereMesh as the parent groundMesh of you have linked the two meshes as parent-child. Now when the parent moves/rotates the child will also move/rotate.  In this case, if you move the sphere (ie: sphere.position.x += 2) then the child mesh (ground) will move with it.

Since you are asking about creating ground - submesh can be used for multi materials.  Here are good links to show what submeshes are:
https://doc.babylonjs.com/features/materials (scroll down to "More Than One Material on a Mesh" for Playground samples)

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A mesh has 1 to n submeshes, which divide its vertex data to n parts. This is useful for many cases (from collision to Octree, to MultiMatrials and co).

When you create a ground mesh, a mesh is being created, and inside of this mesh, a new submesh is created. 

You can see from the signature:

constructor(public materialIndex: number, public verticesStart: number, public verticesCount: number, public indexStart: number, public indexCount: number, mesh: AbstractMesh, renderingMesh?: Mesh, createBoundingBox: boolean = true);

It is receiving information (index-data) of existing vertex data, but has no internal information of its own.

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