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Advice on Mesh class design


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#1 AvengerDr   Members   -  Reputation: 751

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Posted 19 July 2013 - 05:25 AM

Hi all,

I'd like some advice on the design of a mesh class. Currently I have a Mesh class that holds instance data such as local Position, Orientation, etc. I also have a Geometry class which holds the low-level data such as buffers, array of vertices/indices, etc. Geometry objects are unique and there might be a 1:N relationship with mesh objects. Meshes can then be part of a scene tree used to chain transformations.

 

Everything works as long as each model imported consists exactly of one submesh. Should I implement a MeshGroup class having an array of Geometry objects? Another approach that I was thinking of was to let each complex object specify the number of Meshes it needs (each Mesh would be a different field variable). Then for rendering purposes, each class would specify how to properly build a scene tree branch holding the different mesh parts.

 

I am aware that in the long run it is probably better to fuse static meshes together in a bigger VBO, but for the moment I am not even close to needing this kind of optimization. 


--Avengers UTD Chronicles - My game development blog

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#2 L. Spiro   Crossbones+   -  Reputation: 14215

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Posted 19 July 2013 - 06:10 AM

As I have just replied in "Should game objects render themselves, or should an object manager render them?" I have already answered the essence of this question in such a large amount of detail that it should be usable for many future references.

 

One thing is that “position” is part of COrientation.

 

The other part is, what exactly is a “scene tree”?  A “scene graph”?

I am assuming this is what you mean when I reply.

 

Imported models consisting of exactly one sub-mesh is your main problem here.

Not in the least because a model consists of many meshes.

 

But the rest of your question suggests you are trying to use the same tree for both models and the meshes they own.

This is your biggest problem.

 

Think of models are a tree of meshes, and think of a scene as a tree of models.

 

They are 2 separate trees.  This not only improves performance but just makes things logical.  We aren’t even talking about fusing static meshes into a big mesh.  It’s just as simple as a model having its own tree and the scene having a tree of models.  Only when the model’s tree needs updating is it ever updated, hence large portions of tree-updating can be skipped on static models or sleeping physics models.

 

 

L. Spiro


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#3 AvengerDr   Members   -  Reputation: 751

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Posted 19 July 2013 - 07:39 AM

Thanks for the reply, yes I have been reading your posts, they have been very insightful. I want to clarify some aspects: the limitations on one submesh per model is just temporary. I have been experimenting with regular teapots and procedurally generated solid primitives, but of couse the engine needs to handle that situation.

 

Regarding the scene tree vs graph, from my understanding of data structures, a tree has to have only one root node. A graph does not have this restriction. Furthermore a tree can't have as children nodes that jump levels in the hierarchy. I.e. node A can't have as child node B who is a sibling of A's parent. There is also only one parent for each node. So for all logical purposes I'm using a scene tree and not a graph. Isn't this how it is supposed to work? What advantages would a graph have over a hierarchical tree? In my current implementation the absolute world matrix of any object belonging to the tree is computed by traversing the tree from the root to the leaf node. 

 

A tree of meshes is an interesting concept. Yes it makes sense. I didn't think about that because as I said I was only dealing with very simple models but now that I think of it, it makes sense as that is the logical representation one would expect to find in a model. So would it be fine to have a method that allows this tree of meshes to convert itself to an appropriate branch to be inserted in the scene tree?

 

I'm using my tree just as a logical representation of the objects in the scene. It is not used for rendering. It is only needed to update the world matrices of the objects in the scene. I have a CommandManager class that is invoked to compute the most efficient way of rendering the scene. The first time it traverses through the scene tree putting each mesh in a bin according to its material. Then each (flattened) collection of meshes is attached to a RenderCommand (and specialized versions such as InstancingRenderCommand) which renders all objects having the exact same characteristics. The same class also checks for state changes and issues state change commands if needed. Each material defines what the collection of objects it will render should look like. I.e. their vertex layout, the blend/rasterizer/depthStencil state it expects, the shadow algorithm associated with it (if any) and so on. 

 

To display the scene I call CommandManager.Run() and it goes through its linked list firing all commands. Every objects ultimately draws itself. My approach is to have all similar objects drawn together in order to minimize state changes, buffer rebindings and so on.


--Avengers UTD Chronicles - My game development blog




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