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OpenGL Model Loading Performance

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I'm wondering if I'm loading a model in OpenGL to be used in a VBO, is it better to: - read vertex normal data from the model file OR - calculate the normals at runtime If I do go about loading the vertex normals from file, I believe there might be some situations where I need to use a surface normal, like backface culling maybe? Is there any need for surface normals if I'm using vertex normals? Thanks.

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I can't imagine wanting to have to recalculate all of the normals at runtime. If you load from a binary file you can probably read hundreds of thousands of normals in a totally trivial amount of time.

I don't believe you need surface normals for anything, but if I'm wrong someone can correct me. Backface culling is performed in the primitive assembly and I don't think it needs any input from the user to function properly.

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Quote:
Backface culling is performed in the primitive assembly and I don't think it needs any input from the user to function properly.

This is correct, backface culling works by projecting the vertices on the screen and checking, if they are in clockwise or counter clockwise order. The normal(s) are not taken into consideration.

Quote:
What about tangent-space-calculations? There you'd need normals.

True, but you'd need the vertex normals because a) that is how you pass normals to the API and b) there are cases where you want the faces top be "smooth shaded".


You will need surface normals if you are performing collision detection yourself, or you need to place decals etc. For the graphics side alone, I agree with karwosts, I can't think of anything usefull.


About whether or not to store the normals in the file. As was already pointed out, it's not really a matter of performance, although in my experience recalculating them after loading takes a little bit longer.
If the mesh is animated, you'll need to recalculate the normals somehow either way, so I'll assume you are talking about static geometry.

In that case I would either write a dedicated file format for your engine, which your model editor exports and which has all normals stored within. The format should be in a way that loading would just be a bunch of block reads.
If however you wrote the model editor yourself (or wrote an intermediate tool to apply the final touches and settings) then I would reuse the file format of that tool (which should feature things like smoothing groups and face orientations, rather than normals). In that case you can also reuse the loading and normal calculation code from the model editor in your engine and you are sure, that normals look exactly the way they look in your editor.

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Good artist can make manual tweeks to normals of model,
and create better looking models.

And better file size savings come from compressing complite model file
vs. dropping data from file. (pkzip or some custom packing algo.)

Geometry normals (face normals) are not normaly neaded if vertex normals
are given. (normals are extra info descriping surfaces accurate form,
ie. in ultra-hi-poly models (10.000.000+ poly) there is no nead to save
normals, becouse polygons can be used to describe accurate surface.)

/Tyrian

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