Representation of mesh - one vertex (position in space) = one pos, tex coord, normal?

Question

I'm writing the class for storing, loading and rendering the static mesh in my DirectX application.

I know that Box model can uses 8 vertices, but usually it uses 24 or 36 (because one "vertex in space" is de facto 3 vertices with the same positions and different texture coord/normals).

My question is: how about meshes (e.g. character mesh exported from 3ds max or similar application)?

I'm working on the exporter plug-in now for my application. Should I:

• For each vertex in mesh, exports it (as position, normal, texture coordinates) - will it be enough always/for some cases?

or maybe:

• For each face, export 3 vertices (pos, normal, tex coord)

?

I'm not sure if the texture coordinates/normals will be different for the same "vertex" in 3ds max (same position but different normals/tex coord?).

The second aprouch should works fine with D3DPT_TRIANGLELIST, but for 5.000 vertices mesh from 3ds max I will get 15.000 "real" vertices (+/- each 3 will have same pos and different tex coord/normals).

Also, on the example below, the object consists of two parts - top one has #1 smoothing group and bottom one has #2 smoothing group -will my problem looks different for vertex "inside" smoothing group and between two groups (e.g. connection of top/bottom)?

In other words: is the black circled "vertex" one "real" vertex with same pos/tex coord/normals or it's 4 vertices with same positions only (4 vertices with same everything makes sense?)?

And what about red one?

I'm using indexed buffers.

Answer 1

Exporting three vertices for each triangle is always sufficient, but may involve duplicating data unnecessarily. If adjacent triangles have disjoint texture coordinates or other properties, you must duplicate in order to produce correct rendering results. If you want, you can merge duplicate vertices in a post-processing step.

Regarding your reference images, it doesn't look like there is any texture applied to the surface, so the red circled vertex must be two distinct vertices in the buffer in order to create the color discontinuity, as it should be. The black circled vertex doesn't need to be, but it still may be duplicated. In general, you should share vertices within a smoothing group, and not worry about deduplicating across groups. If vertex throughput ends up being an issue, you can look into optimizing it, but for most workloads you'll be pixel or fillrate bound.