Draw calls by two-sided materials

[Tels]

Reading through the article about making LOD models, this caught my attention:

 

If you haven't followed the steps about a texture atlas and created a two-sided material, you will notice that the model uses 4 drawcalls in game, compared to the original model. That means placing many of the new models is actually slower than placing the original models, despite the greatly reduced number of triangles.

The reasons are:

• Without a texture atlas, each patch uses its own material, thus creating two surfaces, using one drawcall each.
  
• With a two-sided material, D3 will upon load duplicate all surfaces and invert their side. This means you end up with 2 drawcalls instead of one.
  

Both issues together will create four drawcalls per model per light, instead one per model per light. Since each model is drawn multiple times (once for each light), this can add up very fast.

 

Isn't that something that could be easily fixed, now that we have the source? Duplicating the surfaces shouldn't nec. add another surface, instead the vertices could be-reused and just new, inverted triangles be attached to the original surface.

 

That would reduce the number of vertices, surfaces and drawcalls for two-sided materials.

 

Here is some more info: http://wiki.thedarkmod.com/index.php?title=Twosided

Edited October 3, 2014 by Tels

[Tels]

Hm, back then we didn't have the source, but now we have and it says:

 

    // duplicate and reverse triangles for two sided bump mapped surfaces
    // note that this won't catch surfaces that have their shaders dynamically
    // changed, and won't work with animated models.
    // It is better to create completely separate surfaces, rather than
    // add vertexes and indexes to the existing surface, because the
    // tangent generation wouldn't like the acute shared edges

 

So maybe it is not easily done or even possible...