Originally Posted by Snader
Yes, a bit.
In the top image, you can see there are 12 chunks of polygons, so 12 draw calls.
Every 'split' in the model is a draw call. Every UV island is an extra draw call, every smoothing group, every instance of a prop in the scene. Everything gets divvied up in to small bite sized pieces.
Having two smaller pieces instead of a large one doesn't cost any more effort to render, but it does take every so slightly more time because instead of the CPU telling the GPU "now render this", it has to say "now render this, and that".
That's not correct. Drawcalls are very simply put, when the Draw() command is called for a specific mesh AND material/shader. DirectX (or any other graphics API) can render any model/object in a single call, but it can only use one material for this call. So if you have two models, that's two drawcalls. If those models have two different materials, that's 2 times 2, so 4 drawcalls, since for every model you'll have to call the draw function for the part with material A, and then for the part with material B.
Smoothing group splits and UV splits do not affect drawcalls. They do increase the FINAL vertexcount, which is more important to judge the performance impact than tri/polycount.
Sorry to detract from the work, but I'd prefer the information in a featured topic to be correct ;)