Cycles - How do i apply different materials to different parts of model?

I have unwrapped model with two vertex groups. And i try to add different materials to each of them. Never thought it can be a problem. I do the usual:

  1. Select vertex group
  2. Assign material.

In preview mode everything looks normal, like this:

But when i render model i have strange artifacts.

I know that i can use UV-Map as mask and just combine two shaders (each for different part of model) in one materal. As described here http://blender.stackexchange.com/questions/516/add-different-materials-to-different-parts-of-a-mesh
I’ve already tried this and it works fine. But still, i want to fint out why there is a problem with vertex groups and materials? Materials assigned to vertex groups is not Cycles ideology, and only fit for Inernal?

UPD: No, second method does not work fine. Artifacts now at another part of model.

I often use different materials on different parts of the same mesh and you dont actually have to have actual vertex groups to do it. Just select the faces for a material and then assign them while in edit mode. The only problem i’ve encountered using multi materials in one mesh is that the cycles surface displace node occasionally goes haywire if you try using that displace in the materials.

Artefacts are quite clearly seen in the first image too.
I doubt you’ll get a definite answer without supplying a test file having same issues first.
What exactly you do when you assign material to the vertex group? Steps? Just curious…

2eppo

>>Artefacts are quite clearly seen in the first image too.
I can’t see any. Can you mark them with red circles?

>>I doubt you’ll get a definite answer without supplying a test file having same issues first.
I’m sorry, i can not post file here, not yet. It is commercial.

>>What exactly you do when you assign material to the vertex group? Steps? Just curious…
Well, the usual:

  1. Create Vertex group
  2. Select part of mesh, assign this part to Vertex group.(Also Ctrl+I and Remove other [art of mesh from Vertex group, just in case).
  3. Add materal, select Vertex group, assign material to Vertex group.

Bunc
Thank you, i will try this approach.
UPD: Yay! And it works! Many thanks!

  1. In a short that would be - Select faces, select Material from a list (or, create a new material), click button Assign.

From this point on, to simplify mesh selection you could use: select Material from a list, click button Select.
Vertex groups in general are not needed for material assignments.

First image desaturated and increased contrast to better show what i see in color:
http://www.pasteall.org/pic/show.php?id=105031

OMG. How do i get rid of those artifacts? Why do they happen? It is simple geometry, with normal map baked in xNormal and simple material and texture. What can go wrong?
I add two subsurf levels. Is artifacts still there?