Part 1: Understanding Skin Partitions - NifSkope
Skyrim armor or clothing meshes use a BSDismemberSkinInstance node and Skin Partitions to divide up the mesh into related body parts. Anyone who's modded Fallout3 may already understand this concept as it follows the same rules. It is completely different than the way Oblivion armor meshes worked so don't try to compare.
What is a Skin Partition?
Any object that uses a skeleton as part of it's mesh (i.e.; any object that moves), such as armor, clothing, creatures, and in the case of Skyrim, a bow, requires the object to be "skinned" to that skeleton. Skinning is defined as rigging (or joining) a mesh to a skeleton. That means, as the skeleton moves the mesh moves along with it. More on skinning in another article.
A Skin Partition is where a mesh is partitioned into submeshes such that each vertex of a submesh is influenced only by a limited and fixed number of bones. Huh? Ok, a skin partition is a collection of vertices ruled by certain bones.
Some pictures will help. Looking at the malebody_1.nif in NifSkope we can see the Skin Partitions defined under BSDismemberSkinInstance of its NiTriShape
You can see there are 3 Partitions in this mesh. If we expand the 3 partitions further we find some interesting things - Body Parts!
What is a Body Part?
The Body Parts represent all the biped slots that can be used in the making of armor/clothing/jewelry, etc. A full list of all the body parts (or biped slots or nodes) can be found here - my reference sheet - or here on the CK wiki. (I don't guarantee it's complete) There's also a list that can be seen in NifSkope if you hover over BSDismemberBodyPartType.
What is this telling us?
The human body in Skyrim requires 3 partitions, and any armor/clothing made for it will require at least 1 of these partitions to match.
Let's look at these partitions more closely. You can see them in NifSkope by looking at the NiSkinPartition block.
This confirms what we already know - that there are 3 Skin Partitions in this mesh. But how do I know what's what? Highlight the first "Skin Partition Blocks". This is the SBP_32_Body partition. See the blue highlighted vertices? All those related vertices of your armor will be influenced by this partition.
Now look at the others. The calves are highlighted here so this is the SBP_38_Calves partition.
The forearms are highlighted here so this is the SBP_34_Forearms partition.
What does this mean so far?
Every NiTriShape requires Partitions. The number of Partitions listed under BSDismemberSkinInstance will equal the number of Skin Partition Blocks in the NiSkinPartition. Also, the number of Partitions you have is equally dependent on your mesh.
Let's look at another example, this time a piece of armor - Dwarven armor. This example follows exactly what I've just explained because the mesh covers all these areas. It has 3 Partitions - Body, Forearms and Calves.
You can see the mesh of the armor confirms this by again looking at the NiSkinPartition blocks.
Another example that doesn't have all 3 Partitions - Steel armor. It only has 2 Partitions - Body and Calves, because the armor mesh does not cover the forearms
And more closely....
However, it has a Skin/body part for the arms. See if you can guess how many Partitions the NiTriShape of the arms has and what the Body Part is called then take a look.
One more example. As I said above armor needs at least 1 Skin Partition, and it's usually the Body.
Hide armor, for example, is like this because the mesh does not cover any part of the calves or forearms.
The Skin/body part does though and that is where you'll find all 3 Skin Partitions - Body, Forearms and Calves.
These were simple examples of armor all with the same body parts/skin partitions. The same principle applies to other types of wearable items such as helmets, gauntlets or boots only with different body parts.
What can you take away from this?
Hopefully a better understanding of the mechanics of Skyrim's armor/clothing and how it relates to the body.
Remember:
- Every NiTriShape of the mesh requires a BSDismemberSkinInstance and Skin Partitions.
- Whatever part of the body the mesh covers requires a Skin Partition.
- Every Skin Partition relates to an appropriate body part.
- The number of BSDismemberSkinInstance Partitions will equal the number of NiSkinPartition Skin Partition Blocks.
- A mesh must have at least 1 Skin Partition.
But doesn't it always? It should, but you need to define them in Blender properly. Move on to Part 2 for more.