Our team has been going back and forth on this for a long time. We have a
single skin / morphing system in place now as a place holder, but want to
move to the next level. I would like to get your opinion on this. Our target
hardware is 700 MHz PC and GeforceII GTS as minimum specs.
The requirements are:
1. The ability to change the clothing and equipment for avatars at will.
2. Be as efficient as possible of texture memory.
3. Allow layers of clothing and jewlery and items to be shown.
4. Allow items to bulk up the character (like a coat).
5. Allow for seperate attached items to be moved in concert with limbs.
6. Allow smooth blended animations.
The proposed solution is this:
1. Body is divided into 21 pieces representing major pieces of the human
anatomy.
2. Animation information is generated using Character Studio with IK
solving. Animation is a series of quaterion rotations applied to a nested
hierarchy of bones.
3. Each animation sequence is time-coded for its step time between
key-states.
4. Multiple animation sequences can be applied at the same time if they are
applied to differnet parts of the bone hierarchy (i.e. upper and lower
body).
5. Transitions from one animation to another are done by calculating
parallel sets of joint rotations for that time state and blending them on
each part of the quaterion vector over a set amount of time, say 500-700ms.
6. As the avatars are modeled, each of the 21 pieces are texture mapped into
a standard pre-determined texture map in a large imaginary texture.
7. If two or more sub-meshes are built for one body part (for bulking for
example) then they must be done in such a way that they can fit with all
other meshes they are adjecent to.
8. Every vertice of every sub-mesh will have a reference to all the bones
that influence it, and the associated weight of each bone.
Texture Building
----------------
At the time the character is loaded, or at the time that a wearable item is
changed we need to determine how the mesh is to be rendered. The idea is
that we will use single textures per sub-mesh where possible, but be
prepared to build a dynamic merged texture for meshes if necessary. Merged
textures will be placed on a single texture in such a way as to allow for
smooth interpolation between sub-meshes. This allows us to share texture
memory for characters in view having the same item, and the ability to build
some items which use anisotropic filtering (for metal) or use a higher res
texture for more detail, but still support layered clothing and jewlery.
1. Each wearable item will be flagged with a set of body parts it covers,
and the class of sub-meshes needed to support it (example is that it covers
upper and lower chest, using Bulk Chest Mesh #2. Each item is also flagged
with whether or not it covers 100 percent of the body parts referenced. So
to recap an "item" is a texture, the names of the body parts it covers, and
the type of mesh (skin tight, medium bulk, big bulk or even specific
sub-mesh) for each body part covered.
2. If an item does not cover 100 percent of the body parts referenced then
the artist built texture should have an alpha mask which indicates which
part of the texture is not covered.
3. The 21 sub meshes are grouped into 2 categories. One category are meshes
which have their own texture. So if the avatar is wearing a long sleeve
shirt, with nothing over it, then all the arms and chest sub-meshes will be
grouped together because they all share the same texture. The second
category are sub-meshes which do not have their own texture because the
items on them only partially cover the underlying texturem requireing a
merge. A dynamic texture is built and used for a single merged sub-mesh
which encompases all of the second category items.
4. Merged textures are built by starting at the base race/gender/complextion
texture per part, laying down scars, cuts, bruises and tattoos, and then
laying down the appropriate item textures on top of them, alpha blending
them together. These little body part textures are arranged on a single
larger texture with as much efficientcy as possible and rendered with the
merged sub-mesh.
Rendering
---------
1. Each frame walk the bones, perform rotations, time interpolation and
rotation blending.
2. Step through all the vertices in all the sub-meshes and apply the bone
weighted transformations.
3. render the groups of sub-meshes which have their own texture.
4. render the rest with the dynamic texture.
Optimizations
-------------
1. If the client machine is weak in CPU but strong in texture memory you can
disable sub-meshing and force all of them into the 2nd category (merged
textures). This would then render as a single deformable mesh, but each
avatar would use a 256x512 texture.
2. If the client is weak in CPU and weak in texture memory, do the same as
in step 1, but scale the avatar texture down by half.
3. If the number of avatars being animated in a viewable area is too large,
start reducing the animations FPS down, starting with characters farther
away from the view. Effectively this would mean that some characters would
recompute their animation every other frame, or even less frequently.
Analysis
--------
1. This should be a very flexible design covering all the requirements.
2. Commonly worn items, if they are 100 percent coverage on their body
parts, should end up being shared textures, conserving tetxure memory
(assuming two or more people wearing the same item are within view)
3. The scheme can be scaled for different machine classes.
4. This supports layering and unique look per character.
Well thats the basic idea. We are going to start coding a prototype, but if
anyone can think of any glaring issues with this then please let me know.
David Yazel
http://www.cosm-game.com/
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