Crystal Space 2.0.0: 4.10.8.1 General concepts of the Animesh

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#### 4.10.8.1 General concepts of the Animesh

This section presents the general concepts that are used to define an Animesh.

#### The envelope

An animesh is at first made of an envelope defining its shape, i.e. a set of vertices and triangles defining the shape of the mesh. Similarly to a Genmesh, it will hold render buffers for the vertices, the triangles, the normals, the UV mapping, the colors, the tangents and the binormals.

#### The animation packet

The animation packets are defined by the classes `CS::Animation::iSkeletonAnimPacket` and `CS::Animation::iSkeletonAnimPacketFactory`.

Animation packets are logical sets of animations, animation nodes, and skeletons that can be loaded or unloaded at once. Only one animation packet can be set per Animesh, but animations and animation nodes can link to some data in other animation packets, so several packets can coexist at the same time for a given Animesh.

That's the animation packet of the Animesh that will define the skeleton in use, you must therefore pass by the animation packet if you want to get access to the skeleton of the Animesh.

#### The skeleton

The skeleton is defined by the classes `CS::Animation::iSkeleton` and `CS::Animation::iSkeletonFactory`.

The skeleton of an Animesh is a tree of articulated bones that can be positioned separately. Changing the pose of the skeleton of an Animesh will affect the envelope of the mesh accordingly. Although the skeleton can be posed manually, it is generally animated through an animation blending tree.

Every bones of a skeleton must have one parent and can have any number of child bones. The root of the skeleton is an exception since it has a null parent (i.e. the ID of its parent is `CS::Animation::InvalidBoneID`).

It is allowed to have several root bones in the skeleton, a skeleton is therefore a graph of bones of the forest type, not only a tree of bones.

If you want to iterate on all the bones of a skeleton, one of the better way to do it is to use the method `CS::Animation::iSkeletonFactory::GetBoneOrderList()`. This method will return the ordered list of all the bones, and will insure both that all the bones that you manipulate are created, and that for a given bone, all children and sub-children bones are treated after the bone (or before, depending on the direction that you use to iterate on the elements of the list).

#### Skeleton coordinate spaces

Several coordinate spaces are defined regardless the skeleton and its bones. These are:

- `‘Absolute space’`: it is defined by the origin of the Animesh. A transform in absolute space is therefore relative to the transform of the mesh, whatever the pose of the skeleton.
- `‘Bone space’`: it is defined by the origin of the parent of a given bone in the skeleton factory. A transform in bone space is therefore relative to the transform of the parent of a bone when the skeleton is in its default pose.
- `‘Bind space’`: it is defined by the origin of a given bone when it is in its default pose. A transform in bind space is therefore relative to the transform of a bone when the skeleton is in its default pose.

The transform of the bones can be get or set using either of these coordinate spaces.

#### Bone influences

In order to deform the envelope of the mesh, i.e. to modify its vertices, normals and bitangents, one must define the bone influences over the vertices of the mesh. This defines in which proportion each vertex of the mesh should be moved regardless the motion of the bones of the skeleton.

The bone influences are defined by selecting up to four bones for each vertex, and by attributing a weight to these bones in order to define in which proportion they affect the vertex. For each vertex, the total sum of each bone weight must be equal or inferior to one. If it is inferior to one, then the vertex will simply be mainly affected by the position of the mesh.

#### Sub-meshes

Similarly to a Genmesh, an Animesh can be split in several sub-meshes that can be displayed separately and with different materials and display properties. This allows to use several materials on a single Animesh, as well as to set up some Level of Detail management, e.g. by enabling or not the visibility of each sub-mesh.

A sub-mesh is defined by the list of the triangles of the envelope of the mesh that are part of it.

#### Morph targets

The morph targets are defined by the class `CS::Mesh::iAnimatedMeshMorphTarget`.

Along with the deformation of the envelope made when changing the pose of the skeleton, the envelope can also be deformed by morphing its vertices. In that goal, one can define morph targets (a.k.a. blend shapes) that are variations of the positions of the vertices of the Animesh. This is useful e.g. to animate the face of a character.

Morph targets are activated by assigning them a weight different than zero. Weigths are generally supposed to be used in the range between zero and one, one being the morph target fully active, but there are no technical obstruction to use a very different value, even a negative one. Various morph targets can be activated at the same time in order to compose more complex deformations.

In order to speed up the morphing process, as well as to reduce the amount of data needed to define a morph target, the vertices of an Animesh are split in several subsets, and the morph targets are defined against these subsets. This is not mandatory to provide manually these subsets, and Crystal Space will compute them internally if they are not provided, resulting mainly in a slower loading time.

#### The skinning process

Every time that the pose of the skeleton or the weight of a morph target is changed, the deformation of the envelope must be re-computed through a step called the skinning process. During that process, the position of the vertices, as well as the normals, tangents and binormals if they are used, are re-computed depending of the bones and morph targets affecting them. Crystal Space uses a higher quality dual quaternion skinning in order to preserve the volume of the envelope.

The skinning process is computed by default on the CPU but can be delegated on the GPU using a dedicated skinning vertex shader, this is however not yet supported natively by Crystal Space.

#### Bone bounding boxes

In order to speed up the update of the global bounding box of the mesh, as well as the collisions and hit beam tests over an Animesh, separate bounding boxes are defined for each bone of the skeleton. Each of these bounding boxes should cover the whole part of the envelope linked to the given bone, i.e. all the vertices that are affected by this bone.

Again, this is not mandatory to provide manually these bone bounding boxes, and Crystal Space will compute them internally if they are not provided, the main drawback being a slower loading time.

#### Sockets

The sockets are defined by the classes `CS::Mesh::iAnimatedMeshSocket` and `CS::Mesh::iAnimatedMeshSocketFactory`.

Sockets can be defined on some bones of an Animesh. This allows to attach external meshes to an Animesh and to have them follow the motion of a given bone, e.g. for a hat, some clothes, or an object in the hands.

#### Decals

Decals are images that are projected on top of the mesh, allowing to personalize it easily. They can be added on Animeshes just like on any other mesh types supporting them, and they will be updated automatically to match the deformations of the Animesh.

An example of setting of decals can be found in the `‘avatartest’` application, in the `‘Frankie’` scene.

#### Clothes and Props

Clothes and props can be attached to an animesh, and be animated physically, either through rigid or soft bodies.

To achieve this, you need to use the `‘Ragdoll’` animation node in order to create and manage the rigid bodies of the skeleton that will be used to attach your clothes or props.

For soft bodies, you would need to use the dedicated Genmesh animation controller, that will deform the mesh depending on the physical simulation.

An example of setting of clothes can be found in the `‘avatartest’` application, in the `‘Krystal’` scene.

#### Hairs and Fur

Hairs and fur can be added using the fur mesh.

#### The Body Mesh

The body mesh is defined by the classes `CS::Animation::iBodySkeleton` and associated.

The body mesh holds the physical description of the skeleton of an animesh. For each relevant bone of the skeleton, one has to define a `CS::Animation::iBodyBone` that will hold the colliders, joint and other physical properties of the bone. Subtrees of the skeleton are defined through the `CS::Animation::iBodyChain` object.

The body mesh is used in several animation nodes such as the `‘Ragdoll’` and the `‘LookAt’`.

#### Creating and importing Animesh meshes

The main ways to import an Animesh into Crystal Space is either through the Blender export script, or the Assimp loader.

#### Animating Animesh meshes

See the specific section on that topic.

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