Vertex animation
Vertex animation is the keyframed animation of individual vertices of a mesh. It's good for very complicated motion like that of cloth, hair, or water. It uses a flex controller bone to move the model through a series of flexes (one for each frame).
Vertex animation was added in Alien Swarm and is available in all games that branch from it, including Portal 2, Counter-Strike: Global Offensive, Source Filmmaker, and Dota 2. To do: Check if vertex animation was actually added in L4D or L4D2

Implementation
To do: CLEANUP! OUTDATED!
- Create a vertex animation in your modelling tool.
- Export a Vertex Cache file. Currently only the official Maya exporter can do this, and only to SMD.
- Add the
vcafile
command to your $model block (see example below).

This produces two flex controllers named after your VCA file. The first defines the blend weight of the animation, the second (prefixed with multi_
) defines the active frame.
A programmer can now control your animation. If you need to map the vertex animation to a standard animation sequence, perform these extra steps:
- Add a new bone which will drive the vertex animation. It doesn't need to have any vertices attached.
- Add two
$boneflexdriver
commands to bind your vertex animation to the new bone (see example). - Add at least one
$sequence
in which the new bone is animated in its X and Y axes. Its X position defines the blend weight of the vertex animation (0-1), and its Y position defines the frame of vertex animation (0-1).Bug: Currently the flex driver sequence must be defined with $upaxis Y. This doesn't affect the rest of the model.
Example
$model "body" "ref_mesh.smd" {
vcafile "vertex_animation.vta"
}
// Optional:
$boneflexdriver "flexAnimBone" tx "vertex_animation" 0 1
$boneflexdriver "flexAnimBone" ty "multi_vertex_animation" 0 1
$upaxis Y // currently flex driver sequences must be Y-up
$sequence "vertex_animation" "flexAnimBone_anim.smd"
Example Videos
- A simple video showing the use of Vertex animation for hair.
- A simple video showing the use of Vertex animation for cloth.
- Example of vertex animation in Source Filmmaker.
- Example of vertex animation in Portal 2's HLMV.