Skinned Mesh Modifier Example
This example shows how to modify vertex positions and recalculate normals of an animating skinned mesh with a surface shader.
Why modifying vertices is difficult
Actually just modifying vertex positions with a surface shader is quite easy with using a custom vertex modification function. However, recalculating normals after modifications is difficult because usually a vertex modifier doesn't know positions of their neighboring vertices.
In this example, we use a mesh converter that encodes positions of neighboring vertices into texture coordinate attributes, so that we can reconstruct the neighboring vertices in a vertex modifier.
How to reconstruct neighbor vertices
If a mesh is completely rigid (not skinned), we can simply store the positions of the neighboring vertices as 3D texture coodinates. This approach is not enough for skinned meshes because the neighboring vertices will be moved from the original positions by a skin deformation.
Instead of directly storing the vertex positions, we transform them from the model space into each tangent space and store them into the texture coordinate attributes. In a vertex modifier, we can transform them back from each tangent space (which is deformed by skinning) to the model space. Although this doesn't reflect the accurate deformations because of skewness of skinning, we can get fairly good approximations of them.
In this example, we use a further simplification; Assuming that a given skinned mesh is flat shaded and all vertices are separated (not shared between triangles). Under this assumption, it's clear that neighboring vertices are laying on its tangent plane, and thus we can ignore the normal axis component (it will be always zero).
We use an editor script that converts a mesh in build time. It encodes centroids into UV2 and neighbor vertices into UV3.
Vertex modifier examples
This shader modifies vertex positions with gradients of a noise field. It reconstructs neighbor vertices and applies the same modification to them, then recalculate the normal of the belonging triangle from the modified vertex positions. Although this approach is not optimal because the modifier will be applied to a same vertex for multiple times, it's faster and more memory-efficient than doing the same operation on the CPU side.
This shader modifies vertex positions toward centroid of belonging triangle.
Things to be improved
This example doesn't have a specialized motion vector pass. It introduces artifacts when using motion vectors (motion blur, TXAA, etc.).
This approach is only useful for flat shaded models. If the model is smooth shaded, it might be better to recalculate normals with an analytical approach.