4 things you should know about Mesh Deform

The Mesh Deform modifier uses a deformation technique called “Harmonic Coordinates” deformation, which was first introduced in Pixar's Ratatouille. Basically, it makes use of a low resolution mesh cage to drive the deformation of a higher resolution model.

BlenRig 5 has a Mesh Deform Cage which is automatically created along with the rig, and which you can fit onto your characters in order to obtain a very fast, but also very organic and high quality deformation.

Here are some basic tips and tricks, that will let you understand some of the techniques behind the character deformation BlenRig 5 offers .

1. Use the “preserve volume” option

The armature modifier has an option called “preserve volume” which actually enables a deformation algorithm called “Dual Quaternions”.

As the name suggests, this option prevents meshes from collapsing at joints when they rotate.

Therefore it is always a good idea to turn this option on. The only drawback is that the Dual Quaternions algorithm can produce some deformation artifacts in very rare cases where bones scale and rotate simultaneously.

But again, this rarely happens, so the option is safe enough for usage, specially for body deformation.

2. Armature deformation does not need to be perfect

One great aspect of Mesh Deform is that the deformation of the Mdef Cage doesn't need to be perfect at all. If you think of the Cage as a Lattice with a customized shape, you will realize that the deformation that will take place in the model will always tend to be organic. In other words, the geometry of the Cage will pull the vertices of the high resolution model in a very smooth and interpolated way.

Therefore, in very complicated places like the shoulders, you can allow the Mdef Cage to have rather abrupt deformations, while obtaining really good looking results in the model underneath.

The following images will give you a clear idea of what this is all about.

3. Quads and Triangles

In the built-in Mdef Cage that BlenRig 5 has, I've made a strategic use of quads and tris. Most of the geometry of the Cage is made out of quad polygons, as topology flow is easier to control that way.

Nevertheless, in certain areas such as shoulders, elbows and knees, having control of volume preservation is a key aspect. That is when tris come into play.

Imagine a single quad polygon. If you select 2 opposing vertices and you pull them down, the surface of the polygon will collapse. If this quad was part of a Mesh Deform Cage, we would be getting a loss of volume in the model.

On the other hand, by using tris, you can determine how the polygon will fold. Therefore, the middle edge of the polygon will remain up, and you will be contributing to volume preservation, as that middle edge will still pull the vertices of the character's model up.

In the images below you will see where the tris are located in the Mesh Deform Cage.

4. Preventing joints from collapsing by using one single face

Last but not least, there is another strategy used for volume preservation, which is the usage of a single face that will shrink when the joints rotate, instead of using multiple faces that could collapse into each other.

Let's see some images to make it clearer.

So, in this first image you see I am using only one face to cover the back of the knee joint.

When the knee rotates, this face is shrinked, and the edges are kept away from the knee. This helps preserve the volume of the geometry that is at the back of the knee. If I used multiple loops in that area, the result would be that those loops would collapse in the direction of the knee, generating a loss of volume in the model.

Well, I hope you've enjoyed these tips and tricks! Stay tuned for the release of BlenRig 5!!

Juan Pablo