Discussion on May 26th 2006 reading group meeting

Paper presented: Kang Hoon Lee, Myung Geol Choi and Jehee Lee. Motion Patches: Building Blocks for Virtual Environments Annotated with Motion Data, to appear in Siggraph 2006 proceedings

• Project page (with paper and video): http://mrl.snu.ac.kr/~zoi/motion_patch/motion_patch.html
• Paper Overview
• Paper Discussion

Paper Overview

The basic idea is to create an environment by assembling a large number of regular building blocks called unit objects, such as a slide, a square ground tile or a cubic block. Some configuration of these unit objects is physically built in a motion-capture studio in order to capture a long sequence of a human navigating this environment.

A model of the physical environment is then used to build motion patches. These are groups of 2 unit blocks (similar up to a rigid transform) together with all the frames of motion that pass through them. Note : The authors say that motion patches can be built with 1 or more unit blocks, although their implementation only use 2 unit block patches except for a single 1 unit block patch. The following discussion is easier to follow if we think of 2 unit blocks patches.

As a pre-process, all the acquired poses are clustered so that two poses fall in the same cluster if they are close enough according to a simple distance metric. Each cluster is assigned an index p. (Incidentally, this clustering is performed in a +/- 200 dimensional space, they use the agglomerative hierarchical k-means algorithm [ref given])

In order to interactively branch between animations, each pose of each motion contained in a patch is binned into a regular grid of cells. A cell occupies a projected square area of about 10cm x 10cm. Also, a cell has 4 dimensions (x, y, theta, p). A motion frame is said to be in the cell with with coordinate (x, y) if its root node falls within the projected square at position x, y. A motion frame falls in the cell with angle theta if the yaw orientation of the character at that frame is equal to theta (within a threshold?). Finally, the motion frame falls in cell with index p if the cluster index of the pose is p.

After each frame of each motion has been binned, the system builds a directed graph where each cell is a node. A link is added between two cells if there exist a segment of motion starting in one cell and ending in the other. When the same cell is occupied by 2 or more frames, the animation can interactively branch at that cell. In order for perform this branching quickly at run-time, the different motions present in the cell are warped to create a smooth branching transition [ref given].

To create a novel environment, motion patches are automatically fitted to the different unit blocks that build the environment. Since motion patch occupies two building block, they will often overlap. When this happens, the cells in the overlapping region are scanned to create links between the motion patches. (The authors don't mention if the motions are warped or if motion blending is used in this case.)

A final type of patch is introduced: a large flat square that can be tiled to produce arbitrarily large interactive walking motions. The idea here is to create a number of regularly spaced entry/exit points along the sides of the square tile. The motion of a subject randomly walking around for about 10 minutes is then captured. This motion is analysed to find paths that would connect each entry point to each exit point. An entry/exit point is annotated by (x, y, theta, p) similarly to the previously introduced cells. A technique to prune the motion graph when obstacle are present on the tilable tile is presented. Path planning and collision avoidance techniques are also discussed.

Paper Discussion

Is this the good way to go?

• Is it such a good idea to have so many very similar motions?
• Wouldn't it be better to do a directed mocap session where each animation performed by the subject will be useful?
• In real life, animations are more smooth and squishy (I think that's what Kevin said) that the authors assumed. Here, only minimal modifications are applied in order to stitch motions together.
• The tilable patch is probably not the best way to create long walking sequence. A nice interactive walking technique would seem more appropriate.
• Although fast and simple, path planning on the tile level still let the character move erratically within each tile, not a very convincing behaviour.

-- PhilippeBeaudoin - 26 May 2006