CSC 471 Final Project - Gambit Animation
by Erik Kitson
Modeling Gambit
Hierarchical modelling allows a three dimensional model to be broken up into a tree where parts of the model are organized into branches. When a transformation is applied to one part of the model, all other parts on that branch are affected. For instance, if you are modelling a leg, a rotation on the thigh will also affect the knee, calf, and foot. However, a rotation to the calf will only affect it and the foot.
I used a modeling program called Wings3D to create my model of Gambit. As a reference, I used this image of Gambit. I used hierarchical modeling to create an animation of Gambit walking.
Figure 1: Gambit model in Wings3D
Figure 2: Model without trench coat
Figure 3: Model rendered in my program
Figure 4: Model rendered with trench coat
Collision Detection
Modeling Gambit’s trench coat would turn out to be the bulk of this entire project. In order to create realistic motion for the coat, I would have to detect whenever any of Gambit’s limbs collided with the coat, and then animate the correct coat vertices. For the detection, I created bounding cylinders around the thighs and calves and bounding spheres around the knees. I ran out of time to do collision detection for the arms. As an example, consider Gambit’s left thigh. Every time the thigh moves, it must check to see if it has collided with any trench coat vertices.
Figure 5: Coat vertices before collision
Figure 6: Coat vertices after collision
First, I calculate its Y and Z range to weed out all vertices that it cannot possibly touch. Then I iterate through all of the remaining vertices and calculate if they are within the cylinder’s radius. If the thigh was not being rotated, I could choose an X-Z coordinate at the center of the radius and choose a Y coordinate equal to the vertex’s Y coordinate. However, because the thigh is being rotated, I must calculate a different Y, so that the normal at that X-Y-Z will strike the trench coat vertex being examined. I then calculate the distance between these two vertices and see if it is less than the bounding cylinder’s radius.
Figure 7: Top-down view of collision detection for thigh
Figure 8: Side view - Y value is the same
Figure 9: Side view - Use the vertex whose normal hits the coat vertex
Cloth Motion
The collision detection for Gambit’s walking animation only moved the vertices that directly collided with the model’s legs. In order to create a more realistic look for the trench coat, I simulated the motion of cloth. Whenever a vertex is moved due to collision detection, it “pulls” all neighboring vertices in the same direction as it was moved. Each of these vertices in turn pulls its neighbors and so on. As the pull passes from one neighbor to the next, it becomes exponentially weaker until it is small enough to be ignored. Every vertex of the cloth also has a “home”, which is the X-Y-Z coordinates it was initially placed at. Whenever a vertex is not at its home, it moves towards that home. The magnitude of this motion decreases as the vertex gets closer to its home, to make it come to a more realistic stop.
Figure 10: Cloth animation
Figure 11: Cloth animation - bottom view
Figure 12: Cloth animation - front view
Issues
Due to time constraints, I was not able to get all aspects of this project functioning correctly. The collision detection for Gambit’s calves appears to be off some. This is most likely a result of the complexity of doing collision detection that is relative to the thigh’s rotation/position. Also, some of the cloth vertices have slightly strange behavior. In the front, some vertices shoot out, creating spikes in the coat. This is most likely a result of having being close to several vertices that were displaced due to collision. All of their pulls are probably accumulating on the vertex and causing its motion to be exaggerated. As I stated above, I ran out of time to do collision detection for the arms, so there is only a basic animation in place for the sleeves which allows the arms to pass through them.
Installing
I used an open source API for reading in files in 3D Studio format and converting them into usable vertices and faces. This must be installed in order to run the program.
1. Download the source code here.
2. Download the API from http://lib3ds.sourceforge.net/ .
3. Extract the lib3ds-1.2.0 directory to your project directory.
4. Open the lib3ds workspace in the msvc directory and build it to build all projects in the folder. Ignore the include file error.
5. In lib3ds-1.2.0/msvc/Build/Debug, there will be a .dll and a .lib. Copy the .dll to the directory the project will run from or to your Windows/system32 directory. Link the .lib in your project (already done in the project files I provide).
6. Build the gambit project from the source code.
7. Make sure the gambit.3ds file (included in gambit.zip) is in the directory the project will run from.
User Controls
The program has only a few basic controls. The user can control the viewing camera or turn a few features on/off.
mouse - click and drag to change where the camera is pointing
w - Move the camera forward
s - Move the camera back
a - Move the camera to the left
d - Move the camera to the right
c - Hide/Show Gambit’s trench coat
p - Render the trench coat as polygons/points
q - Exit the program.
Resources
I used very few references when making this project, as most of them were either too complex for my time frame or too simple to be of much use. However, I did receive some inspiration from these, and they may be of further use to others.
Simplistic cylinder-based collision detection:
http://spiritking.tripod.com/cdtut.htm
Very interesting (and very complex) paper on cloth motion:
http://graphics.snu.ac.kr/publications/journals/cloth.pdf
For all of your X-men trivia needs:
http://www.uncannyxmen.net
