COMP 557 - Fall 2010 - Assignment 1
Transform Hierarchy for Character Posing
Due 12:00 noon Monday 27 September
Download the provided
code from WebCT and dump it into a new java project. Follow the
instructions in assignment zero for setting up your environment.
Note that the provided code contains a jar called mintools.
We will be using more of these helper classes in subsequent
assignments, but for now, we are only using the
TrackBallCamera class. The provided code attaches an
instance of this class to the glCanvas so that mouse events can be
used to alter the 3D view. Using the left mouse button you can
rotate the object, using the middle button you can pan the object in
the viewing plane, and using the right mouse button you can move the
camera further or closer.
Start early! This assignment is not supposed to be hard, or time
consuming, yet it is possible to get caught up on various details.
The purpose of this assignment is to use a hierarchy of
transformations to draw and animate a character. You can use a
variety of objects to build your character. Using glut to draw
spheres, cubes, cones, cylinders, and the occasional teapot will be
easiest. If you also want to draw triangles and quadrilaterals, you
will need to specify their surface normal for lighting to work.
The file A1App.java contains the necessary source code to create
an OpenGL renderer in a windowed frame. A reasonable camera
position and projection matrix is set up for you (see the use of the
TrackBallCamera instance in the display method). The code
also contains TODO comments in the places where you will need
to add code to complete the assignment objectives that are given
Each objective is worth one mark, unless otherwise noted.
Create Scene Graph Nodes
Finish writing the scene graph node class called
BodyNode. The node should have a display method to
draw the contents of the node and all its children (which are
also BodyNodes). In each node, you may want to store:
- position with respect to its parent;
- rotation axis for the joint connecting this node to its parent;
- current rotation angle;
- a list of children (possibly the empty list);
- colour, i.e., material properties of the geometry of this node;
- position, scale, orientation, and geometry information for this node.
Note that as described in the list above, the rotation axis
for the joint, and the position of the node with respect to its
parent, describe a rigid transformation. The transformation
describes the local coordinate frame in which the geometry of
this node and its children are drawn. In the last bullet item,
you can consider the position, scale, and orientation as
additional modeling transforms to help you to place your
desired canonical glut primitives (sphere, torus, cube, etc.).
This is similar to how we saw the scene graph organized in
class, except that we are mixing many transforms, material
properties, and geometry in each node. The idea here is to
simplify things slightly, but feel free to modify this as you
see fit (be careful that your modifications work nicely with
the rest of the objectives of the assignment). For instance,
you may find it conceptually cleaner to keep the geometry in a
separate type of node (i.e., create another class, such as a
GeometryNode, to store the bottom two bullet items in
the above list).
Create and Animate a Chain (two marks)
Write code such that when the 'C' key is pressed, a chain
is created. The chain should be a reasonable length (e.g.,
5 to 10 links), where each link is a torus. Choose your
glutSolidTorus parameters so that you get something that
resembles a chain link, and such that your chain is an
appropriate size to be seen on the screen (i.e., without
need of using the mouse to moving the camera from the
default position). Each link should be rotated 90 degrees
from its parent, and placed such that it appears to be
hanging from its parent (see the screen shot at right).
Each link should rotate as if it is sliding at the point of
contact of its parent. You have two options, and can do
either (or both, though doing both is obviously hardest).
The easier option is to use the axis of rotation that goes
through the center of the torus; this will produce a
rotation which is only visible via the motion of its
children (i.e., the torus will be rotating in place). The
other option is to apply a rotation with an rotation axis
on, and tangent to, the medial
axis of the torus (i.e., at the point at center of the
tube above the point of contact, and tangent to the circle
at the center of the tube); this rotation will be visible
both at the children and the torus itself. Note that the
third axis (the overall direction of the chain) is not a
good choice for rotations in the chain because twists along
the chain direction quickly result in interpenetration.
Use the space bar to toggle the animation mode. The
provided code has an animation mode that sets all the angles
to a sinusoid based on the current time to test your chain.
It should appear to be swinging.
Create a Character (two marks)
Create an articulated character when the user presses the 'D'
key. You can choose to make anything you like (e.g., animal,
human, robot), but it should have at least two legs, two arms,
and a head. In all you should have about 20 degrees of freedom
in your character.
You will likely find it useful to first make a sketch of
your character to figure out how each of its body parts are
connected to one another and where. Note that you will also
need to choose a reasonable root node for the character, such
as the torso. Write code to create the various body parts, and
link them together by adding each child to the appropriate
parent. Be sure to also add all your nodes to the
nodeList! Note that you will want to avoid making a
DAG so that all limbs of your character can be animated
Some joints should have a single degree of freedom (as shown
in the picture above left), such as knees and elbows, while you
should also have other joints which have multiple degrees of
freedom (e.g., hips, shoulders, neck). You can create your
multi-DOF joints any way you like, but it should work with the
posing and animation controls in subsequent steps. One easy
way to achieve a spherical joint would be to create three nodes
all at the same location with different rotation axes. Two of
the three nodes need not have any geometry associated with
them, i.e., you would have two "invisible" nodes at the anchor
position of the ball in socket joint show in the picture above
Posing Controls and Setting Key Frames
The provided code contains a primitive means of changing the
pose of your character. The provided code uses the left and
right arrow keys to walk through the nodeList, and then
use up down arrows to adjust the angle by small increments
(perhaps a few degrees).
You may find it useful to try to find a way to modify the
code so that the currently selected node is drawn in a
different style or colour.
Add code to save the current pose when you press 'K'. Save
the pose into the existing list of "key poses". Code already
exists to erase, load and save your key poses using keys 'E',
'L', and 'S'. Any code you add should work with the provided
code, or you should fix it so that it works with your
Note that using OpenGL picking to select which body part
you want to pose would be a great way to simplify the task of
character posing. This is beyond the scope of this assignment,
but can be implemented for a 5% bonus!
Key Frame Animation
Create a simple key-frame animation. Plan ahead! The key pose
editing functions are very limited (i.e., there is no undo,
redo, insert, replace, delete, etc.). Try to make a reasonable
walk cycle using at least four key poses. Save the animation
and submit the resulting keyposes.javabin file with
your code. Consider adding additional keyboard controls to
increase or decrease the speed of the keyframe animation
(currently the time between keyframes is half a second).
Readme File (two marks)
Create a readme.txt or readme.pdf file to
submit with your assignment. The readme should include any
comments you have with respect to the assignment, and describes
anything you deem to be noteworthy. Try to be brief! Your
readme should also contain written answers to the following
Is setting the joint angles individually always a good
interface for setting key poses? What do you think would be a
useful alternative? Try to think of at least one alternative,
but list several if you can.
For the following pairs of transformation types, do they
commute always, sometimes, or never?
Always means always, while never means only if one or both are
the identity transformation. Sometimes means that the
transforms do not commute in general, however, there are
situations where two transforms will commute. If your answer
is sometimes, then describe, as concisely and as generally as
possible the situation where commutability will occur.
- Rotation and Scale in 2D
- Rotation and Scale in 3D
- Rotation and Rotation in 2D
- Rotation and Rotation in 3D
- Translation and Rotation in 3D
- Translation and Scale in 3D
Great! Be sure your name and student number is in the window
title, in your readme, and in the top comments section of each of
your source files.
Submit your source code and written answers in the readme file
as a zip archive via WebCT. DOUBLE CHECK
your submitted files by downloading them from WebCT. You cannot
receive any marks for assignments with missing or corrupt
Note that you are encouraged to discuss assignments with your
classmates, but not to the point of sharing code and answers.
All code and written answers must be your own.