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- CSCI 4611: Programming Interactive Graphics and Games Assignment 4: Walk the Spline

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Introduction

In this assignment, you will be working with data from the CMU Motion Capture Database.

The data we give you will only be in the form of text files, but with your programming and

math skills, you will bring this data to life!

The CMU motion capture data is typical of all the skeleton-based motion capture data you’ll

find in today’s games and movies. So, gaining some experience with this type of animation is

one of the most important goals of the assignment. The key concept you’ll need to work with

this data is understanding how to compose transformations together as you render a scene

graph. To keep things simple, our rendering will be a pretty simple stick figure.

The other important goal of the assignment is implementing a simple spline curve to define

the path of the character. We will see how existing animation data of a character moving

uniformly in a straight line can be modified to follow a specified trajectory instead.

In this assignment, you will learn:

• How transformations should be composed to create animated characters

• How to create, navigate, and render a hierarchical model

• How to implement a spline curve to represent smooth functions

• How to transform the speed and orientation of a motion-captured animation

CMU Motion Capture Database

The CMU Motion Capture Database contains 2,605 different motions, most recorded at 120Hz.

These motions range from the simple (person walking straight forward), to the complicated

(directing traffic), to the silly (someone doing the “I’m a little teapot” dance).

For this assignment, we’ve found a simple walk motion and chopped up one cycle of it so that

it repeats. Playing this back in a loop while translating the character uniformly along z results

in an infinite walk. First, you will get this to display by drawing the limbs of the character

correctly. Next, you will modify your program to make the character walk along an arbitrary

spline curve instead of along the z-axis. Note that the motion of the character’s right arm is a

little jittery in the original data, so don’t worry about that as long as the rest of the motion is

OK.

You are encouraged to search through the CMU database and see what other motions strike

your fancy. Once you complete the first half of the assignment, you should be able to simply

swap in new motion data files to view them. This can be fun!

The motions in the CMU database use a skeleton specified in .asf files and a separate

motion in an .amc file. The .asf files specify bone names, directions, lengths, and the

skeleton hierarchy. In a bone’s local coordinate system, the translation from the start of the

bone until the end can be found using the bone “direction” and “length” — our support code

provides a shortcut for this, look for getBoneVector() inside the Bone class. The length

of each bone and its resting direction does not change from frame-to-frame, but the angle of

the bone relative to its joint does change. The rotation angles to apply at each frame are

specified in .amc files.

Requirements and Grading Rubric

We provide code that parses and loads the CMU data into a hierarchical skeleton data

structure. We also provide a utility function that returns the bone’s current rotation relative

to its parent. We provide you with code that advances through the time in the animation. You

will need to implement functionality to composite the transformations in this skeleton to

connect the bones together and make your character walk. To accomplish this, you need to

add your own code to the Bone and Character classes in character.hpp. Look for the

draw() routine in each of these classes – this is where most of your code will go. The

comments in the support code should help you get started. Think of Character as the root

node for your character’s scene graph. For a human character, this root node usually

corresponds to the pelvis bone. Therefore, the root node usually has 3 “bones” extending

from it, one for each of the legs and one for the torso. You need to draw each of these bones

within the root node’s coordinate frame, which will change frame-to-frame based on the

animation data. Inside each bone’s draw() routine, you’ll need to draw the bone itself (see

next paragraph) and then draw all of the bone’s children. Look back at the slides from class

and remember how we use glPushMatrix() and glPopMatrix() to render scene

graphs.

To draw a bone, first start with just drawing a line segment from the origin to

the bone vector. Once you get a stick figure made of lines animating, your

next task is to render solid bones instead. To help, we’ve implemented a

function Draw::unitCylinderZ() for you to use; as the name suggests,

it’s a cylinder with unit radius and length, with axis from the origin to (0,0,1).

You will need to apply a transformation to make its axis match the bone

vector. We suggest using a radius of 0.05, and adding spheres of the same

radius at both endpoints to fill in the ends. This creates a 3D shape known

informally as a “capsule”, shown on the right.

Next, you will implement a cubic Hermite spline by adding code to the Spline3 class. The

data the spline contains is simply a collection of control points, each at a specified time and

with a specified value and derivative. In this case, the value and derivative are vec3s

because the data we are interpolating is the translation of the character. You should

implement the getValue() and getDerivative() functions to evaluate the spline

functions at arbitrary times. If you get this correct, you should see the spline drawn on the

ground as a red curve.

Finally, change the code in main.cpp to make the character walk along the spline curve. For

this, you will need to do three things: (i) Translate the character to the current position on the

spline. (ii) Rotate the character about the y-axis to make it face along the current velocity (i.e.

the derivative) instead of along the z-axis. (iii) Change the amount the animation advances

each frame to be proportional to the current speed. This way, the character’s walk cycle

proceeds more slowly if its speed is low, and you won’t see the character’s feet “skate” on the

ground. The original speed of the character was baseSpeed =

glm::length(Config::baseVelocity), so the amount you advance the character

should be dt*currentSpeed/baseSpeed.

Here’s a quick summary of the programming requirements:

1. Hierarchical skeleton transformations: You need to traverse the hierarchical skeleton

model to render the skeleton. You need to composite the various rotations and

translations involved in the model to cause it to come together as a skeleton.

2. Bone rendering: You need to use cylinders capped with spheres to render each bone in

the model.

3. Spline evaluation: You need to complete the implementation of the cubic Hermite

spline, so that it shows up as a smooth curve.

4. Transformed animation: You need to modify the animation so that the character walks

along the spline facing in the direction of motion, with the animation speed adjusted

so that its feet do not skate unrealistically along the floor. (It’s OK if the feet partially

go through the floor, because that’s in the original data.)

Grading for this assignment will be based on the quality of your implementation for each of

these four requirements.

Implementation Notes

Drawing a bone in the right direction. Essentially, you have a shape lying along the z-axis,

and you can obtain a unit vector b in the direction of getBoneVector(). You must perform

a transformation so that the transformed z-axis lies along b. There are at least two different

ways to do this:

a. Directly construct a matrix whose columns are the transformed coordinate axes, as

you did in the midterm. Take an arbitrary vector a that’s not parallel to b, and use it to

construct two more unit vectors orthogonal to b and to each other. Use them as the

first three columns of a mat4 m (what should the fourth column be?) and apply it as a

transformation using glMultMatrix(&m[0][0]). Hint: you could start by taking a

= (1,0,0), then if it is too close to b or −b, change it to (0,1,0).

b. Alternatively, figure out the axis and angle of a rotation that aligns the unit vector z =

(0,0,1) with b. The simplest such rotation is as follows. Imagine the plane containing

the two vectors z and b. If we rotate z along this plane by an angle equal to the angle

between z and b, then it will end up coinciding with b! The axis of this

rotation is simply the normal to the plane, and the angle is something

you know how to get.

For reference, the first frame of the walk animation is shown on the right. If

you comment out the character->advance(…) line in

advanceState(), your character should look like this.

Splines with t0 ≠ 0, t1 ≠ 1. In class, we’ve derived the formula for cubic

Hermite interpolation f(t) with endpoints at 0 and 1. How can we reuse this

formula for a cubic Hermite spline, which has a whole sequence of control

points at different values t0, t1, t2, …? Here’s a simple solution:

1. Find the interval [ti, ti+1] within which the desired time t lies.

2. Rescale the interval so that ti maps to 0 and ti+1 maps to 1. Find the corresponding

value of t, say tr.

3. Now we are working with the standard interval [0, 1]. Apply the cubic Hermite

interpolation formula to evaluate f(tr), and return the result.

There is one thing to be careful about: When you rescale time from [ti, ti+1] to [0, 1], the

derivatives also get rescaled! (That is, the slope of a function changes if you squash it

horizontally.) So you will have to use multiply the derivatives at the endpoints by (ti+1 − ti)

when applying the interpolation formula. Conversely, if you are computing the derivative of

the spline function itself, you will have to divide the result by (ti+1 − ti) to get the correct value

when stretched back to [ti, ti+1].

You can tell that your spline interpolation is working correctly if the default spline defined in

the starter code matches the screenshot on the first page, and the “approximately circular

path” defined in the comments in main.cpp looks like a circle and not a diamond. To verify

that Spline::getDerivative() is working correctly as well, which you’ll need to get

the walking speed right, you could draw a sphere at f(t) (i.e. path->getValue(time)),

another sphere at f(t+1), and an arrow starting from f(t) along the vector f′(t) (i.e.

path->getDerivative(time)). The head of the arrow, f(t) + f′(t), should end up

somewhat close to f(t+1).

Above and Beyond

All the assignments in the course will include great opportunities for students to go beyond

the requirements of the assignment and do cool extra work. We don’t offer any extra credit

for this work — if you’re going beyond the assignment, then chances are you are already

kicking butt in the class. However, we do offer a chance to show off… While grading the

assignments the TAs will identify the best 4 or 5 examples of people doing cool stuff with

computer graphics. After each assignment, the selected students will get a chance to

demonstrate their programs to the class!

There are many ways you can go beyond the requirements in this assignment. You can

improve the model rendering, let multiple people walk (or dance) together, or any number of

other options. If your ideas for going beyond the requirements would make your code more

difficult for the TAs to grade, please help them by submitting a standard version of your

assignment first through the normal website link, and then email the TAs the fancier version

of your assignment.

Support Code

The webpage where you downloaded this assignment description also has a download link

for support code to help you get started. The support code for this assignment is a simple

program using the SDL-based engine, similar to the ones we have used before. You should

also download separately the zip file containing the mocap data files.

The support code defines a program structure and everything you need to read and parse the

mocap data. To make locating data files simpler, we have a header file called config.h

that contains absolute paths to your data files. You should edit this file with the full

path (e.g. “C:\Users\Turing\a4\data” or “/home/turing/a4/data”) where

you’ve placed the data files. We will modify this file appropriately when grading your

assignment.

Handing It In

When you submit your assignment, you should include a README file. This file should

contain, at a minimum, your name and descriptions of design decisions you made while

working on this project. If you attempted any “above and beyond” work, you should note that

in this file and explain what you attempted.

When you have all your materials together, zip up the source files and the README, and

upload the zip file to the assignment hand-in link on our Moodle site. You don’t need to

include the data files, which are pretty large and we have a copy of them anyway. Any late

work should be handed in the same way, and points will be docked as described in our

syllabus.

Data Credits

The motion capture data used in this assignment was obtained from the CMU Motion Capture

Database. This database, along with additional information, can be found at

http://mocap.cs.cmu.edu/.

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