# CSCI 4611 Assignment 2: Car Soccer

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## Introduction

To render compelling 3D computer graphics and games in real time, all graphics data has to
be sent to the GPU using a graphics API such as OpenGL or DirectX. We will be using the
OpenGL API in this course, and this assignment will be your first significant experience with it.

You’ll use OpenGL 2 in immediate mode to render some simple 3D graphics, and you’ll use
the GLM library to work with 3D points and vectors in C++. These will help you to compute the
positions, velocities, and other quantities needed to make a simple 3D interactive game
based on the 2015 game Rocket League (http://rocketleague.psyonix.com/).

The above is a screen shot from an example of the game you will create. The playing field, or
pitch, is rendered out of 3D boxes and line segments. The car is drawn using simple 3D
primitives; in my implementation, it’s just a box. The ball is a sphere. The car can be made to
move around using the arrow keys and go faster by holding Shift. When the car hits the ball,
the ball reacts in a “physically plausible” way, updating its current velocity based on the
direction it was hit and the velocity of the car at the time of impact.

## In this assignment, you should learn to:

• Use a combination of SDL, OpenGL, and GLM to build a 3D graphics program of your
own.
• Draw simple 3D geometry using OpenGL.
• Work effectively with 3D points, vectors, and other “graphics math” primitives.
• Balance the tradeoffs between realism and effective game play by simulating physics
in a “plausible” but not necessarily 100% realistic way.
• Successfully program a first interactive 3D graphics game.

We will post a video of our solution to the assignment so that you have an example of what a
successful implementation might look like. We also provide some support code to help you
get started. This code includes the routines needed to move the car around based on

The game play you are required to implement is simple relative to what it could be. (There’s a
great opportunity here to go above and beyond the assignment to create a much more
compelling game.) In this assignment, all we ask you to do is the following. Have the
computer put the ball into play, kicking off the ball from the center of the pitch toward your
side whenever you press the space bar. Program support for hitting the ball with the car
whenever the two come into contact. You should also detect when the ball hits the goals on
either side of the pitch. To do all this, you’ll need to calculate collisions between the ball and
the ground, walls, and car. Finally, add car-like steering and rotation instead of simply
translating the car across the pitch.

Most computer games modeled after real sports are designed to balance the tradeoff
between physical realism and game play. A completely realistic simulation of, say, soccer
would be quite complex and would make it really difficult to play the game on a 2D computer
screen, especially with the limited amount of control possible using a keyboard/mouse or
controller input. In our case, the playing field is the size of a real soccer pitch, and the car is
about the size of a Mini, but the ball is absurdly large to make it easy to hit. You may also
want to have a larger-than-life gravitational acceleration so that things don’t stay in the air
for an annoyingly long time.

The dimensions of the pitch are shown above in meters, as well as the coordinate system we
will use. The support code is set up to work in units of meters, matching the diagram above.
The ball itself has a radius of 2 meters.

To make the game easier to implement, we will make a few simplifying assumptions about
1. To simulate friction, you can simply decrease the speed of the ball a bit when it hits
anything. For example, you might make the speed after bouncing 0.8 times the speed
before the bounce.

2. To check for collisions between the ball and the car, you should treat them both as
spheres, even if the car is actually drawn on the screen as a box (and would be a much
more complicated model in a real game). This makes it extremely easy to do collision
tests, as we describe below in the Technical Background section. In games, it is typical
to test for collisions using such a “collision proxy” whose geometry is much simpler
than the rendered model.

3. In real life, when two objects collide, they experience equal and opposite forces, and
both their velocities change. To keep things simple, in this assignment we will assume
that in a collision between the ball and the car, only the ball’s velocity changes while
the car is unaffected.

4. Watch out for virtual balls penetrating other virtual objects, i.e. the ground, the walls,
and the car. If you update your simulation once each frame, that means the “time
step” of your simulation (i.e., the time interval dt between consecutive frames) will be
somewhere around 1/30–1/60 second. That’s fast, but still not fast enough to capture
the exact moment when the ball first makes contact with the car. This means that if
you update the position of the ball using p′ = p + v dt, you may have a situation where,
for example, the ball was away from a wall at the last frame, but at the next frame the
ball has not only hit the wall but is partially inside it! Of course, this cannot happen in
real life, but in computer graphics, a virtual ball could actually penetrate the virtual
wall unless you detect this situation and correct for it. The figure below demonstrates
this situation. When you detect this situation, you can correct for it by simply setting
pnew to a value that places the ball just outside the wall, as shown.

A more specific list of requirements follows. We use this list in our grading, so this also serves
as a grading rubric. To get a 100% on the assignment, you need to correctly implement
everything that is listed here. To get a grade in the “A” range, you need to implement
everything in the “C” and “B” ranges, plus some portion of the features in the “A” range, and
so on. You’ll find that the requirements are ordered very conveniently here, almost like a set
of steps that you should take to complete the assignment. We recommend starting at the top
and going down the list as you work.

### Work in the “C” Range Will:

• Draw the 3D soccer pitch and ball with appropriate dimensions and pitch markings.
Feel free to add more functions like rectXY(), circleXY(), etc. to make your life
easier.
• Draw the car as something other than a point. A box of size 3×2×4 meters will do, but
feel free to come up with something more interesting.
• Make the ball move through the air based on a random initial velocity, and relaunch
the ball when the space bar is pressed.
• Update the ball’s position and velocity each time step based on the acceleration due
to gravity.
• Detect a contact between the ball and the ground, and make the ball bounce in the
correct direction. That is, the ball’s velocity vector should be reflected about the
normal of the ground.

### Work in the “B” Range Will:

• Detect when the ball hits the walls and the ceiling and make it bounce off them too.
• Use a similar approach to prevent the car from leaving the bounds of the playing area.
• Decrease the speed of the ball when it bounces. This is due to friction and other
factors, but you do not need to simulate these; just decrease the speed by some
constant factor.
• Give the ball a shadow by drawing a disk on the ground directly under the ball. Give
the shadow a y-coordinate that is just a tiny bit higher than the ground, otherwise the
ground and the shadow will be at exactly the same height, and the graphics engine
will not know which is on top of the other.

### Work in the “A” Range Will:

• Respond to a contact between the ball and the car. To do this correctly — relative to
the car’s spherical collision proxy, at least — you will need to define a “collision
normal”, and reflect the ball’s velocity about the collision normal. See the technical
discussion in the next section.
• Incorporate the velocity of the car into the way the ball responds when hit. For
example, if you drive really fast into the ball it should go much faster than if you nudge
it gently.
• Give the car a more realistic driving model. The car should always move forwards or
backwards relative to the direction it’s facing, but never sideways. The up and down
arrow keys should change its speed, while left and right should turn it at a rate
proportional to its speed. A simple approach for doing this is given at the end of the
technical discussion.
• When the ball hits one of the goals, reset the car to the initial position and relaunch
the ball from the center of the pitch. You don’t have to animate the ball actually going
into the goal, just treat them just as special regions marked out on a flat wall.

### Additional Technical Background and Tips

One of the main challenges in this assignment is handling collisions between the ball and the
car. For collision purposes, we will approximate the car by a sphere, as shown in the figure
below, so we only need to detect whether the two spheres representing the car and the ball
are intersecting. Of course, this may result in us detecting collisions when the car’s rendered
geometry does not actually hit the ball, or vice versa, but as long as the proxy and the render
are not too different it shouldn’t matter too much to the gameplay.

In any collision handling implementation, there are two main steps: first, detecting whether a
collision has occurred, and second, resolving the collision by updating the positions and
velocities of the colliding objects. With spheres, collision detection is easy: two spheres are
colliding if the distance between their centers is less than or equal to the sum of their radii.
For collision resolution, we approximate the neighborhood of the collision by a plane, giving
us a “collision normal”, as you can see above. For a collision between spheres, the collision
normal is simply parallel to the line joining their centers. Now you must do the following
steps:

1. Move pball along n so that the ball is no longer intersecting the “car” sphere, i.e. the
distance between pball and pcar equals rcar + rball.
2. Compute the relative velocity of the ball, vrel = vball – vcar.
3. Reflect the relative velocity about the collision normal.
4. Set the new velocity of the ball, vball = vcar + vrel.

You will not be graded on how you handle multiple simultaneous collisions, such as when the
ball gets squeezed between the car and the wall; this can be a challenge to handle even in
real games. We will only grade you on whether you correctly handle cases where the ball
collides with only one thing at a time. In any case, if you choose the center of the car to be
lower in y than the radius of the ball, the ball will always be able to escape upwards to get out
of the “pinch”.

As for the car’s motion model, many different approaches are possible that vary in realism
and playability. Here’s a simple one you can use, though you are free to come up with your
own. We may store the car’s direction simply in terms of its angle in the xz-plane,
equivalently, how much it is rotated about the y-axis. Separately, we store the speed in the
forward direction, which may be positive or negative. At each frame, assume there is a thrust
force, which is proportional to whether the up or down arrow key is held, and a drag force,
which is proportional to the current speed; therefore, increment the speed by (thrust − drag)
dt. For turning the car, define the turn rate, which is proportional to whether the left or right
arrow key is held, and increment the direction angle by turnRate × speed × dt.

Above and beyond
All of 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 some great opportunities for extra work in this assignment. Turn this program into
a more exciting game! Change the camera position to follow the car and/or point to the ball!
Add a car for a second player! Draw some fireworks when a goal is scored before resetting the
ball! Keep count of the score by drawing some tokens above or next to the goals! (Text is hard
to do in plain OpenGL, so don’t bother with that.)

#### Support Code

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. It includes
some user interface code for moving the car around, and sets up lighting and a camera view
that will work well if you draw the soccer pitch according to the specifications described
above. This should give you a good start on the 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. Any late work should be
handed in the same way, and points will be docked as described in our syllabus.