# Csci 1933 Project #2: A Bouncing Ball Screen Saver

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## Description

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Summary. For Project #2, we will write a screen saver application that animates balls bouncing
and colliding inside a box. This project will require you to use inheritance to extend the Circle
class as well as an AnimationFrame class we have written for you, and will give you practice
applying concepts like composition, arrays, and handling user input.
Part I. Defining and Implementing a Ball class (40 points)
The Ball class should extend the Circle class from the last project (please download our version
from the Moodle site), and should maintain state information and implement the physics of each
bouncing ball. The state information should include the size, color, position and velocity (speed
in both X and Y dimensions) of the ball and all of the methods necessary to interact with a ball
such as checking if two balls have collided and functionality for updating state after a collision.
Methods inherited from Circle:
● Name: setColor; input: color of the shape (type Color); output: none
● Name: setPos; input: x, y position of the center (both doubles) output: none
● Name: setRadius; input: radius (double) output: none
● Name: getColor; input: none; output: color of the shape (type Color);
● Name: getXPos; input: none; output: x position of the center (double)
● Name: getYPos; input: none; output: y position of the center (double)
● Name: getRadius; input: none; output: radius (double)
● Name: calculatePerimeter; input: none; output: perimeter of the circle (type double)
● Name: calculateArea; input: none; output: area of the circle (double)
New methods that you must implement in the Ball class:
● Name: setSpeedX; input: speed of the ball’s movement along the horizontal axis (type
double); output: none; This method should update the ball’s X speed to the input value.
● Name: setSpeedY; input: speed of the ball’s movement along the vertical axis (type
double); output: none; This method should update the ball’s Y speed to the input value.
● Name: getSpeedX; input: none; output: speed of the ball’s movement along the
horizontal axis (type double)
● Name: getSpeedY; input: none; output: speed of the ball’s movement along the
vertical axis (type double)
● Name: updatePosition; input: number of time units passed (type double); output: none;
This method should update the ball’s position based on its current velocity and the time
elapsed.
● Name: intersect; input: another Ball object to check if they have collided (type Ball);
output: Boolean; This method should check if the passed Ball object overlaps with the
current Ball, which will be useful for detecting collisions among Balls.
● Name: collide; input: another Ball object with which the current Ball should collide (type
Ball); output: none; This method should confirm that the balls have collided (use your
“intersect” method to check this) and implement the physics of two balls colliding— the
state (i.e. X and Y speeds) of each ball should be updated accordingly at the end of this
method. See our tutorial on the physics of 2D collisions at the end of this document for
hints on updating the speeds after the balls collide.
● Name: Ball (constructor) ; input: x position (double), y position (double),
radius (double); color (Color) output : object of type Ball
Data members: any that are necessary to maintain the object’s state and implement the
methods above.
Part II. Defining and Implementing a BallScreenSaver class (40 points)
You will need to implement a second class that will actually support the animation of the
bouncing balls. We have implemented an abstract base class, AnimationFrame, that should
help you get started. This class has all of the core functionality necessary for creating an
application frame and updating a window that can animate graphics. You don’t need to worry
about the details of how this class is implemented, but you will need to understand how to make
it draw animation.
The key pieces that enable classes derived from AnimationFrame to implement animation are
the action() and draw() methods. First, an object of this type must be instantiated and the start()
method called. Then, at a rate determined by the member variable, fps (frames per second),
the action() method is periodically called and should perform the logic for updating the animation
frame (e.g. update the positions of all bouncing balls). After each call to action(), the method
draw() is called, and should be used to redraw any graphics inside the frame. As long as the
frames per second rate is higher than ~15, the graphics in the window should look reasonably
animated. Remember that most monitors won’t refresh more than ~60 times per second
anyway, so it will not help to update your graphics more often than that.
Your task is to define and implement a class called BallScreenSaver that extends our
AnimationFrame class to display balls bouncing around inside a box. The methods you will
need to implement/override are:
● action(): input: none; output: none; As described above, this method is called
periodically (default rate 50 times/sec) and actually updates the states of any objects
being animated.
● draw(): input: graphics object reference (type Graphics); output: none; As described
above, this method actually draws the graphics for each frame after the action method is
called to update state.
● processKeyEvent(): input: event object corresponding with a keyboard event (type
KeyEvent); output: none; This method is automatically called whenever a key is
pressed while the animation is running. You will need to override this method to support
the requirements described below.
Hint: all of the above methods should override either existing or abstract methods in the class
AnimationFrame, so the method signatures should be identical to the base class.
● BallScreenSaver (constructor): input: window frame width (int), window frame height
(int), name of the application window (String), the number of bouncing balls to be
animated (int) output : object of type BallScreenSaver
Data members: any that are necessary to implement the methods above.
Requirements for your animation:
Your BallScreenSaver class should support the following functionality:
● Variable number of balls as input (constructor) parameter (Hint: you can assume that all
balls have the same radius and mass)
● Balls should be placed inside a rectangular box, with random initial velocities and
random positions
● Balls should elastically bounce off walls and each other (for hints, see the “Physics of 2D
collisions” section below)
● Keyboard control for object speed: The left and right arrows on the keyboard should be
used to control the speed of all balls in the animation. When the right arrow is pressed,
both the X and Y speed of all balls should be increased by 10% in their current direction.
When the left arrow is pressed, both the X and Y speed of all objects should be
decreased by 10%. (Hint: the processKeyEvent method will be very useful here!)
● Ball color upon collision: In each animation, all balls except one should be green. One
ball should be red, and the red color should “spread” with each collision. Specifically,
anytime a red ball collides with a green ball, the green ball should switch to red. For
example, if you were to let your screen saver run forever, all balls will eventually be red.
Part III. Implementing a CollisionLogger class (20 points)
Finally, you will need to finish an implementation of a CollisionLogger class, which will be used
to count the number of collisions that occurs in each “bucket” of a two-dimensional, discretized
grid. We have provided a skeleton version of this class for you—you just need to add the
appropriate data members and complete the implementation of the methods described below.
You are required to use a two-dimensional array of integers as a data member of this class.
CollisionLogger (constructor): inputs: int screenWidth, int screenHeight, int bucketWidth inputs:
the screen width (type int), the screen height (type int) and the bucketWidth (type int); output: an
instance of the CollisionLogger class.
● collide: inputs: two different Ball objects; output: none. This method should log a collision
at the position of the two balls. You can assume the average X and average Y position
of the two balls for the collision location.
● getNormalizedHeatMap: inputs: none; output: a two-dimensional array of ints. This
should return the current state of the collision log but rescaled such that the bucket with
the largest number of collisions has value 255, and buckets without any collisions have a
value of 0.
Below we provide an example of where different x,y coordinate pairs would be placed for a
given CollisionLogger of the corresponding height, width, and bin size.
You should complete the implementation of the CollisionLogger class we’ve given you, and use
it properly to log collisions in your screen saver. Notice that in our BouncingBall example class
(described below), we demonstrate for you how to set up the CollisionLogger, and we have
configured it such that a PNG image with the collision log heatmap is printed each time the “p”
key is pressed. Your BallScreenSaver implementation should also include this functionality (feel
free to reuse our example code from BouncingBall).
Hints:
An example screen saver class:
To show you a simple example of how you might use our AnimationFrame class, we have
implemented a very simple screen saver class, BouncingBall, that has just one ball bouncing
around inside a box. Our action() and draw() methods look like this:
public void action(){
//This method is called once every frame to update the state of the
BouncingBall.
//update both X and Y positions
ballX+=ballXSpeed/getFPS();
ballY+=ballYSpeed/getFPS();
//handle collisions with the border
if ( ballXgetHeight()-BORDER ){
ballXSpeed*=-1;
}
if ( ballYgetWidth()-BORDER ){
ballYSpeed*=-1;
}
}
public void draw(Graphics g){
//This method is called once every frame to draw the Frame.
//This is how you use the graphics object to draw
g.setColor(Color.black);
g.fillRect(0,0,getWidth(),getHeight());
g.setColor(Color.white);
g.drawRect(BORDER,BORDER,getWidth()-BORDER*2,getHeight()-
BORDER*2);
g.fillOval((int)ballX, (int)ballY, ballSize, ballSize);
}
Your screen saver will be much more exciting, but notice how the methods described above are
implemented to support the animation. Note that to actually use the BouncingBall class, we just
write the following inside a main method:
BouncingBall bb= new BouncingBall(800,800,”Bouncing Ball”);
bb.start();
A tutorial on the physics of 2D collisions:
Since this isn’t a Physics class, we’ll help you out with the physics of 2D collisions. For an indepth discussion of the physics of 2D elastic collisions, see this:
http://en.wikipedia.org/wiki/Elastic_collision . We’ll give you a basic summary and the formulas
you will need to update objects’ velocities after a collision.
Consider two balls of equal radius and the given positions and velocities that are about to
collide. The basic principle we rely on to compute the velocities after the collision is the
conservation of momentum and conservation of kinetic energy (the collision is elastic). A “trick”
which makes the calculation much easier is to translate both balls’ velocities into a coordinate
system aligned along the direction of the collision (Xcol and Xnorm vectors below). Since the ball’s
are of equal mass, the calculation becomes trivial in that coordinate system: the components of
the velocity along the collision direction are simply flipped between the two objects, and the
component perpendicular (Xnorm) stays the same for both objects. The math for this is worked
out in detail below.
First, let’s compute unit vectors for the new coordinate system:
Now, to translate the velocities of both balls into this coordinate system, we need to do the
following matrix multiplication, which simply projects each ball’s velocity vector onto the two
perpendicular directions:
So, these are the actual velocities for balls i and j in the new coordinate system:
Now, we need to actually compute the new velocities for both balls in this new coordinate
system. The second component (corresponding to the Xnorm direction) stays constant for both
balls, but the first component (corresponding to the Xcol direction) flips:
Now, we just need to translate these velocities back into the original coordinate system of our
animation frame, and we’re done. To do this, we simply multiply by the inverse of the matrix
from before, i.e.:
So, to summarize:
1. Compute the distance between the centers of the two balls.
2. Find unit vectors corresponding to our new coordinate system:
Δx = (xi – xj)/D Δy = (yi – yj)/D
3. Redefine the original velocities of the balls according to the new coordinate system:
For each ball: newColVelocity = oldXVelocity * Δx + oldYVelocity * Δy
newNormVelocity = -1 * oldXVelocity * Δy + oldYVelocity * Δx
4. Compute the post-collision velocities of the balls according to the new coordinate
system. This is done simply by swapping the newColVelocities of the balls with each
other. That is, ball i now contains the newColVelocity of ball j, and vice versa.
5. Finally, return to our old, familiar x and y coordinate plane by multiplying the new velocity
vectors by the inverse of the matrix.
For each ball: finalXVelocity = newColVelocity * Δx – newNormVelocity * Δy
finalYVelocity = newColVelocity * Δy + newNormVelocity * Δx
Submitting your finished assignment:
Once you’ve completed Project #2, create a zip file with all of your Java files (.java), including
one that implements your main program, and submit it through Moodle.
Working with a partner:
As discussed in lecture, you may work with one partner to complete this assignment (max team
size = 2). If you choose to work as a team, please only turn in one copy of your assignment. At
the top of your class definition that implements your main program, include in the comments
both of your names and student IDs. In doing so, you are attesting to the fact that both of you
have contributed substantially to completion of the project and that both of you understand all
code that has been implemented.
To give you a sense for the grade your assignment will receive, here are some examples of
what will be required for each grade level:
A-level assignment:
● Properly defined Ball class with correctly implemented methods (passes unit test)
● Properly defined BallScreenSaver class with implementations of the action and draw
methods
● Multiple balls and box and ball collisions detected, states updated properly
● Keyboard control updates ball speeds properly
● Ball colors change upon collisions with the red ball
● Properly implemented CollisionLogger class
● Correct use of the CollisionLogger class in your BallScreenSaver
● Good programming style (spacing and comments)
B-level assignment:
● Properly defined Ball class with correctly implemented methods (passes unit test)
● Properly defined BallScreenSaver class with implementations of the action and draw
methods
● Partially working animation with at least one ball object
● Collision detection working, partially working state updates
● Properly implemented CollisionLogger class
● Partially working use of CollisionLogger inside BallScreenSaver
C-level assignment:
● Properly defined Ball class with correctly implemented methods (passes unit test)
● Partially defined BallScreenSaver class with implementations of the action and draw
methods
● Partially defined CollisionLogger class