The following sequence of assignments reflects the
J-term course given at
Gustavus Adolphus College
in 2013. This content was originally presented with a mixture of lecture and
lab; most of this had verbal explanation. But it's posted here to support future
offerings of the course and for the curious out there who may want to try this
on their own. It's a great way to gain useful coding experience and learn the
First, some thanks directed at sites that sparked this one:
This is a progression starting from very basic Pygame
ideas, through 1D and 2D (pure
Python) physics engines, ending with an application of the
Box2D physics engine.
There is a physics instructor behind this, so of course there will be
references to things like
tables. Time-based physics calculations are separated from any rendering.
The engines calculate object position (meters in floating point) based on
time intervals in the game loop. Euler's integration method is used (rate
calculations). Both impulse and non-impulse collision calculations are made.
The hope here is that students do not need formal training in physics (but
probably do need some high school math).
There is attention given to the issue of stickiness (object-penetration
correction). This uses an intuitive calculation to determine where the objects
would be after the collision had they not penetrated each other.
I wanted the incentive of a tournament behind the course, so there's
This is all aimed at students of a January-term class. The bulk of this
had to be absorbed by the students in about 2.5 weeks (two lectures and
a lab each day). This leaves them about a week to do project work in a team.
Each section below has the following:
An assignment in PDF form that includes a problem statement, reference
material, algorithms, conceptual drawings, and (sometimes obfuscated) code
hints. The good stuff is in the PDFs.
Source code in a raw text file and also a colored HTML version.
A screen-capture video of the code running and rendering in a Pygame
Installation of the Working Environment: PDF
/ (No code)
Note that this download is a snapshot of all the installation
files that were used for the course. More recent versions of these
files can be found, but you can be certain this set will work. These
files are tested on XP, Win7, and Win8. The download link is in
the PDF or here:
Python 2.7, with Pygame, pybox2d, pgu, and PodSixNet
Window's Command Window (cmd) and the Notepad++ Editor: PDF /
This is a small collection of useful Windows and Notepad++ tips
for the beginner and/or those new to Windows.
This first assignment illustrates the display screen and event-handling
features that are used from Pygame. Drawing, erasing, and screen
updating are viewed in the Pygame window. Keyboard and mouse events
are interpreted and used to control the drawing algorithms. A game
loop keeps repeating the process: erase, draw, update the screen.
Holding down the "e" key enables screen erasing every time through
the game loop. Holding down the "f" inhibits the flip operation
that is use to update the screen at the end of the game loop. The
two mouse buttons are used to change the color of the circle.
The video starts with the erase feature. With erasing enabled,
the ball seems animated (the drawing tail does not persist). Later
the flip (screen update) operation is inhibited (these are drawn
in memory but not to the screen). Finally a combination of erasing
and flip inhibiting cause the ball to lag behind and then catch
up to the cursor.
The 1D framework introduces the relationship between the screen
and the physics world. It also brings in the first taste of Euler's
method in animating the motion. There are no car-wall or car-car
collisions here; no gravity. Cars just pass through each other (and
walls) and no acceleration from gravity. OOP classes are used here
to organize the code and prepare for the object nature of the games
The video shows five short demos (press keyboard numbers 1 through
5). The first two are two-car animations. The last three show stacks
of cars spreading out due to the differences in their velocities.
The car in the middle of the stack has zero velocity. At any point
in time, the relative velocity of any pair of cars is proportional
to the separation distance between them (kind of like Hubble's law).
Euler's method comes to life here. Velocities are changing under
the accelerating influence of gravity. There is a drawing in the
PDF that illustrates the idea of penetration/stickiness correction.
(Note that the drawings in the PDFs display with varying quality
depending on the browser; Chrome: great, IE: OK, Firefox: not so
hot.) For now, we'll just do the stickiness correction; later there
will be several video demos of this issue. A coefficient of restitution
is used to model the energy loss in the wall collisions. Notice
the apparent settling of the cars at the end of the video.
(Note: You can watch in full-screen mode using the YouTube controls
in the lower right corner of each video. Esc to get back to this
Car-car collision physics and car-car stickiness correction are
added here. This script uses the "c" and the "s" keys to toggle
two algorithmic features that help to illustrate the collisions
and stickiness. The "c" key toggles the "color-transfer" feature
which, when enabled, causes the colors of two colliding cars to
swap. So when two cars start to settle near a wall under the influence
of gravity, they collide frequently and the colors will swap quickly
to show the cars are still colliding. If this feature is off the
cars will appear to settle, but the cars really don't settle in
our basic physic engine! (Later with Box2D we will show true settling.)
The "s" toggles the stickiness correction on and off. So if you
turn off the correction, the cars will be pulled into each other
as they settle. Hit the "s" key again and they will unstick with
a little pop. A combination of the "c" and "s" key is used at the
tail end of the video. Sorry, this one runs a little long (kind
of like watching paint dry).
The user gets to interact with the objects on the screen. Cursor
spring and drag forces are calculated based on the separation distance
between the cursor and the selected car (and the velocity of the
car). Cars are selected by clicking on the car or holding down the
mouse button and letting the car run over the cursor. Cursors attach
at the center of the car. Each mouse button invokes a different
cursor tether (with a different spring constant and car drag coefficient).
The left mouse button is medium, the right mouse button is stiff,
and the center (roller) button has the softest spring. The video
closes with me trying to pull the car into the wall. The color transfer
("c") is turned on so the frequent collisions are illustrated. Note
you can again turn off the stickiness correction here ("s") and
pull the cars into the wall; then toggle it off and watch them pop
out (not so much "paint drying" here since you can pull them in
pretty quickly, especially with the stiffer of the three cursor
Gui here, not just sticky (ha ha). Controls have been added for
stickiness and color-transfer toggles as well as a gravity slider
(for simulating that bad cruise-liner experience) and a button to
freeze the cars. If gravity is set to zero, a freeze operation will
stop the cars and they will stay that way until...
Car mass is visualized here by hollowing out the lighter cars.
The video shows 10 of the 13 demos. The demo number is indicated
in the window title (upper part of the Pygame window frame). Demo
#4 shows the inelastic collisions between a set of cars where the
total momentum of the set is zero before and after the collision
(a reverse explosion). Most of the other demos make use of the color-transfer
feature to highlight the transfer of momentum through cars (like
Newton's cradle). A description of each demo is in the PDF.
The video shows the server window and one client's game-pad window
(also running on the server's computer). Another client (running
on a networked laptop) is connected but not visible in the video.
The state (U:up or D:down) of the a-s-d-w keys of each client are
also rendered on the server screen. Please note that this client
works only for this assignment.
The last 200 mouse locations (when mouse button is down) are
drawn each game loop. This causes the dynamic tails effect.
One Friday we ran this server on the computer that hosts the
projector in the lecture room. Many students connected with the
client. For a while we tried this with no verbal communication.
It was interesting to see how cooperative this became in spite of
The video shows a vector sandbox that is based on the vector
class. There are seven demos that are run (start these using the
number keys above the keyboard). Each demo uses a set of vectors
ranging in set size from 2 to 140. Vectors can be selected with
the mouse (click and drag over the arrow head). Components of selected
vectors can be displayed ("c" key toggles this on/off). Components
include x, y, unit normal (red), unit perpendicular (red), and the
projection of the selected vector onto a second vector. Vector rotation
can be toggled on/off with the "f" key. The "a" key toggles the
display of the add vector (grand total), which is shown in green.
In "add" mode each vector in the sum series is rendered head to
tail. The "t" key toggles on/off the display of a tail drawn from
the head of the total vector. The tail is represented with a 350
point FIFO list of the most recent points. The tail can be shown
as points or lines (toggle back and forth with the "l" key). Zoom
in/out with the "h" and "n" keys.
This assignment merges the 1D engine with the 2D vector class
and the multiplayer module. The spring class is also introduced
here. There are drawings in the PDF that explain 2D collisions,
stickiness corrections, and the client-server relationship.
The video shows two cursors, one from the local client and the
other from a network client on a laptop. Left and right hands are
working the two cursors. It's surprisingly natural (we've got two
hands). One section of the video shows the two cursors' tethers
pulling two balls together and watching the transition from stable
to unstable as the cursors cross each other; once unstable, the
balls flip their positions to get back to stability. Stickiness
behavior is toggled on/off with the "p" key; the black background
turns grey when the stickiness corrections are off.
The client script that is used here works with all the following
assignments (except the first two in the Box2D section).
A raw tube and a jet tube are associated with a client-controlled
puck. The association causes the tubes to move with (are drawn relative
to the center of mass of) the pucks. A new three-point red polygon
is used to represent the jet's exhaust flame. The video shows the
local client and a network client controlling (rotating) tubes on
the two pucks. Python inheritance is introduced in this exercise:
jet-tube is derived from the tube base class.
A thrust force is added to coincide with the jet's flame. This
brings the total to four forces that are being processed in the
physics engine: gravity, cursor-tether tension, spring-damper, and
now jet force.
The "w" key turns on the jet forces that are applied to the host
puck along the direction of the jet-tube axis. This jet-force vector
is added to the puck_forces_2d_N (net-force-on-puck) vector and
processed in the Euler's method calculations.
The video show a cursor tether restraining a puck under continuously
applied jet forces while the jet tube is turned at a constant rate.
The physics of gun
is modeled with an impulse force. Each bullet firing causes an opposing
impulse force to be delivered to the gun. The magnitude of this
force is equal to the change in momentum of the bullet divided by
the duration of the firing interval (one step in time by the physics
calculations in the game loop). Bullet clean-up is implemented by
assigning a birth time to each fired bullet. Bullet objects older
than 3 seconds are deleted.
This assignment adds features to make a playable game. For example,
a hit counter (a hit by another player makes your puck flash red)
is the basis for establishing the health of each player. Health,
or rather the lack of it, is illustrated with an expanding red circle.
Too many hits and your puck pops (hence the name of the game: Puck
Popper). Shields prevent bullets from hitting your puck; when shields
are up, you can't fire. The "s" and "k" keys are used to flip the
tube directions into an orientation opposing the current motion
of the puck; this can be useful in breaking (stopping). The "f"
and "g" keys can add an interesting dimension to the game by the
local user (user on the server computer); try it.
The jello simulation that's shown in some of the other videos
has been turned into a game here. You scramble the jello (this starts
the timer) for your opponent then press the "p" key to freeze the
physics engine and the corresponding timer on the screen. When your
opponent is ready to try and straighten out the jello, they press
the "p" key again. That resets and starts the timer and physics
engine. When the jello is straightened (no puck collisions), the
timer will stop. A useful tool to use in the straightening is repeated
use of the "f" key. This momentarily sets the velocity of each puck
to zero. Low score (time) wins. Players take turns.
The video shows me playing Puck Popper solo with something heavy
sitting on the "i" key of my laptop. So the red player is pretty
much a sitting duck in the corner. But you get the idea of the shields,
health circles, and the popping pucks. I also do one round of Jello
Madness and got a de-tangling time of 22.15 seconds (a big part
of this is to really make a mess of it for the other guy). Jello
Madness requires an I5 processor or better; this will crash (become
unstable) on slower machines.
The perfect-kiss algorithm is a refinement to the overlap-correction
calculations described in the 2D-Physics Engine Framework
assignment. This refinement offers true contact-normal calculations
and corresponding ideal modeling of 2D puck collisions. The video
is annotated to show three categories of puck collisions: (1) raw
(no overlap correction), (2) overlap correction using the approximate-contact
normal, and (3) overlap correction using the idea-contact normal.
The speed of the incoming puck is chosen to produce large overlaps
and its initial position is randomized to show a variety of responses.
The third category of runs shows a consistent collision prediction
that is independent of the amount of overlap at the collision detection
point. To illustrate the correction process two intermediate steps
are drawn with special colors: both overlapping pucks are drawn
in red; both kissing pucks are drawn in cyan; the final corrected
position is drawn in normal puck colors. There are cases where some
of the intermediate puck images are not visible because they lie
directly underneath a subsequent drawing.
This script is based on the test_BodyTypes.py file in the examples
directory of the pybox2d distribution. This depends on the Pygame
framework in that distribution so all the framework files must be
in the same directory as this file. (This script works best if you
run it in the "...box2d_source_files\box2d_jdm" folder of the zip
distribution provided above. I fixed a few things in their framework
files and those fixes are only available in the box2d_jdm folder.)
This file has been modified to support bullet shooting. (I looked
through their examples for one that had some hinged object that
would work for aiming the projectile stream.) The main adders here
are: (1) bullet aiming and firing, (2) masking of objects (bullets
pass through the gun base without a collision), (3) age limits on
the bullets so they clean up after themselves (the simulation would
labor if the collection of bullets kept growing), (4) the cursor
gun, (5) something interesting to shoot at (target generation):
pyramids, circles, and squares.
This script is based on the simple_01.py example in the pybox2d
example directory. Unlike the example above, this one does not depend
on the framework files, so it will run anywhere. I've extracted
mouse-joint and zooming features as well as some basic polygon rendering
facilitated by Pygame. This demonstrates the body-transform overload
operation on a vertex of a polygon (to get its physics-world coordinates).
There are also two force points defined on one of the cars (keyboard
The video shows an air-track type environment, except of course
this is 2D and the cars are allowed to behave very badly and can
end up off the track. Cars can be dragged with the mouse. Zoom the
view with the mouse wheel; pan the view with the right mouse button.
Control the two force points with the "f" and "g" keys. When a force
is applied at a force point, the point lights up green. (View in
full-screen mode at 480p to see the force points.)
Finally, here is an integration of the box2d engine into our
own 2D framework. This is like taking the engine out of our Honda
Civic and replacing it with a 426 Hemi. This opens the door to non-spherical
object collisions, object rotation, torque (and rotational drag),
surface friction, and true settling behaviors.
The basic idea here is that we define an interface between our
air-table framework and the pybox2d engine using force points. So
we keep all the force generating objects such as springs and cursor-tethers
and communicate their forces to the Box2D world. Then we let Box2D
take care of collisions and object motion and let it inform our
rendering functionality as to the state/position of the objects.
A few notes on the interface: gun and jet controls are the same
as before. The "t" key will torque a selected object. Shift-t will
torque it the other way. The "f" key still freezes the translational
motion, but now there is also the "r" key which will stop the rotation
of all the objects in the world. Cursor tethers attach, by default,
to the center of mass of the object, but with the shift key down,
you can attach cursor tether anywhere on the object. The "h" and
"n" keys zoom the view in and out; if the control key is down, mouse
movement acts to pan the view. Various demos are initiated as usual
with the keyboard number keys (i.e. above the letters). In this
video I have one network client, a laptop, connected (red cursor);
the regular cursor is controlled by the server.