Lab 9B: Super 6.009 Adventure

The questions below are due on Wednesday December 13, 2017; 05:00:00 PM.

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Table of Contents

1) Preparation

This is Part 2 of a two-part lab. For Part 2 (lab9B), both the submission and checkoff are due at the final deadline: Wednesday 12/13, at 5p. Note: due to Faculty Rules it is not possible extend the final deadline using late days, so the Part 2 submission and checkoff must be completed by the deadline.

This lab assumes you have Python 3.5 or later installed on your machine.

The following file contains code and other resources as a starting point for this lab:

Most of your changes should be made to, which you will submit at the end of this lab. Importantly, you should not add any imports to the file. You may submit portions of the lab late (see the grading page for more details), but the hard deadline is the end of our last class period on Wednesday, the last week of class (following MIT rules).

This lab is worth a total of 4 points. Your score for the lab is based on:

  • passing the test cases from under the time limit (2 points), and
  • a brief "checkoff" conversation with a staff member to discuss your code (2 points).

For this lab, you will only receive credit for a test case if it runs to completion in under 10 seconds on the server.

Please also review the collaboration policy before continuing.

2) Introduction

This zip contains tests for both the first week and second week to ensure that additions you make this week don't break last week's work!

You should start by copying over your from Week 1 -- that code will have to be in good working order to complete this week's tasks.

Several of the following parts use notions of "vertical" and "horizontal" collisions. A collision is vertical if the corresponding minimal translation vector is vertical, and it's horizontal otherwise.

3) Jumps [unit test: Test07]

Our initial implementation of jumps was rather simplistic. Let's fix it:

⇨ Change the event handling for "up" so that pressing "up" sets the player's vertical velocity to Constants.PLAYER_JUMP_SPEED for the next Constants.PLAYER_JUMP_DURATION rounds.

This requires a bit of clarification. If the player starts at y = 0, and the user jumps at time t = 0, then here is how y evolves, assuming that PLAYER_JUMP_SPEED is 62, PLAYER_JUMP_DURATION is 3, GRAVITY is -9, and MAX_DOWNWARDS_SPEED = 48

Step n: vy is … and y is…
0       0       0
1       53      53   # Initial upwards accel.; velocity is JUMP_SPEED - GRAVITY
2       53      106  # Initial upwards accel.; velocity is JUMP_SPEED - GRAVITY
3       53      159  # Initial upwards accel.; velocity is JUMP_SPEED - GRAVITY
4       44      203  # Deceleration (upwards velocity is decreasing)
5       35      238
6       26      264
7       17      281
8       8       289
9       -1      288
10      -10     278
11      -19     259
12      -28     231
13      -37     194
14      -46     148
15      -48     100  # Downwards velocity saturates at -MAX_DOWNWARDS_SPEED
16      -48     52
17      -48     4
18       0      0    # Hit the floor

This implementation still allows for repeated jumps; let's fix that too.

⇨ Change the event handling so that jumping twice in close succession does not do anything. More precisely, change the jump handling so that after each jump, the player cannot jump again until they have vertically collided with a hard blob from above. Concretely, this means that pressing "up" continuously in the example above would not make a difference until turn 19. It you're having timing issues, use the STEP button in the UI while continuously pressing the "up" key.

Note that this implementation allows you to jump (once!) after falling off a platform (see the superjump map). This is a tribute to Donkey Kong Country's "super jumps." Bonus question: how would you make sure that these super-jumps are not allowed?

4) Foes [unit test: Test08]

The game feels a bit lonely, doesn't it? Let's add a few foes. Here's what we need to change to make this work:

  • We need to add support for new types of blobs. We'll do bees, fire, storms, and evil mushrooms.

  • We need to do more in collision detection; until now, we only moved the player when it hit a hard blob; now, we also need to act based on collisions (exactly what happens depends on the foe). This requires particular care, since collision resolution might create new collisions between soft blobs (thus, we process soft-blob-against-soft-blob collisions after resolving collisions against hard blobs).

  • We need to be able to mark blobs as alive or dead; for example, if we bounce on a mushroom, the mushroom is squashed, and it dies. Dead blobs disappear from the board (except for the player, which changes textures). In general (though mushrooms are a special case), if the player touches a foe, they are defeated (and thus the game is over at the end of that turn).

Here are the rules for the four new blobs:

  • Fire is the simplest. It does not move on its own (but it's subject to gravity, and it stops if it hits a hard blob), and it cannot be killed (unless it falls off the board); if the player touches it, the game is over at the end of that turn. Fire also kills mushrooms, but it doesn't affect bees.

  • Storms are simple too; they don't move at all (they are not subject to gravity) and they can't be killed; the only difference is that they blink. They only affect the player; they don't affect other foes. More precisely, their texture starts as Textures.Storm, and after Constants.STORM_LIGHTNING_ROUNDS rounds it becomes Textures.Rain, and then after Constants.STORM_RAIN_ROUNDS rounds it goes back to Textures.Storm, etc. The player is only defeated when colliding with a storm with a texture of Textures.Storm; the player is not defeated by colliding with a storm with a texture of Textures.Rain.

Concretely (with 5 rounds of lightning and 10 of rain), it looks like this:

Game.render → Storm
Game.render → Storm
Game.render → Storm
Game.render → Storm
Game.render → Storm
Game.render → Rain
… (Rain appears 10 times total)
Game.render → Storm
… etc.
  • Bees are trickier: they travel at a constant vertical velocity Constants.BEE_SPEED (starting towards the top), until they collide vertically with a hard blob (or fall off the board). If they collide with a hard blob, their velocity is reversed. Bees are not subject to gravity. They cannot be killed (in particular, they are immune to fire). They kill the player but not other blobs.

  • Mushrooms are the most complex: they travel at a constant velocity Constants.MUSHROOM_SPEED (starting towards the right), and their horizontal velocity is reversed every time they hit a hard blob horizontally. Unlike bees, fires, and storms, mushrooms can be squished: the mushroom is squished if the player collides with it vertically, from above (that's equivalent to the minimal displacement vector having y > 0). Touching a mushroom from the side or from below kills the player.

⇨ Implement support for fire, storms, mushrooms, and bees.

Note: Dead blobs (trees and mushrooms touched by fire, squished mushrooms) do not participate in collisions and are not displayed onscreen (thus render should not return them). The player blob is a bit different: if it dies, it stays onscreen, but with texture Textures.PlayerLost.

5) Special items [unit test: Test09]

The only significant part that we're missing is collectible items. These items disappear after the player comes in contact with them.


The helicopter blob (h) grants the player flying powers. After passing over a helicopter, the player's texture should change to Textures.Helicopter (until the end of the game, at which point the usual Textures.PlayerWon or Textures.PlayerLost should be displayed), and checks making sure that multiple jumps can't happen in mid-air should be disabled. Players don't get bored when they're a helicopter!

⇨ Add support for the helicopter blob. This is enough to play flappy and flappy-bees!!

Water and boat

The water blob is just like the floor blob, with one exception: when the player is touching water, its sprite should change to Textures.Boat but when the player jumps from water or moves horizontally off the water, its texture should revert to the regular player texture. Players don't get bored when they're a boat! Other moving blobs interact with it just like a floor (mushrooms walk on water, and bees change direction when they hit it).

⇨ Add support for the water blob.


Our last item is the fireball. Upon collecting a Sun blob (o, with texture Textures.Sun), the player can shoot a total of Constants.SUN_POWER fireballs. Fireballs are shot using either the x key (to shoot right) or the z key (to shoot left). They move at Constants.FIREBALL_SPEED towards the right or the left. Pressing both x and z shoots two fireballs (if the player has only one fireball left, ignore z and only shoot towards the right).

The starting position of each fireball is exactly the position of the player on the time step when the fireball is shot, before the player moves. Note that the fireball is created and added to the game while processing player actions in a given timestep. That means that fireballs will be affected by the subquent motion phase of the timestep, i.e., fireballs will move in the step in which they are created.

Fireballs die against all hard blobs as well as mushrooms. If a fireball touches a mushroom or a tree, that blob dies too (bees are immune to fire and fireballs). Fireballs pass through other blobs without affecting them and are not affected by gravity. There is no limit on how many unused fireballs a player can collect at a time.

⇨ Add support for fireballs.

6) Efficient collision detection [unit test: Test10]

The algorithm that we implemented in week 1 for collision detection works OK, but it's much too slow once large numbers of blobs are involved. There are two commonly used tricks to make things better:

  • Don't simulate blob motion outside of the field of view. This works, though it may make some things feel unnatural. It's good for timing enemy actions to the player's arrival, though. We won't implement this (but feel free to experiment on your own!).

  • Improve the collision detection algorithm. Our original implementation considers all pairs of blobs, which is much too slow. You'll need to make the test pass in under 10 seconds to receive credit for this part.

⇨ Implement a fast collision detection algorithm.

You are free to use a well-known data structure like a quadtree, or to implement your own solution tailored to this problem (our solution is just over 15 lines of pretty simple code; don't overthink it!). Unlike week 1's implementation, your solution doesn't need to work with arbitrary rectangles — we will only test it with actual game scenes. Concretely, this means that you need to be able to select the w2-tests-39-many-collisions map in the UI, and things should be fluid.

Now go explore the w2-game-* maps, and enjoy the smoothness of your efficient collision detector. :)

7) Extending the game

We've worked hard to make the game extensible. You can add new game maps to resources/maps/ (each game map is a simple text file). You can also add new actions (your game receives all keystrokes, not just x, z, left, right, and up). Finally, you can add new blob types; you can give them any behavior you want. Some ideas:

  • A tree that continuously produces new mushrooms.
  • A beanie that lets you jump higher.
  • A friend-zapper that turns evil mushrooms into friendly mushrooms; upon touching a friendly mushroom, evil mushrooms turn into friendly mushrooms.
  • Smarter fireballs (that you could throw in various directions).
  • Moving platforms.
  • Platforms that disappear and reappear.
  • Smarter enemies.
  • Larger or smaller blobs: instead of a "pos", you can include a "rect" ([x, y, w, h]) in the serialized output.
  • A helicopter implementation that points left or right depending on the direction the helicopter is flying (you can use a negative width in the serialized "rect" to ask the UI to reverse a picture).
  • An evil twin that follows the player with a 20-cycle delay; the evil twin performs exactly the same actions as the player, but with a delay; touching it causes the player to loose. This forces the player to keep moving and not backtrack.

And if you're feeling motivated, you could even make it a multiplayer game (it should be pretty easy: just handle A, W, and D to move a different "player" blob around)!

⇨ Implement your own extension of the game. This could be a different behavior, a new map, etc. There are no tests for this, but we want to see what you came up with at debriefing time (we won't grade you on this; we'll just check that you did implement an extension — it doesn't need to be fancy; something simple will do).

To add a new blob, you'll want to extend the Textures class and the Constants.TEXTURE_MAP dictionary. You can refer to resources/emoji_table.txt for a list of Emoji codes.

If you design a new map, feel free to share it with your classmates and the staff on Piazza! And congrats on completing the lab!

8) Appendix: blob list

The following table lists all blob types that we ask you will implement in this lab:

Blob Hard? Feels gravity? Character in maps Textures
Bee e e
Building B B
Castle C C
Cloud c c
Fire f f
Fireball F
Floor = =
Helicopter h h
Mushroom m m
Player p p (normal), b (boat), h (flying), bored, defeat, victory
Storm s s (storm with thunder), r (rain)
Sun o o
Tree t t
Water w w

9) Code Submission

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10) Checkoff

Once you are finished with the code, please come to a tutorial, lab session, or office hour and add yourself to the queue asking for a checkoff. You must be ready to discuss your code and test cases in detail before asking for a checkoff.

You should be prepared to demonstrate your code (which should be well-commented, should avoid repetition, and should make good use of helper functions). In particular, be prepared to discuss:

  • the internal representation for the new blobs
  • the implementation of timestep processing to other non-player blobs
  • player jump logic
  • the implementation for fireballs
  • strategy and implementation for efficient collision checking
  • game extension


You have not yet received this checkoff. When you have completed this checkoff, you will see a grade here.