Gravity Waves…

Many games utilise a common sense notion of gravity as part of the overall game mechanics. In Line Rider, the sledge rides the line under the force of gravity, for example; in Launchball, the ball is affected by gravity; and in the Newtoon physics game, gravity (as well as electrostatic forces) can be modeled within the game.

The Phun 2D physics sandbox also provides an environment for creating animations that explore a gravity filled, abstract digital word:

(Unfortunately, phun isn’t released for Macs…. yet…)

At the simplest level, the effect of gravity can be seen in games where things ‘drop’ from higher up in a room to the ‘floor’, and maybe bounce around a little when they land. The speed (or more correctly, velocity) of the item increases as it drops – that is, it accelerates due to the force of gravity. If the item bounces when it hits the ground, the height it bounces up to is related to the height from which it was dropped in the first place – another consequence of the “physical laws” implemented by the game (in this case, a consideration of the kinetic energy of the object as it hits the ground, and the angle at which it does so).

So far, so basic physics…

I bet you always wondered what physics was good for, didn’t you? Well this is it – creating games and game engines! If you would like to learn more about the details, such as how gravity works “in the real world”, and how gravity can be modeled mathematically (so that it can be implemented by a game’s physics engine, for example), try working through the OpenLearn unit Motion under gravity, or the more mathematical Describing motion along a line.

So – how do we add gravity to a Game Maker game? Very easily, in fact.

Create a new game, add a couple of sprites (such as the clown or a simple monster sprite, and a wall sprite), a couple of objects and an empty room.

Configure one object as a wall object (no events or actions required – just a solid object), and add a wall along at least the bottom edge of the room:

Turning now to the ‘dropping object’, configure it with a Create Event that is handled by a Set Gravity action. When you start running the ‘game’, you may wish to experiment with the gravity settings to see what happens… If there’s anything you don’t understand, raise it in a comment…

When the dropping object collides with the floor/wall object, we need it to do two things: firstly, we need it to bounce:

Secondly, we need to model the change in velocity, and in particular lose energy. If the collision was elastic, then no energy would be lost during the collision and the object would bounce back to its original height. In an inelastic collision, however, energy is lost from the dropped object, and it will bounces to a lower height with each bounce.

Place several instances of the dropping object in the room at different heights above the solid floor, and run the game. Does each instance bounce to a different height?

One of the popular exercises in basic animation is to animate a bouncing ball. In Game Maker, the internal ‘physics engine’ handles most of the animation work for us. To make the ball look more realistic, it is often set spinning. See if you can design, or find, a multi-image sprite to simulate a spinning ball.

Experiment with various gravity settings in the initial object creation event. What does the direction setting determine (and how do you know which direction gravity is acting in?) Also experiment with the vert speed setting in the collision action. How significant is that setting in terms of the mechanics of the bounce ‘feeling right’?

Create a room that contains a series of platforms, made out of wall objects. The player character should enter the room at the top!

Configure the player character object so that is can be controlled using the left and right arrow keys. If the player character walks off the edge of wall platform, it should fall under gravity until it lands on a wall object underneath. (In the player object defined below, I have inherited the gravity behaviour form a parent object.)

To add an element of risk to the game, add one or more instances of a poison object to the room that will kill the player and send it back to the start. Experiment with adding one or two more instances of the dropped/player object to the room, all of which will be controlled by the same keyboard commands. The aim of the game now is to get all of the characters out of the room without killing any of them…

Alternatively, you may wish to add some treasure to the room (in which case you’ll also need to keep track of your score!). To add a little bit more tension to the game, how about creating a key object that must be collected from one of the platforms halfway up the room that will unlock a door leading out of the room on the ground floor platform?

What “issues” does your game have in terms of its playabilty? Do the objects bounce appropriately if they hit the wall at an angle? If the door out of the room is not on the lowest platform level, is there any way for that player character to get out of the room?

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5 Responses to “Gravity Waves…”


  1. 1 Dan moore November 18, 2009 at 6:17 pm

    I am trying those exercises in gamemaker and i dont understand why setting the gravity to 180 for the direction doesnt mean that gravity is pointing straight down. like wise 360 sends the sprite off to the right while 90 sends it to the left. If this is the direct that the gravity flows in, surely 180 would mean it is pointing straight down….

  2. 2 zenozo June 2, 2011 at 10:20 am

    i have that to set it to zero and it go’s down but vibrates on the spot

    90 left
    360 right
    0 down
    180 up

    is what i found

    z

    ps
    ^ ^
    (*.*)
    (> >) any one like the bunny

  3. 3 Daniel Franklin June 3, 2011 at 5:37 pm

    Maybe it should be 270, which way does the object go when gravity is 180?, or possibly it should be zero, I don’t know as I haven’t tried this yet

  4. 4 Laura Johnson November 25, 2011 at 2:31 am

    For my player object I set gravity to direction 270 and gravity .30 and it drops straight down. If it’s not bouncing check that you’ve added the ”bounce against solid objects” event to the collsion with the wall.

    Hope this helps!


  1. 1 Why Don’t Things Fall Up? Making Use of Physics Engines « Digital Worlds - Interactive Media and Game Design Trackback on July 21, 2008 at 11:40 am

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