After playing around with Box2D, I tried to make a simple platformer. After setting up a static body to represent the ground, and a dynamic box to represent the character, I ran into a problem – how do you know whether your character can jump? Essentially you need to know whether he is standing on the ground. If he is on the ground, and the player presses some button, you can do your jumping physics (set a velocity or apply a force/impulse). One way to approach this problem is to use sensors. Sensors are fixtures that don’t affect the result of an actual collision, but can detect when a collision would have occurred. If you place a sensor under your character’s main area, you can detect when there is something right under him. Here is a diagram:
Continue reading Sensors in Box2D (or how to make a basic platformer)
Category Archives: Moai
Handling Collisions in Box2D
You can read about how collisions in Box2D work over in this tutorial, but in Moai you will find that a lot of the functions referenced there don’t actually exist. There is no way to get the full contact list of everything that is colliding through getContactList(). However, Moai does provide one way to respond to collisions – the MOAIBox2DFixture’s setCollisionHandler() function.
Here is an example of how you can call it:
function handler(phase, fixtureA, fixtureB, arbiter)
print("we had a collision!")
end
myFixture:setCollisionHandler(handler, MOAIBox2DArbiter.END)
Box2D Basics
Box2D is a simple open source 2D physics engine. It was written as part of a GDC talk in 2006, and a lot of famous iPhone games like Angry Birds use it. You can find more information about it over at its website. I also found a great set of tutorials here, which introduce a lot of the concepts behind Box2D. These tutorials are in C++, but you can get the general idea and apply it to Moai/Lua. Moai has built in support for Box2D, so you can use it without referencing any additional files.
I’m not going to go too deep about how to use Box2D, but I’ll provide some general guidance to get you started. In this post I’ll show you enough to demonstrate a simple ball bouncing on a floor.
Moai Main Event Loop
The previous Moai example I posted before only setup a scene, but it didn’t have a main game loop. The main loop can be important if you want to try to do something at a regular interval, like polling for input or advancing some game logic. Here’s a basic example you can add to the bottom:
gameOver = false
local mainLoop = MOAICoroutine.new ()
mainLoop:run (
function ()
while not gameOver do
-- Do processing here
coroutine.yield ()
end
print("Game Over!")
os.exit()
end
)
Basic Moai Program Example
I’ll walk through a really basic Moai program to show how to get setup. The first thing you need to do is setup a new window and a viewport:
local width = 400 local height = 400 MOAISim.openWindow ( "Hello World", width, height ) MOAIGfxDevice.setClearColor(1,1,1,1)