Erase Box: the tutorial

Some days ago I blogged about Erase Box, a Box2D game made with my tutorials, now it’s time to publish Rodrigo‘s tutorial.

« On a game I was making, I needed a way to detect if a body was leaving the screen.

That can be done by checking the x and y positions of it, but I also wanted to explore Box2D features.

That was actually good, because I found another way to do that, and it can be of great help on other projects.

Sensors are bodies that won’t directly interact with others.

They won’t make a moving box stop when they hit, i.e.

That being said, they are still useful.

Box2D will keep track of it’s “contacts” just as if it were a normal body, so they can be used as exactly what their name means, a sensor.

Also, Box2D allows you to “attach” some information to a body, so that it can be distinguished from others.

Let’s see an example:

This is a modified version of Emanuele’s script: Box2D: tutorial for the absolute beginners

So I will only comment the changes:

Line 14: Creating a variable to keep track of the number of boxes created.

Line 40: Defining the_box as a sensor.

Lines 41 and 45: Setting up the userData for both sensors, so Box2D will be able to distinguish between them and other bodies.

Line 63: Setting the randomly generated box userData to the variable “i” (that keeps track of the number of boxes created).

Line 67: Increasing “i

Lines 73 to 86: Setting up the contact listener:

73: Checking if m_contactList is not null.

75: Defining variable contact.

77 and 82: m_conctactList will also check for contact between boxes. This will make sure that it will only execute the lines below on the bodies that we want.

78: Tracing the box userdata (“i” at the time it was created)

83 and 84: Defining body2 as the second object in contact and then destroying it. It is also possible to add “&& contact.GetShape2().GetBody().GetUserData() == 5” to the if function, so it will only execute on the fifth body, i.e.

It is also possible to create non-0-density boxes as sensors, and attach them to bodies, but this is for another tutorial :) »

This is the result:

Download the source code.

  • ThomasTan

    Cool! I never thought of that! I thought i would just check if the corner furthest away from the side would touch the side… Great one!

  • ThomasTan

    seems like Emanuele NEVER CHECKS HIS MAIL…

  • Is it maybe possible to add an listener for the sensors instead checking it with the onenter frame and IF stuff? :)

  • ThomasTan

    No way… He really does not check his mail…

  • hi,could you show me a example about
    man Walking animation?thank you

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