在box2d Javascript示例中简单的一般rayCasting

Question

I've been looking through various tutorials on rayCasting with Box2D, but I haven't seen any clear examples. I was hoping someone familiar with box2dweb would be able to give a clear example of how one would go about setting up a simple function that would end up looking something like this:

var myRayCastFunction = function(p1,p2,maxFraction){
    //Code here
}

The idea being that it would be usable like this:

var retVal = myRayCastFunction(p1,p2,maxFraction)
var fixture = retVal.fixture
var point = retVal.point
var normal = retVal.normal
var fraction = retVal.fraction

(in this case, I'm simply returning 1 intersection, say the nearest one, but would want to know how to make a similar one where retVel is a list of these outputs for each intersection)

I've been trying to understand all of the details of how RayCasting works in box2D, and I understand that this requires making a custom callback function (I think?), but I never figured out where that function needs to be placed, and what IT should be as well.

Answer 1

I started to answer this question because I happened to be about to add raycasting to my current project, and I realized there were actually some bugs in box2dweb that needed to be fixed before I could get it done. I'll link to the details instead of cluttering up this post: http://www.iforce2d.net/box2dweb-fixes.txt

Here is how I've used the raycast callback successfully. Declare your callback class and give it a ReportFixture function:

var RaycastCallback = function() {
    this.m_hit = false;
}
RaycastCallback.prototype.ReportFixture = function(fixture,point,normal,fraction) {

    if ( ... not interested in this fixture ... ) 
        return -1;

    this.m_hit = true;
    this.m_point = point;
    this.m_normal = normal;
    return fraction;
};

Now make an instance of that to pass to the worlds RayCast function:

var rayStart = ...;
var rayEnd = ...;

var callback = new RaycastCallback();

world.RayCast(callback, rayStart, rayEnd);
if ( callback.m_hit ) {
    ... use callback.m_point etc ...
}