Question

I’m using Cocos2d iPhone with Box2D to create a basic physics engine.

Occasionally the user is required to drag around a small box2D object.

Creation of touchjoints on small objects is a bit hit and miss, with the game engine seeing it as a tap on blank space as often as actually creating the appropriate touchjoint. In practice this means the user is constantly mashing their fingers against the screen in vain attempts to move a stubborn object. I want the game to select small objects easily without this ‘hit and miss’ effect.

I could create the small objects with larger sensors around them, but this is not ideal because objects above a certain size (around 40px diameter) don’t need this extra layer of complexity; and the small objects are simply the big objects scaled down to size.

What are some strategies I could use to allow the user experience to be better when moving small objects?

Here’s the AABB code in ccTouchBegan:

b2Vec2 locationWorld = b2Vec2(touchLocation.x/PTM_RATIO, touchLocation.y/PTM_RATIO);

    b2AABB aabb;
    b2Vec2 delta = b2Vec2(1.0/PTM_RATIO, 1.0/PTM_RATIO); 
    //Changing the 1.0 here to a larger value doesn't make any noticeable difference.
    aabb.lowerBound = locationWorld - delta;
    aabb.upperBound = locationWorld + delta;
    SimpleQueryCallback callback(locationWorld);
    world->QueryAABB(&callback, aabb);

    if(callback.fixtureFound){
      //dragging code, updating sprite location etc.
    }

SimpleQueryCallback code:

    class SimpleQueryCallback : public b2QueryCallback
    {
    public:
    b2Vec2 pointToTest;
    b2Fixture * fixtureFound;

    SimpleQueryCallback(const b2Vec2& point) {
        pointToTest = point;
        fixtureFound = NULL;
    }

    bool ReportFixture(b2Fixture* fixture) {
        b2Body* body = fixture->GetBody();
        if (body->GetType() == b2_dynamicBody) {
            if (fixture->TestPoint(pointToTest)) {
                fixtureFound = fixture;
                return false;
            }
        }        
        return true;
    }        
};

Answer 1

What I ended up doing – thanks to iforce2d, was change ReportFixture in SimpileQueryCallback to:

bool ReportFixture(b2Fixture* fixture) {
    b2Body* body = fixture->GetBody();
    if (body->GetType() == b2_dynamicBody) {

        //if (fixture->TestPoint(pointToTest)) {
        fixtureFound = fixture;

        return true;
        //}
    }        
    return true;
} 

And increase the delta to 10.0/PTM_RATIO.