Cocos2dx - Unable to set velocity = 0.0

问题:

I'm making an pool game with cocos2dx. First, i setup the edgeBox with this parameters PhysicsMaterial(1.0f, 1.0f, 0.8f) And then these 2 balls PhysicsMaterial(1.0f, 1.0f, 0.5f)

On the update function, i want slow down balls time by time without gravity (like making ground friction) by adding physicsBody->setLinearDamping(0.3);

On the update function, i set the minimum velocity, if the velocity of each ball reaches lower than 15, reset velocity to 0,0

auto MV = 15;
auto v1 = player1->getPhysicsBody()->getVelocity();
auto v2 = player2->getPhysicsBody()->getVelocity();
if (v1.x > MV || v1.x < -MV ||
    v1.y > MV || v1.y < -MV) {
} else if(v1 != Vec2(0,0)) {
    player1->getPhysicsBody()->setVelocity(Vec2(0,0));
    CCLOG("sx 1 : %f %f",v1.x,v1.y);
}

if (v2.x > MV || v2.x < -MV ||
    v2.y > MV || v2.y < -MV) {
} else if(v2 != Vec2(0,0)) {
    player2->getPhysicsBody()->setVelocity(Vec2(0,0));
    CCLOG("sx 2 : %f %f",v2.x,v2.y);
}

Everything works fine except when the balls stand next to the wall or each other. I see the small blue glue to these objects, this is when the contact has been made.

And in these situation, i can't set the velocity to 0,0. I think there is some kind of force constantly changing the velocity. You can see the image below to see the blue glue and keep setting velocity = 0.0 like forever.

回答1:

Firstly reset forces before setting velocity to zero: player2->getPhysicsBody()->resetForces();

Also gravity can be a cause that bodies continue to move. So you can set gravity to zero for whole physics world. For example:

auto scene = Scene::createWithPhysics();
scene->getPhysicsWorld()->setGravity(Vec2(0, 0));

or just for one particular body:

player2->getPhysicsBody()->setGravityEnable(false);

or you can customize velocity function:

#include "chipmunk.h"

cocos2d::PhysicsBody * pBody = player2->getPhysicsBody();
pBody->getCPBody()->velocity_func = customVelFunc;

where customVelFunc could be defined as:

void customVelFunc(cpBody *body, cpVect gravity, cpFloat damping, cpFloat dt)
{
    cpBodyUpdateVelocity(body, cpvzero, damping, dt);
}