introduce substeps for accurate physics

CHANGELOG

@@ -1,4 +1,5 @@
 cocos2d-x-3.3??      ??
+    [NEW]           PhysicsWorld: add setSubsteps() and getSubsteps()
     [NEW]           ActionManager: added removeAllActionsByTag()
     [NEW]           GLViewProtocol: added getAllTouches()
     [NEW]           Node: added stopAllActionsByTag()

cocos/physics/CCPhysicsWorld.cpp

@@ -879,6 +879,18 @@ void PhysicsWorld::setGravity(const Vect& gravity)
     _info->setGravity(gravity);
 }
 
+void PhysicsWorld::setSubsteps(int steps)
+{
+    if(steps > 0)
+    {
+        _substeps = steps;
+        if (steps > 1)
+        {
+          _updateRate = 1;
+        }
+    }
+}
+
 void PhysicsWorld::step(float delta)
 {
     if (_autoStep)
@@ -914,10 +926,14 @@ void PhysicsWorld::update(float delta, bool userCall/* = false*/)
         _updateTime += delta;
         if (++_updateRateCount >= _updateRate)
         {
-            _info->step(_updateTime * _speed);
-            for (auto& body : _bodies)
+            const float dt = _updateTime * _speed / _substeps;
+            for (int i = 0; i < _substeps; ++i)
             {
-                body->update(_updateTime * _speed);
+                _info->step(dt);
+                for (auto& body : _bodies)
+                {
+                    body->update(dt);
+                }
             }
             _updateRateCount = 0;
             _updateTime = 0.0f;
@@ -936,6 +952,7 @@ PhysicsWorld::PhysicsWorld()
 , _updateRate(1)
 , _updateRateCount(0)
 , _updateTime(0.0f)
+, _substeps(1)
 , _info(nullptr)
 , _scene(nullptr)
 , _delayDirty(false)

cocos/physics/CCPhysicsWorld.h

@@ -141,7 +141,15 @@ public:
     inline void setUpdateRate(int rate) { if(rate > 0) { _updateRate = rate; } }
     /** get the update rate */
     inline int getUpdateRate() { return _updateRate; }
-    
+    /**
+     * set the number of substeps in an update of the physics world.
+     * One physics update will be divided into several substeps to increase its accuracy.
+     * default value is 1
+     */
+    void setSubsteps(int steps);
+    /** get the number of substeps */
+    inline int getSubsteps() const { return _substeps; }
+
     /** set the debug draw mask */
     void setDebugDrawMask(int mask);
     /** get the bebug draw mask */
@@ -150,7 +158,7 @@ public:
     /**
      * To control the step of physics, if you want control it by yourself( fixed-timestep for example ), you can set this to false and call step by yourself.
      * Defaut value is true.
-     * Note: if you set auto step to false, setSpeed and setUpdateRate won't work, you need to control the time step by yourself.
+     * Note: if you set auto step to false, setSpeed setSubsteps and setUpdateRate won't work, you need to control the time step by yourself.
      */
     void setAutoStep(bool autoStep){ _autoStep = autoStep; }
     /** Get the auto step */
@@ -194,6 +202,7 @@ protected:
     int _updateRate;
     int _updateRateCount;
     float _updateTime;
+    int _substeps;
     PhysicsWorldInfo* _info;
     
     Vector<PhysicsBody*> _bodies;