Implement COCOS2D-X Tracking Bullets

By Cocos2dx

Reproduced from: http://blog.csdn.net/zhaixh_89/article/details/24031363

Requirements

Tracking bullets is a very important part of the game Elves. The implementation of bullet tracking is essentially to adjust the linear velocity and angular velocity of the bullet in real time, so that it constantly moves in the direction approaching the target.

Analysis

Bullet speed is divided into two categories, one is linear velocity and the other is angular velocity. The so-called tracking is to dynamically calculate the positional relationship between the bullet and the target in each frame of bullet update, and then update the current linear velocity of the bullet, and rotate the bullet itself around its center point by an angle, so that the bullet is facing and The bullet speed direction is the same.

here our bullets per frame The rotation angle of is set to 1 degree. The direction is clockwise or counterclockwise according to the position between it and the target. For example, if the target is at the top right of the bullet, the bullet (per frame) will rotate one degree clockwise, and if it is at the top left, it will rotate counterclockwise. Of course, in the actual calculation process, we can decide the direction according to the vector operation.

The line speed of the bullet needs By calculating the angle between the bullet and the target, if the angle between the bullet and the target is not 0, then the bullet needs to adjust the direction of its speed in real time. We also set it to rotate 1 degree per frame (this value Just like the rotation of the bullet itself, in order to keep the bullet always moving in front of itself), the velocity of the bullet (modulus of velocity) must remain constant during the entire movement.

In addition, we need to add some restrictions Conditions (natural or artificial restrictions):

  1. When the bullet is launched, it needs to find the target, the bullet will find the closest to itself by default, and Does not meet the goals of 4.1 and 4.2 conditions.

  2. If the bullet is launched without finding the target, it will fly at the initial speed.

  3. If the bullet loses its target during the movement, it will fly at the current speed.

  4. Once the bullet locks onto the target, it will keep tracking the target until it hits the target or when the target is abandon and re-find the target when the following conditions are met:

    4.1 The target leaves the screen range

    4.2The angle between the bullet and the target is greater than 90 degrees

Implement

the key part Realization includes: 1. Search for the target 2. Calculate the angle between the speed direction and the target 3. Calculate the direction that the bullet needs to rotate to hit the target 4. The bullet motion function, string things together here

1. Search for enemies

C++ Code:

/** * Find the target, all the enemies are stored in a CCArray, you need to traverse from it to get the one that meets the conditions. * @author [KC](http://i.kimiazhu.info) */ void HTTrackBullet::seekEnemy< span style="border-width:0px;vertical-align:baseline;">() { mTarget = NULL; //The square value of the distance between the target and the bullet, the square value is used, because here as long as the size is judged, there is no square.  float minDistanceSQ = mMinDistanceSQ; //Get an array of all enemies CCArray< /span>* array = getArrayForEnemy(); for   span>(int  i = 0; i < array->count(); i++) { //The enemy inherits self-defined HTCollidablePart collidable parts HTCollidablePart* enemy =   span>(HTCollidablePart *) array->objectAtIndex(i);  if (enemy->isCollidable()  && enemy->isInsideWindow< /span>() && calcAngle(enemy->getPosition()) < (PI / 2)) { /* * Judging to meet the conditions: * 1. The enemy can collide (that is, it can be hit) * 2. The enemy is within the visible range of the current window* 3. The angle between the target and the enemy is less than 90 degrees*/ float distanceSQ = ccpDistanceSQ (enemyenemy span>->getPosition(), this->getPosition()); if (distanceSQ < minDistanceSQ< /span>) { minDistanceSQ = distanceSQ;< /span> mTarget = enemy; } }< span style="color:rgb(0,0,0);border-width:0px;vertical-align:baseline;"> } }

2.Calculate the angle

C++ Code:

/** * Calculate the angle between the speed direction and the target* @author [KC] (http://i.kimiazhu.info) */ float HTTrackBullet::calcAngle(CCPoint target)  span>{ float r = PI; span> CCPoint p2 < span style="border-width:0px;vertical-align:baseline;">= ccpSub(target, this< span style="border-width:0px;vertical-align:baseline;">->getPosition()); r = ccpAngle(speed< span style="border-width:0px;vertical-align:baseline;">, p2); //The calculated angle r is a radian value, not an angle value< /span> return r; }

3. Calculate the direction of rotation of the bullet itself

C++ Code:

/** * Calculate the direction that the bullet needs to rotate to hit the target* @author [KC](http:/ /i.kimiazhu.info) */ RotateDirection HTTrackBullet::calcDirection(CCPoint target span>) { CCPoint p2 = ccpSub(target, this->getPosition ()); if (ccpCross(speed, p2) > 0) { // In the opengl right-handed coordinate system, the vector cross product is greater than 0 to indicate counterclockwise direction  return COUNTERCLOCKWISE;  } else if (ccpCross(speedspeed span>, p2) < 0) { return CLOCKWISE; } else { return NO_ROTATE; } }

4. The bullet moves every frame

C++ Code:

/** * Bullet Movement* @author [KC](http://i.kimiazhu.info) */  void HTTrackBullet::move< span style="border-width:0px;vertical-align:baseline;">() { do { if (mTarget) { if ( mTarget->isDamaged() || (mTarget->getPositionX() <= 0) || (mTarget->getPositionY() <= 0)) { mTarget = NULL; break; } //step 1:确定角度 //计算夹角r是弧度值，不是角度值 float _rad = calcAngle(mTarget->getPosition()); if (_rad && _rad >= PI / 2) { //角度大于90的时候放弃，不追踪 mTarget = NULL; break; } float _deg = CC_RADIANS_TO_DEGREES(_rad); float deltaR = _rad < mDeltaRadians ? _rad : mDeltaRadians; float deltaD = _deg < mDeltaDegree ? _deg : mDeltaDegree; //step 2:确定方向 switch (calcDirection(mTarget->getPosition())) { case COUNTERCLOCKWISE: { speed = ccpRotateByAngle(speed, ccp(0,0), deltaR); this->setRotation(this->getRotation() - deltaD); break; } case CLOCKWISE: { speed = ccpRotateByAngle(speed, ccp(0,0), -deltaR); this->setRotation(this->getRotation() + deltaD); break; } default: break; } } else {  seekEnemy(); } } while (0); this->setPosition(ccpAdd(getPosition(), speed)); }

转自：http://blog.csdn.net/zhaixh_89/article/details/24031363

需求

追踪子弹是游戏中的一个相当重要的精灵。子弹追踪的实现本质上是要实时调整子弹的线速度和角速度，使其不断地照着接近目标的方向移动。

分析

子弹速度分为两类，一个线速度，一个是角速度。所谓追踪，就是在子弹更新的每一帧中，动态计算子弹和目标之间的位置关系，然后更新子弹当前的线速度，和把子弹本身绕着自己的中心点旋转一个角度，使子弹朝向和子弹速度方向一致。

这里我们的子弹每帧的旋转角度设定为1度。方向为顺时针或是逆时针根据其与目标之间的位置决定，例如目标在子弹的右上方，则子弹（每帧）顺时针旋转一度，如果在左上方，则逆时针旋转。当然实际计算过程我们可以根据向量运算决定方向。

而子弹的线速度需要通过计算子弹和目标之间的夹角得到，如果子弹和目标之间夹角不为0，那么子弹需要实时调整自己的速度的方向，我们这里也设定为每帧可以旋转1度（这个值和子弹自身的旋转要一样，才能保持子弹看来永远朝着自己的前方运动），在整个运动过程中，子弹的速率（速度的模值）必须是保持不变的。

另外我们需要加一些限制条件（自然的或者人为的限制）：

  1. 子弹发射时需要寻找目标，子弹默认会寻找离自己最近的，并且不满足4.1和4.2两个条件的目标。

  2. 子弹如果没有找到目标的情况下发射出去了，则按照初始速度飞行。

  3. 子弹在运动过程中如果失去目标，则按照当前速度飞行。

  4. 子弹一旦锁定目标，就会一直追踪打击目标，直到击中目标或者当目标满足下面条件时放弃并重新寻找目标：

    4.1 目标离开屏幕范围

    4.2 子弹与目标之间的夹角大于90度角度

实现

关键部分的实现，包括： 1. 搜索目标 2. 计算速度方向和目标之间的夹角 3. 计算子弹自身要打击到目标需要旋转的方向 4. 子弹运动函数，在这里把东西全串起来

1.搜寻敌人

C++ Code:

 

 

/** * 寻找目标,所有的敌人都保存在一个CCArray中，需要从中遍历得到满足条件的一个。 * @author [K.C.](http://i.kimiazhu.info) */ void HTTrackBullet::seekEnemy() { mTarget = NULL; //目标和子弹之间距离的平方值，之所以用平方值，因为这里只要判断大小，就不开方了。  float minDistanceSQ = mMinDistanceSQ; //获取保存所有敌人的数组 CCArray* array = getArrayForEnemy(); for (int i = 0; i < array->count(); i++) { //敌人都继承自我自定义的HTCollidablePart可碰撞部件 HTCollidablePart* enemy = (HTCollidablePart*) array->objectAtIndex(i); if (enemy->isCollidable() && enemy->isInsideWindow() && calcAngle(enemy->getPosition()) < (PI / 2)) { /* * 判断满足条件： * 1. 敌人可碰撞（就是可以被打击） * 2. 敌人在当前窗口可视范围内 * 3. 目标和敌人的角度小于90度 */ float distanceSQ = ccpDistanceSQ(enemy->getPosition(), this->getPosition()); if (distanceSQ < minDistanceSQ) { minDistanceSQ = distanceSQ; mTarget = enemy; } } } }

2.计算角度

C++ Code:

 

/** * 计算速度方向和目标之间的夹角 * @author [K.C.](http://i.kimiazhu.info) */ float HTTrackBullet::calcAngle(CCPoint target) { float r = PI; CCPoint p2 = ccpSub(target, this->getPosition()); r = ccpAngle(speed, p2); //计算夹角r是弧度值，不是角度值 return r; }

3.计算子弹自身旋转方向

C++ Code:

 

/** * 计算子弹自身要打击到目标需要旋转的方向 * @author [K.C.](http://i.kimiazhu.info) */ RotateDirection HTTr ackBullet::calcDirection(CCPoint target) { CCPoint p2 = ccpSub(target, this->getPosition()); if (ccpCross(speed, p2) > 0) { // 在opengl的右手坐标系中，向量叉乘大于0表示逆时针方向 return COUNTERCLOCKWISE; } else if (ccpCross(speed, p2) < 0) { return CLOCKWISE; } else { return NO_ROTATE; } }

4.子弹每帧移动

C++ Code:

 

/** * 子弹运动 * @author [K.C.](http://i.kimiazhu.info) */ < span style="border-width:0px;vertical-align:baseline;">void HTTrackBullet::move() { do { if (mTarget) { if (mTarget->isDamaged() || (mTarget->getPositionX() <= 0) || (mTarget->getPositionY() <= 0)) { mTarget = NULL; break; } //step 1:确定角度 //计算夹角r是弧度值，不是角度值 float _rad = calcAngle(mTarget->getPosition()); if  (_rad && _rad >= PI / 2) { //角度大于90的时候放弃，不追踪 mTarget = NULL; break; } float _deg = CC_RADIANS_TO_DEGREES(_rad); float deltaR = _rad < mDeltaRadians ? _rad : mDeltaRadians; float deltaD = _deg < mDeltaDegree ? _deg : mDeltaDegree; //step 2:确定方向 switch (calcDirection(mTarget->getPosition())) { case COUNTERCLOCKWISE: { speed = ccpRotateByAngle(speed, ccp(0,0), deltaR); this->setRotation(this->getRotation() - deltaD); break;  } case CLOCKWISE: { speed = ccpRotateByAngle(speed, ccp(0,0), -deltaR); this->setRotation(this->getRotation() + deltaD); break; } default: break; } } else { seekEnemy(); } } while (0); this->setPosition(ccpAdd(getPosition(), speed)); }

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