确定顺时针或逆时针旋转

Question

确定顺时针或逆时针旋转

3

我需要确定用户是进行顺时针旋转手势还是逆时针。我有起始向量位置以及当前和上一个触摸点。虽然我认为起始向量可能没有太大用处，因为用户还可以在旋转过程中更改旋转方向。就像旋转x-box的d-pad一样。为了像Dead Trigger 2开发人员那样实时演示想法，屏幕上没有按钮，只需使用手势在屏幕上旋转即可。我该如何确定它？

- Sachin Sharma

1个回答

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- Blindman67 · Accepted Answer

为了确定一个二维输入是顺时针旋转还是逆时针旋转，需要计算从旋转中心到两个向量的叉积，并判断其正负性。其中一个向量来自前一次输入，另一个向量来自当前输入。

伪代码如下：

centerX = screenCenterX;  // point user is rotating around
centerY = screenCenterY;
inputX = getUserX();   // gets X and Y input coords
inputY = getUserY();   //
lastVecX = inputX - centerX;  // the previous user input vector x,y
lastVecY = inputY - centerY;  //      

while(true){ // loop 
    inputX = getUserX();  // gets X and Y input coords
    inputY = getUserY();  //

    vecInX = inputX - centerX;  // get the vector from center to input
    vecInY = inputY - centerY;  // 

    // now get the cross product
    cross = lastVecX * vecInY - lastVecY * vecInX;

    if(cross > 0) then rotation is clockwise
    if(cross < 0) then rotation is anticlockwise
    if(cross == 0) then there is no rotation

    lastVecX = vecInX;  // save the current input vector
    lastVecY = vecInY;  // 
} // Loop until the cows come home.

您需要对向量进行归一化，然后叉乘是角度变化的正弦值。

以下是伪代码：

vecInX = inputX - centerX;  // get the vector from center to input
vecInY = inputY - centerY;  // 

// normalized input Vector by getting its length
length = sqrt(vecInX * vecInX + vecInY * vecInY);

// divide the vector by its length
vecInX /= length;
vecInY /= length;

// input vector is now normalised. IE it has a unit length

// now get the cross product
cross = lastVecX * vecInY - lastVecY * vecInX;

// Because the vectors are normalised the cross product will be in a range
// of -1 to 1 with < 0 anticlockwise and > 0 clockwise

changeInAngle = asin(cross);  // get the change in angle since last input
absoluteAngle += changeInAngle; // track the absolute angle

lastVecX = vecInX;  // save the current normalised input vector
lastVecY = vecInY;  //       
// loop