计算触摸点的角度和旋转它的Android [英] Calculate angle of touched point and rotate it in Android
问题描述
数学再次击败我。这是这样一个简单的任务，但我不能设法把它完成。
Math has defeated me once again. This is such a simple task, but I can't manage to get it done.
情景：我画上一个SurfaceView一个圆形的图像。用户倒是像边境的一个点，并开始将它拖到adround。我需要根据用户的移动旋转一圈图像。 我有两个信息重要的一块，图像中心X，Y坐标和触摸点坐标。
Scenario: I draw on a SurfaceView a round image. The user touches a point on image border and starts to drag it adround. I need to rotate the circle image according to user movement. I have two important piece of information, the image center X,Y coordinates and the touched points coordinates.
正如你可以在图片中看到，用户触摸的点，根据我的抽签触摸点的角度应在40。我不能设法正确地计算它。
As you can see in the image, the user touched a point, according to my draw the touched point angle should be around 40. I can't manage to calculate it properly.
我试图用这个公式：
angle = Math.atan2(touchedY - centerY, touchedX - centerX) * 180 / Math.PI
我不能设法了解我应该怎么计算角度，因为它是现在，它不能正常工作和值都不好。例如，在图像的情况下，该角度计算为-50。
I can't manage to understand how I should calculate the angle, as it is now, it doesn't work properly and values are not good. For instance, in the case of the image, the angle calculate is -50.
感谢您的时间，任何的信息是很乐意考虑。
Thank you for your time, any informations is gladly taken.
LE：其实我做了一个错误，我想，如下所述。如果圆圈是这样的：
LE: Actually I did a mistake I think, as mentioned below. Should the circle be like:
推荐答案
让我们改写这个问题：你想找到两个向量的夹角。第一载体是upvector会straigt从您的中心点（ U ），第二个载体是从中心点到触摸点的向量（ v ）
Let's reformulate the problem: You want to find the angle between two vectors. The first vector is the upvector going straigt up from your center-point (u), and the second vector is the vector from the center point to the touch point (v).
现在我们可以回想一下（或谷歌）的
Now we can recall (or google) that
COS的在 = UV /（| U | * | V |）
cos a = uv / (|u|*|v|)
其中的在的是向量之间的夹角| U |是一个矢量的长度。该upvector， U ，是（0,1）和长度为1。
Where a is the angle between the vectors and |u| is the length of a vector. The upvector, u, is (0, 1) and has length 1.
用手乘以向量取消的X项，为我们提供了这样的事情。
Multiplying the vectors by hand cancels the x-term and gives us something like this.
double tx = touch_x - center_x, ty = touch_y - center_y;
double t_length = Math.sqrt(tx*tx + ty*ty);
double a = Math.acos(ty / t_length);
请注意如何购买V矢量减去所述触摸点的中心点获得。请记住，如果需要转换为度。
Note how the v vector is obtained by subtracting the center point from the touch point. Remember to convert to degrees if needed.
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