I have 3 points on my screen:
a = a point which is (c.x, 0) makes a line pointing straight up
b = a user input touch, can be anywhere on the screen
c = a moving object
a
_______.________
| | |
| | |
| b | |
| . | |
| \ | |
| \ | |
| \| |
| | c |
|______._______|
I have drawn some lines so that you can see the vectors.
I want to be able to get the angle between a and b. I have tried this, but it doesn't work, does anyone know what I'm doing wrong?:
//v1 moving object
float boxX = this.mScene.getLastChild().getX();
float boxY = this.mScene.getLastChild().getY();
//v2 user touch
float touchX = pSceneTouchEvent.getX();
float touchY = pSceneTouchEvent.getY();
//v3 top of screen
float topX = boxX;
final float topY = 0;
float dotProd = (touchX * topX) + (touchY * topY);
float sqrtBox = (touchX * touchX) + (touchY * touchY);
float sqrtTouch = (topX * topX) + (topY * topY);
double totalSqrt = sqrtBox * sqrtTouch;
double theta = Math.acos(dotProd / Math.sqrt(totalSqrt));
The answer I usually get is between 0 and 1. How do I fix this so that I get the angle in degrees?
Answer
You are looking for the wondrous atan2.
// v1 moving object
float boxX = this.mScene.getLastChild().getX();
float boxY = this.mScene.getLastChild().getY();
// v2 user touch
float touchX = pSceneTouchEvent.getX();
float touchY = pSceneTouchEvent.getY();
double theta = 180.0 / Math.PI * Math.atan2(boxX - touchX, touchY - boxY);
Normally it is used as atan2(y,x)
but since you are looking for the angle with the vertical line, you need to use atan2(-x,y)
instead.
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