How to do hit test on a rotated image in Android?

Question

I am trying to do hit test on a rotated image object drawn on the canvas in surface view. When image is not rotate we can directly use left, top , width and height as boundaries to check whether the point lies within image rectangle. But how to do it when the object is rotated to some angle?

I am using: canvas.rotate(angle, pivotX ,pivotY); to draw the rotated image.

I could not get the rotated left and top of the image object. I tried to take original left and top of the image and when i tap on the screen i rotate the touch point back with same angle using:

angledTouchX = (float) (eventX * Math.cos(-objectAngle) - eventY * Math.sin(-objectAngle));
angledTouchY  = (float) (eventY * Math.sin(-objectAngle) + eventX * Math.cos(-objectAngle));

It does not work because it rotates the point wrt (0,0), but i want it wrt center of the image object.

Sirisilp Kongsilp Sirisilp Kongsilp · Accepted Answer · 2014-07-20T09:02:58

Was working on similar problem. In my case, I want to do a hottest between touchpoint and rotated and scaled view. So I transform touch point co-ordinate system according to view's rotation and check if its inside non-rotated view boundary or not.

Step

Get touch point in screen co-ordinate system which received from event.getRawX() and event.getRawY()

Target view location in screen location which received from

 int location[] = new int[2];
 view.getLocationOnScreen(location);
 int viewX = location[0];
 int viewY = location[1];

Create vector as if view origin is the space origin

float[] vec = new float[2];
vec[0] = event.getRawX() - viewX;
vec[1] = event.getRawY() - viewY;

Rotate the vector

Matrix matrix = new Matrix();
matrix.setRotate(-1 * view.getRotation());
matrix.mapVectors(vec);

Change rotated vector to screen space

vec[0] = vec[0] + viewX;
vec[1] = vec[1] + viewY;

Check if rotated touch co-ordinate is in view unrotated boundary

if(( vec[0] > viewX 
    && vec[0] < (viewX + (view.getWidth() * view.getScaleX()) )) 
    &&( vec[1] > viewY 
    && vec[1] < (viewY + (view.getHeight() * view.getScaleY()) ))){
        isPointInTextView = true;
} else {
        isPointInTextView = false;
}

How to do hit test on a rotated image in Android?

2 Answers