Android: how to detect touch location on ImageView if the image view is scaled by matrix?

android matrix touch imageview scale
hzxu picture hzxu · May 18, 2011 · Viewed 13.3k times · Source

I set OnTouchListener of an ImageView and implement onTouch method, but if the image is scaled using matrix, how do I calculate the location of the touch?

Or does the motion event automatically takes that into account when returning getX() and getY() ?

Answer

zorgbargle picture zorgbargle · Sep 14, 2011

getX and getY will return the touch location in the ImageView's coordinate system. If you're looking for the point within the image's coordinate system, you can use the inverse matrix of the matrix used by the ImageView. I've done something like the following:

// calculate inverse matrix
Matrix inverse = new Matrix();
imageView.getImageMatrix().invert(inverse);

// map touch point from ImageView to image
float[] touchPoint = new float[] {event.getX(), event.getY()};
inverse.mapPoints(touchPoint);
// touchPoint now contains x and y in image's coordinate system

Related questions

Android: How to rotate a bitmap on a center point

I've been looking for over a day for a solution to this problem but nothing helps, even the answers here. Documentation doesn't explain anything too. I am simply trying to get a rotation in the direction of another object. The …

Read More
Android image view matrix scale + translate

I am trying to manually get an image inside an imageview centered and fitting the screen. I need to do it with a matrix (I will later dynamically change the matrix transformation). Problem is I can't get the image centered …

Read More
Android: Matrix -> what is the different between preconcat and postconcat?

I'm using Matrix to scale and rotate Bitmaps. Now I'm wondering what the difference between preconcat & postconcat is, or more precisely the difference between: postRotate preRotate setRotate From what I could figure out so far setRotate always overwrites the …

Read More