Question

I have an image, I’m using it as an image map. If the image was fixed then there would be no problem but I need to zoom and drag this image and get and use the coordinates of where the image clicked.

Do I need to keep track of exactly how much this image has moved and has been resized or can I get the 0x0 point of my image(the top left corner of my image).

Is there another way to do it

I should add I’ve based my image manipulation on this excellent tutorial http://www.zdnet.com/blog/burnette/how-to-use-multi-touch-in-android-2/1747?tag=rbxccnbzd1

Answer 1

You can get the point using the same transformation matrix that is being applied to the image. You want to transform the point between the screen’s coordinate system to the image’s coordinate system, reversing the effect of the original matrix.

Specifically, you want to transform the x,y coordinates where the user clicked on the screen to the corresponding point in the original image, using the inverse of the matrix that was used to transform the image onto the screen.

A bit of pseudocode assuming matrix contains the transformation that was applied to the image:

// pretend user clicked the screen at {20.0, 15.0}
float x = 20.0;
float y = 15.0;

float[] pts[2];

pts[0] = x;
pts[1] = y;

// get the inverse of the transformation matrix
// (a matrix that transforms back from destination to source)
Matrix inverse = new Matrix();
if(matrix.invert(inverse)) {

    // apply the inverse transformation to the points
    inverse.mapPoints(pts);

    // now pts[0] is x relative to image left
    //     pts[1] is y relative to image top
}