What is the unit of the coordinate system of Canvas? - android

Question

I find that the unit of the coordinate system of Canvas is different from that of screen.

For example in my case as below:

For one particular point, its on-screen coordinate obtained from ImageView.getX() and ImageView.getY() is (336, 578).

Then by trial and error, I draw a dot on the Canvas so that this dot falls EXACTLY the same position as the ImageView. I called canvas.drawCircle(330, 440, radius, paint); to achieve this.

Here comes the question:

Why would the 2 coordinates, (336, 578) and (330, 440), different?

Is it because the screen and the canvas use different units?

Is it an issue regarding pixel, dp and all that?

Answer 1

You could try converting to and from window coordinates using View.getLocationInWindow this way you'll always be sure you are using the same coordinates system.

Coordinates of ImageView.getX an Y are relative to the parent. Canvas coordinates are relative to the view. This could cause the differences

Edit: I assume in one place you get the coordinates of the view (using getX and getY). Convert these to window coordinates. Call these viewXY[].

When you need to use them, to draw in your canvas, also convert the top and left coordinates to window coordinates. We call these canvasXY[].

Now, the X position to draw at would be viewXY[0] - canvasXY[0], and the Y position would be viewXY[1] - canvasXY[1].