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].

Answer 2

A view's position co-ordinates are always relative to it's parent. It's like saying, where with respect to it's parent is this view placed.

There in another thing, screen Co-ordinate. This says where with respect to the window top/left (think actual physical screen) is this object placed. This can be obtained through View.getLocationOnScreen.

If you want to compare two locations, they will only be equivalent if the parent view of both these locations is same or you're using absolute screen co-ordinates. Otherwise, you'll only get results that "seem" different.

If you want to unify the two either take them to a common parent or use absolute co-ordinates.