How can I get the coordinates of a mouse click in a scalable canvas element?

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I'm trying to build a knockoff of r/Place, and I've got quite a bit done. But what I'm stuck on is how I can get the coordinates of a mouse click in a canvas element that can be scaled up or down.

What I have tried:

I've tried this code from geeksforgeeks.org but it changes the coordinates completely when you zoom in or out:

function getMousePosition(canvas, event) {
    let rect = canvas.getBoundingClientRect();
    let x = event.clientX - rect.left;
    let y = event.clientY - rect.top;
    console.log("Coordinate x: " + x,
                "Coordinate y: " + y);
}

let canvasElem = document.querySelector("canvas");

canvasElem.addEventListener("mousedown", function(e)
{
    getMousePosition(canvasElem, e);
});

I was also thinking of making a huge <map> element, but with a 250x250 grid that I might turn into something like 500x500 (or more) later, I don't think that's a good solution.

Edit: canvas scaling is handled by a scrollwheel event and the transform: scale() CSS attribute

Posted 8-Jun-23 19:45pm

A-Games

Updated 9-Jun-23 5:49am

v2

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Andre Oosthuizen · Answer 1 · 2023-06-09T00:32:00

You can make use of the 'devicePixelRatio property' which should solve your issue when a user zoom their page - Window: devicePixelRatio property [^]

I made a fiddle that seems to work irrespective of zoom percentage - [^]

I used the code inside the body element with 'document.onclick' to call the function also inside the body element -

<html>
<head>
  <title>Mouse Click Position</title>
  
</head>
<body>
<p>
Hi
</p>
<script>
    function getMousePosition(event) {
      const zoomLevel = window.devicePixelRatio || 1; // Get the zoom level of the page

      let x = event.clientX / zoomLevel; // Adjust the X coordinate by the zoom level
      let y = event.clientY / zoomLevel; // Adjust the Y coordinate by the zoom level

      alert("Mouse Click Position - X: " + x + ", Y: " + y);
    }

    document.onclick = getMousePosition;
  </script>
</body>
</html>