HTML5 Canvas Notes for Professionals

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This example will show how to get the mouse position relative to the canvas, such that (0,0) will be the top-left hand corner of the HTML5 Canvas. The e.clientX and e.clientY will get the mouse positions relative to the top of the document, to change this to be based on the top of the canvas we subtract the left and right positions of the canvas from the client X and Y.
var canvas = document.getElementById("myCanvas"); var ctx = canvas.getContext("2d");
ctx.font = "16px Arial";
canvas.addEventListener("mousemove", function(e) {
var cRect = canvas.getBoundingClientRect(); // Gets CSS pos, and width/height var canvasX = Math.round(e.clientX - cRect.left); // Subtract the 'left' of the canvas var canvasY = Math.round(e.clientY - cRect.top); // from the X/Y positions to make ctx.clearRect(0, 0, canvas.width, canvas.height); // (0,0) the top left of the canvas ctx.fillText("X: "+canvasX+", Y: "+canvasY, 10, 20);
});

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