canvas上のマウス座標を出力する

by s_yoshiki

HTML

<input type="file" id="file">
<hr>
<canvas id="canvas"></canvas>
<hr>
<pre id="debug"></pre>
<hr>
<textarea id="textarea"></textarea>

CSS

#canvas {
	background-color : #f3f3f3;
}

#textarea {
	width: 300px;
	height: 100px;
}

JavaScript

const canvas = document.getElementById('canvas');

document.getElementById("file").addEventListener("change", function (e) {
	var file = e.target.files;
	var reader = new FileReader();

	reader.readAsDataURL(file[0]);

	reader.onload = function () {
		var src = reader.result;
		drawCanvas(src);
	};
}, false);

canvas.addEventListener("mousemove", function(e){
	var borderWidth = 1;
	var rect = e.target.getBoundingClientRect();
	var x = e.clientX - rect.left - borderWidth;
	var y = e.clientY - rect.top - borderWidth;
	document.getElementById("debug").innerHTML = `${x}:${y}`
});

canvas.addEventListener("click", function(e){
	var borderWidth = 1;
	var rect = e.target.getBoundingClientRect();
	var x = e.clientX - rect.left - borderWidth;
	var y = e.clientY - rect.top - borderWidth;
	document.getElementById("textarea").value += `${x}:${y}\n`
});

function drawCanvas(source) {
	if (canvas.getContext('2d')) {
		var context = canvas.getContext('2d');
		var image = new Image();
		image.src = source;
		image.onload = function () {
			canvas.width = image.width;
			canvas.height = image.height;
			context.drawImage(image, 0, 0);
		};
	}
}