canvas-learning/scale-image.html at master · luogao/canvas-learning · GitHub

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<html>

<head>
    <style>
        #canvas {
            border: 1px solid #ccc;
        }
    </style>
</head>

<body>
    <canvas id="canvas" width="400" height="400"></canvas>
    <canvas id="zoom" width="300" height="227"></canvas>
    <input type="checkbox" id="smoothbtn">
    <img src="54903.jpg" id="img" alt="">
    <script>
        let img = document.getElementById('img')
        window.onload = function () {
            draw(img)
        }

        function draw(img) {
            const canvas = document.getElementById('canvas')
            const ctx = canvas.getContext('2d')
            ctx.drawImage(img, 0, 0, canvas.width , canvas.height)
            img.style.display = 'none'

            let zoomctx = document.getElementById('zoom').getContext('2d');
            let smoothbtn = document.getElementById('smoothbtn');
            let toggleSmoothing = function (event) {
                zoomctx.imageSmoothingEnabled = this.checked;
                zoomctx.mozImageSmoothingEnabled = this.checked;
                zoomctx.webkitImageSmoothingEnabled = this.checked;
                zoomctx.msImageSmoothingEnabled = this.checked;
            };
            smoothbtn.addEventListener('change', toggleSmoothing);

            let zoom = function (event) {
                let x = event.layerX;
                let y = event.layerY;
                zoomctx.drawImage(canvas,
                    Math.abs(x - 50),
                    Math.abs(y - 50),
                    10, 10,
                    0, 0,
                    200, 200);
            };

            canvas.addEventListener('mousemove', zoom);
        }
    </script>
</body>

</html>