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HTML 

              
                <h3>2. マウス座標を起点とした拡大・縮小</h3>
<canvas id="myCanvas" width="480" height="320">
    Your browser doesn't support HTML5 canvas tag.
</canvas>
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CSS
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JS 

              
                const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');
const image = new Image();
const MAX_SCALE = 5;
const SCALE_STEP = 0.2;
let imageScale = 1, imageScaleIndex = 0;

// マウス座標
let mouseX, mouseY;

// 拡大・縮小後の画像表示領域
let zoomWidth, zoomHeight, zoomLeft = 0, zoomTop = 0;

canvas.addEventListener('mousewheel', canvasZoom);
canvas.addEventListener('mouseover', disableScroll);
canvas.addEventListener('mouseout', enableScroll);

function draw() {
    image.addEventListener("load", function(){
        // 画像のスケーリング表示
        ctx.drawImage(image, zoomLeft, zoomTop, canvas.width / imageScale, canvas.height / imageScale, 0, 0, canvas.width, canvas.height);

        // 倍率の描画
        ctx.font = '30px "arial black"';
        ctx.fillStyle = 'white';
        ctx.fillText('x' + imageScale.toFixed(1), 390, 300);
        ctx.strokeStyle = 'black';
        ctx.lineWidth = 2;
        ctx.strokeText('x' + imageScale.toFixed(1), 390, 300);
        
        // 枠の描画
        ctx.lineWidth = 3;
        ctx.strokeRect(0, 0, canvas.width, canvas.height);
        
    });
    image.src = "https://kano.arkoak.com/images/harinezumi.jpg";
}

function canvasZoom(e) {
    // Canvas上マウス座標の取得
    let rect = e.target.getBoundingClientRect();
    mouseX = e.clientX - rect.left;
    mouseY = e.clientY - rect.top;
    
    if (e.wheelDelta > 0) {
        imageScaleIndex++;
        imageScale = 1 + imageScaleIndex * SCALE_STEP;
        if (imageScale > MAX_SCALE) {
            imageScale = MAX_SCALE;
            imageScaleIndex--;
        } else {
            zoomWidth = canvas.width / imageScale;
            zoomHeight = canvas.height / imageScale;

            zoomLeft += mouseX * SCALE_STEP / (imageScale * (imageScale - SCALE_STEP));
            zoomLeft = Math.max(0, Math.min(canvas.width - zoomWidth, zoomLeft));

            zoomTop += mouseY * SCALE_STEP / (imageScale * (imageScale - SCALE_STEP));
            zoomTop = Math.max(0, Math.min(canvas.height - zoomHeight, zoomTop));
        }
    } else {
        imageScaleIndex--;
        imageScale = 1 + imageScaleIndex * SCALE_STEP;
        if (imageScale < 1) {
            imageScale = 1;
            zoomLeft = 0;
            zoomTop = 0;
            imageScaleIndex = 0;
        } else {
            zoomWidth = canvas.width / imageScale;
            zoomHeight = canvas.height / imageScale;
            
            zoomLeft -= mouseX * SCALE_STEP / (imageScale * (imageScale + SCALE_STEP));
            zoomLeft = Math.max(0, Math.min(canvas.width - zoomWidth, zoomLeft));
            
            zoomTop -= mouseY * SCALE_STEP / (imageScale * (imageScale + SCALE_STEP));
            zoomTop = Math.max(0, Math.min(canvas.height - zoomHeight, zoomTop));
        }
    }
    
    draw();
}

// Cnavas上ではブラウザのスクロールを無効に
function disableScroll() {document.addEventListener("mousewheel", scrollControl, { passive: false });}
function enableScroll() {document.removeEventListener("mousewheel", scrollControl, { passive: false });}
function scrollControl(e) {e.preventDefault();}

draw();
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999px

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