js使用canvas实现图片鼠标滚轮放大缩小拖拽预览，显示像素坐标，显示像素值
html代码
todo 实现画矩形框，圆形roi
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Canvas Image Viewer</title>
<style>canvas {border: 1px solid black;}
</style>
</head>
<body>
<div><button id="loadImageButton">Load Image</button>
</div>
<canvas id="mainCanvas" width="900" height="800"></canvas><div><span>坐标</span><span id="imageXY"></span><div><span id="imagePix"></span></div>
</div><script>const canvas = document.getElementById('mainCanvas');const ctx = canvas.getContext('2d');ctx.imageSmoothingEnabled = false;let imageObj = new Image();let imageWidth;let imageHeight;let scale = 1;let offsetX = 0;let offsetY = 0;let pixdata;canvas.addEventListener('mousedown', mouseDownListener);canvas.addEventListener('mouseup', mouseUpListener);canvas.addEventListener('mousemove', mouseMoveListener);canvas.addEventListener('mousewheel', mouseWheelListener);document.getElementById('loadImageButton').addEventListener('click', loadImage);function initPixData(image){const canvasHide = document.createElement('canvas');const context = canvasHide.getContext('2d');canvasHide.width = image.width;canvasHide.height = image.height;context.drawImage(image, 0, 0);const imageData = context.getImageData(0, 0, image.width, image.height);pixdata = imageData.data;console.log(pixdata)}function getPixelValue(x, y) {let pixelValue;if (pixdata.length === 4) {pixelValue = {red: pixel[0],green: pixel[1],blue: pixel[2],alpha: pixel[3]};} else if (ipixdata.length === 1) {pixelValue = {gray: pixel[0]};} else {throw new Error('Unsupported image format');}return pixelValue;}function loadImage() {const input = document.createElement('input');input.type = 'file';input.accept = 'image/*';input.click();input.onchange = e => {const file = e.target.files[0];const reader = new FileReader();reader.onload = readerEvent => {const url = readerEvent.target.result;imageObj.src = url;};reader.readAsDataURL(file);};}let isDragging = false;let lastX;let lastY;function mouseDownListener(e) {let rect = canvas.getBoundingClientRect();isDragging = true;lastX = e.clientX - rect.left;lastY = e.clientY - rect.top;}function mouseUpListener(e) {isDragging = false;}function mouseMoveListener(e) {if (isDragging) {let rect = canvas.getBoundingClientRect();offsetX += (e.clientX - rect.left) - lastX;offsetY += (e.clientY - rect.top) - lastY;lastX = e.clientX - rect.left;lastY = e.clientY - rect.top;ctx.clearRect(0, 0, canvas.width, canvas.height);drawImage();}showImageXY(e);}function mouseWheelListener(e) {let delta = Math.max(-1, Math.min(1, (e.wheelDelta || -e.detail)));if (scale > 0.1 && scale < 10 && delta < 0) {scale *= 0.9;}if (scale > 0.1 && scale < 10 && delta > 0) {scale *= 1.1;}ctx.clearRect(0, 0, canvas.width, canvas.height);drawImage();}function createBlock(a, b, r) {let rect = canvas.getBoundingClientRect();let canvasX = (a * scale) + offsetX + rect.left;let canvasY = (b * scale) + offsetY + rect.top;let radius=r*scale;ctx.beginPath();ctx.fillStyle = 'red';ctx.arc(canvasX, canvasY, radius, 0, Math.PI * 2);ctx.fill();}function drawImage() {ctx.drawImage(imageObj, offsetX, offsetY,imageWidth * scale,imageHeight * scale);createBlock(200,200,200)}//计算当前鼠标坐标对应的图片里的像素坐标function showImageXY(e) {let rect = canvas.getBoundingClientRect();let x=(e.clientX - rect.left- offsetX)/scalelet y=(e.clientY- rect.top - offsetY)/scalex=Math.round(x)y=Math.round(y)if(x<0 || x>imageWidth){x="-"}if(y<0 || y>imageHeight){y="-"}if(!(x=='-'|| y=='-')){showImagePixVal(x,y);}document.getElementById("imageXY").innerText = `x=${x} y=${y} xInCanvas=${offsetX} yInCanvas=${offsetY} width=${imageWidth} height=${imageHeight}`;}function showImagePixVal(x,y){let idx=((y * (imageWidth * 4)) + (x * 4));if(pixdata){let r=pixdata[idx]let g=pixdata[idx+1]let b=pixdata[idx+2]let a=pixdata[idx+3]document.getElementById("imagePix").innerText = `r=${r} g=${g} b=${b} a=${a}`;}}imageObj.onload = () => {imageWidth = imageObj.width;imageHeight = imageObj.height;drawImage();initPixData(imageObj);}
</script>
</body>
</html>