简介：在Web开发中，有时候我们需要在图片上进行一些交互式操作，比如绘制区域、标记等。这种场景下，我们可以使用HTML5的<canvas>元素来实现。Canvas 是 HTML5 提供的一种图形绘制接口，可以通过 JavaScript 在网页上绘制图形、动画和其他视觉效果。这里来记录一下，如何在一张图片上，绘制区域

先看一下效果：

如何使用Canvas在图片上绘制区域？

初始化Canvas

一. 首先，我们需要初始化三个canvas画布

initCanvas() {// 初始化canvas画布let canvasWrap = document.getElementsByClassName("canvas-wrap");this.wrapWidth = canvasWrap[0].clientWidth;this.wrapHeight = canvasWrap[0].clientHeight;this.imgCanvas = document.getElementById("imgCanvas");this.imgCtx = this.imgCanvas.getContext("2d");// 绘制canvasthis.drawCanvas = document.getElementById("drawCanvas");this.drawCtx = this.drawCanvas.getContext("2d");// 保存绘制区域 saveCanvasthis.saveCanvas = document.getElementById("saveCanvas");this.saveCtx = this.saveCanvas.getContext("2d");
}

1. imgCanvas用于绘制原始图片
2. drawCanvas用于临时绘制区域
3. saveCanvas用于保存最终绘制的区域

二. 计算并设置canvas的宽高比例，以适应图片尺寸

initImgCanvas() {// 计算宽高比let ww = this.wrapWidth; // 画布宽度let wh = this.wrapHeight; // 画布高度 let iw = this.imgWidth; // 图片宽度let ih = this.imgHeight; // 图片高度if (iw / ih < ww / wh) {// 以高为主this.ratio = ih / wh;this.canvasHeight = wh;this.canvasWidth = (wh * iw) / ih;} else {// 以宽为主 this.ratio = iw / ww;this.canvasWidth = ww;this.canvasHeight = (ww * ih) / iw;}// 初始化画布大小this.imgCanvas.width = this.canvasWidth;this.imgCanvas.height = this.canvasHeight;this.drawCanvas.width = this.canvasWidth; this.drawCanvas.height = this.canvasHeight;this.saveCanvas.width = this.canvasWidth;this.saveCanvas.height = this.canvasHeight;// 图片加载绘制let img = document.createElement("img");img.src = this.imgUrl;img.onload = () => {console.log("图片已加载");this.imgCtx.drawImage(img, 0, 0, this.canvasWidth, this.canvasHeight);this.renderDatas(); // 渲染原有数据};
}

这里先计算画布和图片的宽高比,根据比例关系决定以宽为主还是以高为主进行等比缩放。然后设置三个canvas的宽高,并在图片加载完成后将其绘制到imgCanvas上。renderDatas函数用于渲染已有的绘制数据(如果有的话)。

开始绘制

三. 绘制的主要逻辑

startDraw() {// 绘制区域if (this.isDrawing) return;this.isDrawing = true;// 绘制逻辑this.drawCanvas.addEventListener("click", this.drawImageClickFn);this.drawCanvas.addEventListener("dblclick", this.drawImageDblClickFn);this.drawCanvas.addEventListener("mousemove", this.drawImageMoveFn);
}

我们在drawCanvas上监听click、dblclick和mousemove事件,分别对应点击、双击和鼠标移动三种绘制交互。

四. 点击事件用于开始一个新的区域绘制

drawImageClickFn(e) {let drawCtx = this.drawCtx;if (e.offsetX || e.layerX) {let pointX = e.offsetX == undefined ? e.layerX : e.offsetX;let pointY = e.offsetY == undefined ? e.layerY : e.offsetY;let lastPoint = this.drawingPoints[this.drawingPoints.length - 1] || [];if (lastPoint[0] !== pointX || lastPoint[1] !== pointY) {this.drawingPoints.push([pointX, pointY]);}}
}

这里获取鼠标点击的坐标,并将其推入drawingPoints数组中,用于临时保存当前绘制区域的点坐标。

五. 鼠标移动事件用于实时绘制区域

drawImageMoveFn(e) {let drawCtx = this.drawCtx;if (e.offsetX || e.layerX) {let pointX = e.offsetX == undefined ? e.layerX : e.offsetX;let pointY = e.offsetY == undefined ? e.layerY : e.offsetY;// 绘制drawCtx.clearRect(0, 0, this.canvasWidth, this.canvasHeight);// 绘制点drawCtx.fillStyle = "blue";this.drawingPoints.forEach((item, i) => {drawCtx.beginPath();drawCtx.arc(...item, 6, 0, 180);drawCtx.fill(); //填充});// 绘制动态区域drawCtx.save();drawCtx.beginPath();this.drawingPoints.forEach((item, i) => {drawCtx.lineTo(...item);});drawCtx.lineTo(pointX, pointY);drawCtx.lineWidth = "3";drawCtx.strokeStyle = "blue";drawCtx.fillStyle = "rgba(255, 0, 0, 0.3)";drawCtx.stroke();drawCtx.fill(); //填充drawCtx.restore();}
}

这里先清空drawCanvas,然后遍历drawingPoints数组,绘制已经点击的点。接着再绘制一个动态区域,即从第一个点开始,连线到当前鼠标位置,形成一个闭合多边形区域。

六. 双击事件用于完成当前区域的绘制

drawImageDblClickFn(e) {let drawCtx = this.drawCtx;let saveCtx = this.saveCtx;if (e.offsetX || e.layerX) {let pointX = e.offsetX == undefined ? e.layerX : e.offsetX;let pointY = e.offsetY == undefined ? e.layerY : e.offsetY;let lastPoint = this.drawingPoints[this.drawingPoints.length - 1] || [];if (lastPoint[0] !== pointX || lastPoint[1] !== pointY) {this.drawingPoints.push([pointX, pointY]);}}// 清空绘制图层drawCtx.clearRect(0, 0, this.canvasWidth, this.canvasHeight);// 绘制区域至保存图层this.drawSaveArea(this.drawingPoints);this.drawedPoints.push(this.drawingPoints);this.drawingPoints = [];this.isDrawing = false;// 绘制结束逻辑this.drawCanvas.removeEventListener("click", this.drawImageClickFn);this.drawCanvas.removeEventListener("dblclick", this.drawImageDblClickFn);this.drawCanvas.removeEventListener("mousemove", this.drawImageMoveFn);
}

双击时,先获取双击的坐标点,并将其推入drawingPoints数组中。然后清空drawCanvas,并调用drawSaveArea方法,将当前绘制区域渲染到saveCanvas上。

七. 遍历区域点坐标的方法

drawSaveArea(points) {if (points.length === 0) return;this.saveCtx.save();this.saveCtx.beginPath();points.forEach((item, i) => {this.saveCtx.lineTo(...item);});this.saveCtx.closePath();this.saveCtx.lineWidth = "2";this.saveCtx.fillStyle = "rgba(255,0, 255, 0.3)";this.saveCtx.strokeStyle = "red";this.saveCtx.stroke();this.saveCtx.fill(); this.saveCtx.restore();
}

drawSaveArea方法会遍历当前区域的所有点坐标,并在saveCanvas上绘制一个闭合的多边形区域,边框为红色,填充为半透明的紫色。接下来,将当前绘制区域的点坐标数组drawingPoints推入drawedPoints数组中,用于保存所有已绘制的区域数据。然后,重置drawingPoints和isDrawing的状态,并移除所有绘制事件的监听器。

至此,一个区域的绘制就完成了。如果需要继续绘制新的区域,只需再次调用startDraw方法即可。

保存和渲染数据

八. 保存数据：我们需要将绘制的区域数据保存下来，以及从已有数据中渲染出区域

savePoints() {// 将画布坐标数据转换成提交数据let objectPoints = [];objectPoints = this.drawedPoints.map((area) => {let polygon = {};area.forEach((point, i) => {polygon[`x${i + 1}`] = Math.round(point[0] * this.ratio);polygon[`y${i + 1}`] = Math.round(point[1] * this.ratio);});return {polygon: polygon,};});this.submitData = objectPoints;console.log("最终提交数据", objectPoints);
}

这里遍历所有已绘制的区域drawedPoints,将每个区域的点坐标根据ratio进行缩放(实际图片尺寸),并转换成一个polygon对象的形式,最终保存在submitData中。

九. 渲染数据

renderDatas() {// 将提交数据数据转换成画布坐标this.drawedPoints = this.submitData.map((item) => {let polygon = item.polygon;let points = [];for (let i = 1; i < Object.keys(polygon).length / 2 + 1; i++) {if (!isNaN(polygon[`x${i}`]) && !isNaN(polygon[`y${i}`])) {points.push([polygon[`x${i}`] / this.ratio,polygon[`y${i}`] / this.ratio,]);}}this.drawSaveArea(points);return points;});
}

渲染数据的逻辑是，遍历submitData中的每个polygon对象，根据ratio将其坐标值转换成canvas的坐标值，并调用drawSaveArea方法将其渲染到saveCanvas上。至此，我们就完成了在canvas上绘制图片区域的全部逻辑。可以根据具体需求进行相应的调整和扩展。

十. 执行过程

let canvasWrap, wrapWidth, wrapHeight, imgCanvas, imgCtx, drawCanvas, drawCtx, saveCanvas, saveCtx;
let ratio, canvasWidth, canvasHeight, imgWidth, imgHeight, imgUrl;
let isDrawing = false;
let drawingPoints = [];
let drawedPoints = [];
let submitData = [];// 1. 初始化Canvas画布
function initCanvas() {// 获取canvas容器元素并设置宽高canvasWrap = document.getElementsByClassName("canvas-wrap")[0];wrapWidth = canvasWrap.clientWidth;wrapHeight = canvasWrap.clientHeight;// 获取canvas元素并获取2D绘图上下文imgCanvas = document.getElementById("imgCanvas");imgCtx = imgCanvas.getContext("2d");drawCanvas = document.getElementById("drawCanvas");drawCtx = drawCanvas.getContext("2d");saveCanvas = document.getElementById("saveCanvas");saveCtx = saveCanvas.getContext("2d");
}// 2. 初始化图片Canvas
function initImgCanvas() {// 计算画布和图片的宽高比let ww = wrapWidth;let wh = wrapHeight;let iw = imgWidth;let ih = imgHeight;if (iw / ih < ww / wh) {ratio = ih / wh;canvasHeight = wh;canvasWidth = (wh * iw) / ih;} else {ratio = iw / ww;canvasWidth = ww;canvasHeight = (ww * ih) / iw;}// 设置三个canvas的宽高imgCanvas.width = canvasWidth;imgCanvas.height = canvasHeight;drawCanvas.width = canvasWidth;drawCanvas.height = canvasHeight;saveCanvas.width = canvasWidth;saveCanvas.height = canvasHeight;// 加载图片并绘制到imgCanvas上let img = document.createElement("img");img.src = imgUrl;img.onload = () => {console.log("图片已加载");imgCtx.drawImage(img, 0, 0, canvasWidth, canvasHeight);renderDatas(); // 渲染已有数据};
}// 3. 开始绘制
function startDraw() {if (isDrawing) return;isDrawing = true;// 监听drawCanvas的click、dblclick和mousemove事件drawCanvas.addEventListener("click", drawImageClickFn);drawCanvas.addEventListener("dblclick", drawImageDblClickFn);drawCanvas.addEventListener("mousemove", drawImageMoveFn);
}// 4. 清空所有绘制区域
function clearAll() {saveCtx.clearRect(0, 0, canvasWidth, canvasHeight);drawedPoints = [];
}// 5. 获取并加载图片
function getImage() {imgUrl = "需要渲染的图片地址";imgWidth = 200;imgHeight = 300;imgUrl && initImgCanvas();
}// 6. 点击事件,记录点坐标
function drawImageClickFn(e) {if (e.offsetX || e.layerX) {let pointX = e.offsetX == undefined ? e.layerX : e.offsetX;let pointY = e.offsetY == undefined ? e.layerY : e.offsetY;let lastPoint = drawingPoints[drawingPoints.length - 1] || [];if (lastPoint[0] !== pointX || lastPoint[1] !== pointY) {drawingPoints.push([pointX, pointY]);}}
}// 7. 鼠标移动事件,实时绘制区域
function drawImageMoveFn(e) {if (e.offsetX || e.layerX) {let pointX = e.offsetX == undefined ? e.layerX : e.offsetX;let pointY = e.offsetY == undefined ? e.layerY : e.offsetY;drawCtx.clearRect(0, 0, canvasWidth, canvasHeight);drawCtx.fillStyle = "blue";drawingPoints.forEach((item, i) => {drawCtx.beginPath();drawCtx.arc(...item, 6, 0, 180);drawCtx.fill();});drawCtx.save();drawCtx.beginPath();drawingPoints.forEach((item, i) => {drawCtx.lineTo(...item);});drawCtx.lineTo(pointX, pointY);drawCtx.lineWidth = "3";drawCtx.strokeStyle = "blue";drawCtx.fillStyle = "rgba(255, 0, 0, 0.3)";drawCtx.stroke();drawCtx.fill();drawCtx.restore();}
}// 8. 双击事件,完成当前区域绘制
function drawImageDblClickFn(e) {if (e.offsetX || e.layerX) {let pointX = e.offsetX == undefined ? e.layerX : e.offsetX;let pointY = e.offsetY == undefined ? e.layerY : e.offsetY;let lastPoint = drawingPoints[drawingPoints.length - 1] || [];if (lastPoint[0] !== pointX || lastPoint[1] !== pointY) {drawingPoints.push([pointX, pointY]);}}drawCtx.clearRect(0, 0, canvasWidth, canvasHeight);drawSaveArea(drawingPoints);drawedPoints.push(drawingPoints);drawingPoints = [];isDrawing = false;drawCanvas.removeEventListener("click", drawImageClickFn);drawCanvas.removeEventListener("dblclick", drawImageDblClickFn);drawCanvas.removeEventListener("mousemove", drawImageMoveFn);
}// 9. 绘制区域到saveCanvas
function drawSaveArea(points) {if (points.length === 0) return;saveCtx.save();saveCtx.beginPath();points.forEach((item, i) => {saveCtx.lineTo(...item);});saveCtx.closePath();saveCtx.lineWidth = "2";saveCtx.fillStyle = "rgba(255,0, 255, 0.3)";saveCtx.strokeStyle = "red";saveCtx.stroke();saveCtx.fill();saveCtx.restore();
}// 10. 保存绘制数据
function savePoints() {let objectPoints = [];objectPoints = drawedPoints.map((area) => {let polygon = {};area.forEach((point, i) => {polygon[`x${i + 1}`] = Math.round(point[0] * ratio);polygon[`y${i + 1}`] = Math.round(point[1] * ratio);});return {polygon: polygon,};});submitData = objectPoints;console.log("最终提交数据", objectPoints);
}// 11. 渲染已有数据
function renderDatas() {drawedPoints = submitData.map((item) => {let polygon = item.polygon;let points = [];for (let i = 1; i < Object.keys(polygon).length / 2 + 1; i++) {if (!isNaN(polygon[`x${i}`]) && !isNaN(polygon[`y${i}`])) {points.push([polygon[`x${i}`] / ratio, // 根据ratio换算canvas坐标polygon[`y${i}`] / ratio,]);}}drawSaveArea(points); // 调用drawSaveArea将区域绘制到saveCanvas上return points;});
}// 使用方式
initCanvas(); // 1. 初始化Canvas画布
getImage(); // 5. 获取并加载图片 
startDraw(); // 3. 开始绘制