Canvas绘制图片和区域（前端使用Canvas绘制图片，并在图片上绘制区域）

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canvas
  this.drawCanvas = document.getElementById("drawCanvas");
  this.drawCtx = this.drawCanvas.getContext("2d");

  // 保存绘制区域 saveCanvas
  this.saveCanvas = document.getElementById("saveCanvas");
  this.saveCtx = this.saveCanvas.getContext("2d");
}

imgCanvas用于绘制原始图片
drawCanvas用于临时绘制区域
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

具体全部的执行顺序如下:

初始化Canvas
- 调用initCanvas()方法初始化三个Canvas画布
- 调用initImgCanvas()方法计算并设置画布宽高比例，加载并绘制图片
开始绘制
- 调用startDraw()方法
- 监听drawCanvas的click、dblclick、mousemove事件
- 点击时,在drawImageClickFn中记录点坐标
- 移动时,在drawImageMoveFn中实时绘制区域
- 双击时,在drawImageDblClickFn中完成当前区域绘制，保存至saveCanvas
保存和渲染数据
- 调用savePoints()方法,将绘制区域的点坐标数据转换并保存到submitData中
- 调用renderDatas()方法,将submitData中的数据转换并渲染到saveCanvas上

简单来说，就是先初始化画布，然后开始绘制区域的交互，最后保存和渲染数据。

十一. 当然，如果想使用原生JS实现，可以改成像下面这样

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

renderDatas函数的作用是将已有的绘制数据(submitData)转换成canvas坐标，

并调用drawSaveArea方法将其渲染到saveCanvas上。

该函数遍历submitData中的每个polygon对象，

根据ratio将其坐标值转换成canvas的坐标值，

然后调用drawSaveArea方法绘制该区域。

最终返回一个包含所有区域点坐标的数组drawedPoints。

最后，需要按顺序调用initCanvas() -> getImage() -> startDraw()等方法,分别完成初始化画布、加载图片和开始绘制的功能。