微信小程序canvas绘制图片不变形的方法

项目中需要在canvas中绘制图片，图片大小未知
已这张图片为例

如果直接绘制图片将被压缩

根据官方文档：https://developers.weixin.qq.com/miniprogram/dev/api/canvas/CanvasContext.drawImage.html
可以设定这些参数来保证图片不会变形

上代码：

wxml:

<view class="share">
  <view style="padding-top: 65px, padding-bottom: 180rpx">
    <view class="canvas" style="height: 475px">
      <canvas canvas-id="firstCanvas"></canvas>
    </view>
  </view>
</view>

wxss:

.share{
  width: 100%;
  min-height: 100vh;
  background: #000;
  z-index: 132;
}
.canvas{
  width: calc(100% - 60upx);
}
canvas{
  width: 100%;
  height: 100%;
  margin-top: 30upx;
  margin-left: 30upx;
  overflow: hidden;
}

js:

const app = getApp()
Page({

  /**
   * 页面的初始数据
   */
  data: {

  },

  onLoad() {
    wx.showLoading({
      title: '分享图生成中…'
    })
    let _this = this
    setTimeout(function() {
      _this.lotteryCanvas()
    }, 500)
  },
  getImageInfo: function(url) {
    return new Promise((resolve, reject) => {
      wx.getImageInfo({
        src: url,
        success: function (res) {
          resolve(res);
        }, fail(res) {
          reject('fail');
        }
      });
    })
  },
  // 绘制canvas
  lotteryCanvas: function() {
    const query = wx.createSelectorQuery().in(this);
    query.select('.canvas').boundingClientRect(res => {
      console.log(res)
      var ctx = wx.createCanvasContext('firstCanvas', this)
      let canvasWidth = res.width
      let canvasHeight = res.height
      let thumb = 'https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1602735806949&di=e064594d86b54c2addb3ba5f986ab4b7&imgtype=0&src=http%3A%2F%2Ft8.baidu.com%2Fit%2Fu%3D3571592872%2C3353494284%26fm%3D79%26app%3D86%26f%3DJPEG%3Fw%3D1200%26h%3D1290'
      let getImageData = {}
      // 获取图片
      this.getImageInfo(thumb).then(res => {
        getImageData.thumbImg = res
          
        // 开始画图
        ctx.save();
        ctx.setFillStyle("#E8554E");//填充背景色
        ctx.fillRect(0, 0, canvasWidth , canvasHeight );//画出矩形红色背景
        // 绘制缩略图
        this.drawImage(ctx, canvasWidth * 0.8, 150, getImageData.thumbImg.path, getImageData.thumbImg.width, getImageData.thumbImg.height, canvasWidth * 0.1, 160)
        ctx.draw()
        wx.hideLoading()
      }).catch(res => {
        console.log('获取缩略图失败')
      })
    }).exec();
  },
  /** 等比例图片方法
    ctx  画布
    bg_w  图片绘制区域的宽
    bg_h  图片绘制区域的高
    imgPath  图片路径
    imgWidth  图片的原始宽度
    imgPath  图片的原始高度
    x  图片绘制的左上角x坐标
    y  图片绘制的左上角y坐标
  **/
  drawImage(ctx, bg_w, bg_h, imgPath, imgWidth, imgHeight, x, y) {
    let dWidth = bg_w/imgWidth;  // canvas与图片的宽度比例
    let dHeight = bg_h/imgHeight;  // canvas与图片的高度比例
    if (imgWidth > bg_w && imgHeight > bg_h || imgWidth < bg_w && imgHeight < bg_h) {
      if (dWidth > dHeight) {
        ctx.drawImage(imgPath, 0, (imgHeight - bg_h/dWidth)/2, imgWidth, bg_h/dWidth, x, y, bg_w, bg_h)
      } else {
        ctx.drawImage(imgPath, (imgWidth - bg_w/dHeight)/2, 0, bg_w/dHeight, imgHeight, x, y, bg_w, bg_h)
      }
    } else {
      if (imgWidth < bg_w) {
        ctx.drawImage(imgPath, 0, (imgHeight - bg_h/dWidth)/2, imgWidth, bg_h/dWidth, x, y, bg_w, bg_h)
      } else {
        ctx.drawImage(imgPath, (imgWidth - bg_w/dHeight)/2, 0, bg_w/dHeight, imgHeight, x, y, bg_w, bg_h)
      }
    }
  }
})

最后的结果：