小程序上传图片加水印

<canvas class='canvas' canvas-id="firstCanvas"></canvas>

.canvas{ 
   
  border: 2rpx solid pink;
  background: pink;
  width: 100%;
  height: 100%;
  //position:fixed;left:100%; 想隐藏画布单又想执行画布方法，用这个css；用display会不执行画布
}

//点击方法上传
  chooseImg: function () { 
   
    var that = this;
    wx.showActionSheet({ 
   
      itemList: ['手机拍摄', '图片上传'],
      success: function (res) { 
   
        //if (res.tapIndex == 0) { 
   
          wx.chooseImage({ 
   
            count: 3, // 默认9
            sizeType: ['original', 'compressed'], // 可以指定是原图还是压缩图，默认二者都有
            sourceType: ['camera'], // 可以指定来源是相册还是相机，默认二者都有
            success: function (res) { 
   
              // 返回选定照片的本地文件路径列表，tempFilePath可以作为img标签的src属性显示图片
              var tempFilePaths = res.tempFilePaths;//
              for (let item of tempFilePaths) { 
   
                //that.uploadimg(item);//这是直接上传原图
              }
              if (that.data.imgList.length != 0) { 
   
                that.setData({ 
   
                  imgList: that.data.imgList.concat(res.tempFilePaths)
                })
              } else { 
   
                that.setData({ 
   
                  imgList: res.tempFilePaths
                })
              }
              that.watermark();//调方法加水印
            }
          })
      },
    })
  },
  //加水印
	watermark:function() { 
   
    let that = this
    console.log(that.data.imgList,11111)//上传的图片
		//获取图片详细信息
		for (let item of that.data.imgList) { 
   
			wx.getImageInfo({ 
   
				src: item,
				success: (ress) => { 
   
					console.log("获取图片详情", item)
          let ctx = wx.createCanvasContext('firstCanvas')
					that.setData({ 
   
						canvasHeight: ress.height,
						canvasWidth: ress.width
          })
					//将图片src放到cancas内，宽高为图片大小
					ctx.drawImage(item, 0, 0, 56, 56)
					//将声明的时间放入canvas
					ctx.setFontSize(17) //注意：设置文字大小必须放在填充文字之前，否则不生效
					ctx.setFillStyle('grey')
					ctx.fillText("水印", 0, 30)
          			ctx.strokeText("水印", 0, 30)
					ctx.draw(false, function () { 
   
					wx.canvasToTempFilePath({ 
   
						canvasId: 'firstCanvas',
						success: (res) => { 
   
                			that.uploadimg(res.tempFilePath);//上传图片，加了水印的图片
							},
							fail: (e) => { 
   
								console.log(e)
							}
						})
					})
				}
			})
		}
  },
  //上传图片
	 uploadimg(localUrl) { 
   
    let that = this;
    wx.showLoading({ 
   
      title: "正在上传图片",
      mask: true
    })
    var tempFilePaths = [];
    tempFilePaths.push(localUrl)
    //这是公司封装的上传方法，就是ajax上传而已
    ygPost.uploadFile(tempFilePaths, function (res) { 
   
      wx.hideLoading();
      if (res) { 
   
        wx.showToast({ 
   
          title: '图片上传成功',
          icon: 'success',
          duration: 1000
        })
        let arr = []
        arr.push(res)
        that.setData({ 
   
          uploadList: that.data.uploadList.concat(arr)//加了水印的图片合集
        })
        console.log(that.data.uploadList, 1111)
        wx.hideLoading();
      }
    })
  },
  //测试end

大概思路