uniapp 给照片加水印

<template>
    <view>
        <button type="primary" @tap="takePhoto">拍照</button>
        <image mode="widthFix" :src="src" @click="preview"></image>
        <view style="position: absolute;top: -999999px;">
            <view>
                <canvas :style="{'width':w,'height': h}" canvas-id="firstCanvas"></canvas>
            </view>
        </view>
    </view>
</template>

<script>
    export default {
        data() {
            return {
                src:'',
                w:'',
                h:''
            }
        },
        methods: {
            preview(){
                uni.previewImage({
                    urls:[this.src],
                    current:0
                })
            },
            takePhoto() {
                var that = this;
                 uni.chooseImage({
                    count:1,
                    success(res) {
                        uni.getImageInfo({
                          src: res.tempFilePaths[0],
                          success: (ress) => {
                            that.w = ress.width/3+'px';
                            that.h = ress.height/3+'px';
                            let ctx = uni.createCanvasContext('firstCanvas');    /** 创建画布 */
                            //将图片src放到cancas内，宽高为图片大小
                            ctx.drawImage(res.tempFilePaths[0], 0, 0, ress.width / 3, ress.height / 3)
                            ctx.setFontSize(18) 
                            ctx.setFillStyle('#8a2929')
                            ctx.rotate(30 * Math.PI / 180);
                            let textToWidth = ress.width / 3 * 0.5; 
                            let textToHeight = ress.height / 3 * 0.3;
                            ctx.fillText('我是水印', textToWidth, textToHeight)  
                                                         /** 除了上面的文字水印，这里也可以加入图片水印 */
                                                        //ctx.drawImage('/static/watermark.png', 0, 0, ress.width / 3, ress.height / 3)
                            ctx.draw(false,()=>{
                                setTimeout(() => {
                                    uni.canvasToTempFilePath({
                                      canvasId: 'firstCanvas',
                                      success: (res1) => {
                                            that.src=res1.tempFilePath;
                                      }
                                    });
                                  }, 1000);
                            });
                          }
                        })
                    }
                 });
              }
        }
    }
</script>

<style>
</style>