//Page data initialization add parameter: issigncanvas show

//Convert the image to jpg through canvas, making the image generate a white background for easy viewing and Preview

//List is the original image array list. index indicates the current image subscript.
//imgList represents the list of pictures that have been converted through canvas
    trasformImgType(list,index,imgList){
        this.setData({
            isSignCanvasShow:true
        });
        index=index?index:0;
        const that=this;
        let img=list[index].fileUrl; 
        img=img.replace(/http/,'https');
        tip.loading('Opening picture');
        //Get picture information,
        wx.getImageInfo({
            src: img,
            success (res) {
                //Draw in canvas
                const context = wx.createCanvasContext('picCanvas');
                that.resetCanvas(context);
                context.drawImage(res.path,0, 0);
                context.draw(false,function(drawed){
                    // console.log(drawed);
                    wx.canvasToTempFilePath({
                        x: 0,
                        y: 0,
                        width: 414,
                        height: 300,
                        destWidth: 414,
                        destHeight:300,
                        fileType: 'jpg',
                        canvasId: 'picCanvas',
                        success(imgRes) {
                            tip.loaded();
                            imgList.push(imgRes.tempFilePath);
                            if(index<list.length-1){
                                that.trasformImgType(list,index+1,imgList)
                                return;
                            }
                            that.setData({
                                isSignCanvasShow:false
                            })
                            wx.previewImage({
                                current: '', //Icon current subscript
                                urls: imgList, // http link list of pictures to preview
                                fail:function(res){
                                    tip.alert('Pictures need to be refreshed after expiration');
                                },
                            }) 
                        },
                        fail() {
                            that.setData({
                                isSignCanvasShow:false
                            })
                            tip.loaded();
                            tip.alert('Pictures need to be refreshed after expiration');
                        }
                    })
                } )
            }
        })
    },    
    //Redraw Sketchpad
    resetCanvas(context){
        context.rect(0, 0, this.data.screenWidth, this.data.screenHeight - 4);  //Sketchpad size
        context.setFillStyle('#fff '); / / background fill
        context.fill()  //Set fill
        context.draw()    //Draw a picture

    },

wxml The following code needs to be added to the file:

<view hidden="{{!isSignCanvasShow}}">
　　<canvas canvas-id="picCanvas" id='picCanvas' class="pic-canvas" ></canvas>
</view>

Method interpretation:

When previewing a picture through wx.previewImage, when the format is png and the preview background is black, the picture can not be viewed clearly. You can convert it to a white background through the following methods to facilitate viewing.

1. Use wx.getImageInfo to download the picture to the local area and get the picture information;

2. Draw the image into canvas and generate the temporary image address;

3. Fill in the address generated by canvas into imgList for caching;

4. After all the pictures are converted, call wx.previewImage to view the pictures.

5. When converting a picture, draw canvas again.