let canvas = document.createElement('canvas');

        canvas.width = width;

        canvas.height = height;

        let ctx = canvas.getContext('2d');

        let gpTexture = new cc.RenderTexture(); //绘制

        //@ts-ignore

        gpTexture.initWithSize(width, height, cc.game._renderContext.STENCIL_INDEX8);

        this.gpCamera.targetTexture = gpTexture;

        this.gpCamera.render(this.node);//相机绘制，将屏幕上的内容更新到renderTexture中

        let data = gpTexture.readPixels();//读取renderTexture中的数据

        // data = this.changeAlpha(data);

        let rowBytes = width * 4;

        for (let row = 0; row < height; row++) {

            let srow = height - 1 - row;

            let imageData = ctx.createImageData(width, 1);

            let start = srow * width * 4;

            for (let i = 0; i < rowBytes; i++) {

                imageData.data[i] = data[start + i];

            }

            ctx.putImageData(imageData, 0, row);

        }

let _tempFilePath = await canvas.toTempFilePathSync(

                {

                    x: 0,

                    y: 0,

                    width: width,

                    height: height,

                    destWidth: 380,

                    destHeight: 380,

                    fileType: 'png',

                    quality: 1

                });

            wx.previewImage({

                urls: [_tempFilePath] 

            })

明明是透明部分 怎么就变成黑色了