为啥使用canvas绘制完成后会出现黑色边框[图片] 代码：  <canvas type="2d" id="canvasChessBoard"></canvas>   const query = wx.createSelectorQuery()     query.select('#canvasChessBoard')       .fields({ node: true, size: true })       .exec((res) => {         const canvas = res[0].node         const ctx = canvas.getContext('2d')         // ctx.drawImage(dataTemp.strPathImgChessboard, 0, 0, dataTemp.nCanvChessBoardWidth, dataTemp.nCanvChessBoardHeight)         const dpr = wx.getSystemInfoSync().pixelRatio         canvas.width = res[0].width * dpr         canvas.height = res[0].height * dpr         ctx.scale(dpr, dpr)         ctx.fillRect(0, 0, 100, 100)       })