我想做的事我想知道如何使背景顏色變白。我用畫布構(gòu)建了一個繪圖應(yīng)用程序。您可以通過單擊下載按鈕下載您繪制的畫布圖像。但它的背景顏色是黑色（技術(shù)上透明）。怎么改成白色？我試過的我將以下代碼添加到我的代碼中，但效果不佳。我什么也畫不出來。ctx.fillStyle = "#fff";ctx.fillRect(0, 0, canvas.width, canvas.height);這是我的代碼const canvas = document.querySelector('#draw');const ctx = canvas.getContext('2d');ctx.strokeStyle = '#BADA55';  ...function draw(e) {  if (!isDrawing) return;  console.log(e);  ctx.strokeStyle = `hsl(${hue}, 100%, 50%)`;  ctx.beginPath();  ctx.moveTo(lastX, lastY);  ctx.lineTo(e.offsetX, e.offsetY);  ctx.stroke();  [lastX, lastY] = [e.offsetX, e.offsetY];  ...}canvas.addEventListener('mousedown', (e) => {  isDrawing = true;  [lastX, lastY] = [e.offsetX, e.offsetY];});canvas.addEventListener('mousemove', draw);  ...downloadBtn.addEventListener('click', downloadImage);function downloadImage() {  if (canvas.msToBlob) {    const blob = canvas.msToBlob();    window.navigator.msSaveBlob(blob, filename);  } else {      downloadLink.href = canvas.toDataURL('image/png');      downloadLink.download = filename;      downloadLink.click();  }}我想將下載圖像的背景顏色設(shè)為白色。