async function drawCanvas() {
    const mainFile = document.getElementById('mainImg').files[0];
    const bgFile = document.getElementById('bgImg').files[0];
    if (!mainFile) return alert('请上传主图');

    // 加载图片
    const main = await loadImage(URL.createObjectURL(mainFile));
    const titleText = document.getElementById('title').value;
    const descText = document.getElementById('desc').value;
    const descLines = descText.split('\n');

    // --- 动态高度计算 ---
    const padding = 15; // 边缘空隙
    const imgW = 1080 - (padding * 2);
    const imgH = (main.height * imgW) / main.width; // 按比例缩放图片高度
    
    // 预留：标题(60px) + 文字间距(20px) + 描述(每行30px) + 底部信息(150px)
    const contentH = 60 + 20 + (descLines.length * 30) + 150;
    const totalH = padding + imgH + 20 + contentH + padding;

    canvas.width = 1080;
    canvas.height = totalH;

    // --- 绘图 ---
    ctx.fillStyle = "#ffffff"; 
    ctx.fillRect(0, 0, canvas.width, canvas.height);
    
    // 绘制背景图
    if (bgFile) {
        const bg = await loadImage(URL.createObjectURL(bgFile));
        ctx.drawImage(bg, 0, 0, 1080, totalH);
    }

    // 绘制主图 (居中，左右空隙15px)
    ctx.drawImage(main, padding, padding, imgW, imgH);

    // 绘制标题 (紧凑排列)
    const textTop = padding + imgH + 30; // 标题离图片只有30px
    ctx.fillStyle = "#1e293b";
    ctx.font = "bold 50px sans-serif";
    ctx.fillText(titleText, padding, textTop);
    
    // 绘制描述
    ctx.font = "28px sans-serif";
    descLines.forEach((line, i) => {
        ctx.fillText(line, padding, textTop + 50 + (i * 35));
    });

    // 底部右对齐信息
    const qr = await loadImage('https://album.880200.xyz/image/qrcode_fengcblog.png'); 
    const bottomY = totalH - 140;
    if(qr) ctx.drawImage(qr, 1080 - 125, bottomY - 10, 110, 110);
    
    ctx.textAlign = "right";
    ctx.font = "24px sans-serif";
    ctx.fillText("Shared from fengc's Blog", 1080 - 145, bottomY + 30);
    ctx.fillText("https://fengcblog.880200.xyz", 1080 - 145, bottomY + 70);
}