diff --git a/CamScanner.cs b/CamScanner.cs
index 5fb7085..de0ca89 100644
--- a/CamScanner.cs
+++ b/CamScanner.cs
@@ -332,22 +332,24 @@ public static class DocumentScanner
         Cv2.CvtColor(src, gray, ColorConversionCodes.BGR2GRAY);
 
         // --- b: 背景估计（形态学闭运算）---
-        // 闭运算 = 先膨胀后腐蚀，效果是填平暗区域（文字），保留亮区域（背景）
-        // 比高斯模糊更好：不会把远处的亮区域扩散到阴影处
-        int morphSize = Math.Max(gray.Width, gray.Height) / 25;
-        if (morphSize < 25) morphSize = 25;
-        if (morphSize % 2 == 0) morphSize++;
+        // 闭运算 = 先膨胀后腐蚀，填平暗区域（文字），保留亮区域（背景）
+        // 核要足够大，能完全覆盖文字笔画和行间距，否则文字密集区域背景被拉低
+        // 用多次迭代小核代替单次超大核（性能更好）
+        int morphSize = 41;
+        int iterations = Math.Max(gray.Width, gray.Height) / 200;
+        if (iterations < 3) iterations = 3;
 
         Mat morphKernel = Cv2.GetStructuringElement(MorphShapes.Ellipse,
             new OpenCvSharp.Size(morphSize, morphSize));
         Mat background = new Mat();
-        Cv2.MorphologyEx(gray, background, MorphTypes.Close, morphKernel);
+        Cv2.MorphologyEx(gray, background, MorphTypes.Close, morphKernel,
+            null, iterations);
         morphKernel.Dispose();
 
-        // 对背景估计做轻度模糊，消除形态学的块状伪影
-        int smoothSize = morphSize / 2;
+        // 对背景估计做模糊，消除形态学的块状伪影
+        int smoothSize = morphSize * iterations / 2;
         if (smoothSize % 2 == 0) smoothSize++;
-        if (smoothSize < 3) smoothSize = 3;
+        if (smoothSize < 21) smoothSize = 21;
         Cv2.GaussianBlur(background, background, new OpenCvSharp.Size(smoothSize, smoothSize), 0);
 
         // --- c: 减法提取墨迹 ---