[转] [C#] 解决生成的缩略图模糊的问题

一、问题场景

针对一张正方形图片生成 48px × 48px 的缩略图并保存为图片文件，但发现生成的缩略图很模糊。

生成的模糊的缩略图如下：

原始图片（300px × 300px, png格式）：

代码中调用的是 System.Drawing.Image. GetThumbnailImage() 方法，主要实现代码如下：

 1 private void SaveThumbnail(Bitmap bitmap, int width, int height, string directory, string filename, string extension)
 2 {
 3     var physicalPath = directory + filename + extension;
 4     using (var thumbnail = bitmap.GetThumbnailImage(width, height, () => { return true; }, IntPtr.Zero))
 5     {
 6         using (var encoderParameters = new EncoderParameters(1))
 7         {
 8             encoderParameters.Param[0] = new EncoderParameter(Encoder.Quality, 100L);
 9             thumbnail.Save(physicalPath,
10                         ImageCodecInfo.GetImageEncoders()
11                             .Where(x => x.FilenameExtension.Contains(extension.ToUpperInvariant()))
12                             .FirstOrDefault(),
13                         encoderParameters);
14         }
15     }
16 }

二、解决方法

改为调用 System.Drawing.Graphics.DrawImage() 方法， 主要实现代码如下：

 1 private void SaveThumbnail(Bitmap originBitmap, int width, int height, string directory, string filename, string extension)
 2 {
 3     var physicalPath = directory + filename + extension;
 4
 5     using (var newImage = new Bitmap(width, height))
 6     {
 7         using (var graphic = GetGraphic(originBitmap, newImage))
 8         {
 9             graphic.DrawImage(originBitmap, 0, 0, width, height);
10             using (var encoderParameters = new EncoderParameters(1))
11             {
12                 encoderParameters.Param[0] = new EncoderParameter(Encoder.Quality, 100L);
13                 newImage.Save(physicalPath,
14                             ImageCodecInfo.GetImageEncoders()
15                                 .Where(x => x.FilenameExtension.Contains(extension.ToUpperInvariant()))
16                                 .FirstOrDefault(),
17                             encoderParameters);
18             }
19         }
20     }
21 }
22
23 private Graphics GetGraphic(Image originImage, Bitmap newImage)
24 {
25     newImage.SetResolution(originImage.HorizontalResolution, originImage.VerticalResolution);
26     var graphic = Graphics.FromImage(newImage);
27     graphic.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
28     graphic.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
29     graphic.PixelOffsetMode = System.Drawing.Drawing2D.PixelOffsetMode.HighQuality;
30     graphic.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;
31     return graphic;
32 }

生成的缩略图效果如下：

三、参考资料

Cropping image using jQuery, Jcrop and ASP.NET

Resizing an Image without losing any quality