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：

private void SaveThumbnail(Bitmap bitmap, int width, int height, string directory, string filename, string extension)
{
    var physicalPath = directory + filename + extension;
    using (var thumbnail = bitmap.GetThumbnailImage(width, height, () => { return true; }, IntPtr.Zero))
    {
        using (var encoderParameters = new EncoderParameters(1))
        {
            encoderParameters.Param[0] = new EncoderParameter(Encoder.Quality, 100L);
            thumbnail.Save(physicalPath,
                        ImageCodecInfo.GetImageEncoders()
                            .Where(x => x.FilenameExtension.Contains(extension.ToUpperInvariant()))
                            .FirstOrDefault(),
                        encoderParameters);
        }
    }           
}

二、解决方法

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

private void SaveThumbnail(Bitmap originBitmap, int width, int height, string directory, string filename, string extension)
{
    var physicalPath = directory + filename + extension;
            
    using (var newImage = new Bitmap(width, height))
    {
        using (var graphic = GetGraphic(originBitmap, newImage))
        {
            graphic.DrawImage(originBitmap, 0, 0, width, height);
            using (var encoderParameters = new EncoderParameters(1))
            {
                encoderParameters.Param[0] = new EncoderParameter(Encoder.Quality, 100L);
                newImage.Save(physicalPath,
                            ImageCodecInfo.GetImageEncoders()
                                .Where(x => x.FilenameExtension.Contains(extension.ToUpperInvariant()))
                                .FirstOrDefault(),
                            encoderParameters);
            }
        }
    }            
}

private Graphics GetGraphic(Image originImage, Bitmap newImage)
{
    newImage.SetResolution(originImage.HorizontalResolution, originImage.VerticalResolution);
    var graphic = Graphics.FromImage(newImage);
    graphic.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
    graphic.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
    graphic.PixelOffsetMode = System.Drawing.Drawing2D.PixelOffsetMode.HighQuality;
    graphic.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;
    return graphic;
}

 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