故事：用户上载将添加到相册的图像。作为上传过程的一部分，我们需要A)将图像存储在Web服务器的硬盘驱动器上，以及B)将图像的缩略图存储在Web服务器的硬盘驱动器上。

此处的"最佳"定义为

• 相对容易实现，理解和维护
• 产生质量合理的缩略图

性能和高质量缩略图是次要的。

解决方案

回答

我想最好的解决方案是使用.NET Image类中的GetThumbnailImage。

// Example in C#, should be quite alike in ASP.NET
// Assuming filename as the uploaded file
using ( Image bigImage = new Bitmap( filename ) )
{
   // Algorithm simplified for purpose of example.
   int height = bigImage.Height / 10;
   int width = bigImage.Width / 10;

   // Now create a thumbnail
   using ( Image smallImage = image.GetThumbnailImage( width, 
                                                       height,
                                                       new Image.GetThumbnailImageAbort(Abort), IntPtr.Zero) )
   {
      smallImage.Save("thumbnail.jpg", ImageFormat.Jpeg);
   }
}

回答

这是VB.NET中图像类的扩展方法

Imports System.Runtime.CompilerServices

Namespace Extensions
   ''' <summary>
   ''' Extensions for the Image class.
   ''' </summary>
   ''' <remarks>Several usefull extensions for the image class.</remarks>
   Public Module ImageExtensions

      ''' <summary>
      ''' Extends the image class so that it is easier to get a thumbnail from an image
      ''' </summary>
      ''' <param name="Input">Th image that is inputted, not really a parameter</param>
      ''' <param name="MaximumSize">The maximumsize the thumbnail must be if keepaspectratio is set to true then the highest number of width or height is used and the other is calculated accordingly. </param>
      ''' <param name="KeepAspectRatio">If set false width and height will be the same else the highest number of width or height is used and the other is calculated accordingly.</param>
      ''' <returns>A thumbnail as image.</returns>
      ''' <remarks>
      ''' <example>Can be used as such. 
      ''' <code>
      ''' Dim _NewImage as Image 
      ''' Dim _Graphics As Graphics
      ''' _Image = New Bitmap(100, 100)
      ''' _Graphics = Graphics.FromImage(_Image)
      ''' _Graphics.FillRectangle(Brushes.Blue, New Rectangle(0, 0, 100, 100))
      ''' _Graphics.DrawLine(Pens.Black, 10, 0, 10, 100)
      ''' Assert.IsNotNull(_Image)
      ''' _NewImage = _Image.ToThumbnail(10)
      ''' </code>
      ''' </example>
      ''' </remarks>
      <Extension()> _
      Public Function ToThumbnail(ByVal Input As Image, ByVal MaximumSize As Integer, Optional ByVal KeepAspectRatio As Boolean = True) As Image
         Dim ReturnImage As Image
         Dim _Callback As Image.GetThumbnailImageAbort = Nothing
         Dim _OriginalHeight As Double
         Dim _OriginalWidth As Double
         Dim _NewHeight As Double
         Dim _NewWidth As Double
         Dim _NormalImage As Image
         Dim _Graphics As Graphics

         _NormalImage = New Bitmap(Input.Width, Input.Height)
         _Graphics = Graphics.FromImage(_NormalImage)
         _Graphics.DrawImage(Input, 0, 0, Input.Width, Input.Height)
         _OriginalHeight = _NormalImage.Height
         _OriginalWidth = _NormalImage.Width
         If KeepAspectRatio = True Then
            If _OriginalHeight > _OriginalWidth Then
               If _OriginalHeight > MaximumSize Then
                  _NewHeight = MaximumSize
                  _NewWidth = _OriginalWidth / _OriginalHeight * MaximumSize
               Else
                  _NewHeight = _OriginalHeight
                  _NewWidth = _OriginalWidth
               End If
            Else
               If _OriginalWidth > MaximumSize Then
                  _NewWidth = MaximumSize
                  _NewHeight = _OriginalHeight / _OriginalWidth * MaximumSize
               Else
                  _NewHeight = _OriginalHeight
                  _NewWidth = _OriginalWidth
               End If
            End If
         Else
            _NewHeight = MaximumSize
            _NewWidth = MaximumSize
         End If
         ReturnImage = _
            _NormalImage.GetThumbnailImage(Convert.ToInt32(_NewWidth), Convert.ToInt32(_NewHeight), _Callback, _
                                    IntPtr.Zero)
         _NormalImage.Dispose()
         _NormalImage = Nothing
         _Graphics.Dispose()
         _Graphics = Nothing
         _Callback = Nothing
         Return ReturnImage
      End Function
   End Module
End Namespace

抱歉，代码标签不喜欢vb.net代码。

回答

我们可以使用Image.GetThumbnailImage函数为我们完成此操作。

http://msdn.microsoft.com/zh-cn/library/system.drawing.image.getthumbnailimage.aspx(.NET 3.5)

http://msdn.microsoft.com/zh-cn/library/system.drawing.image.getthumbnailimage(VS.80).aspx(.NET 2.0)

public bool ThumbnailCallback()
{
  return false;
}

public void Example_GetThumb(PaintEventArgs e)
{
  Image.GetThumbnailImageAbort myCallback = new Image.GetThumbnailImageAbort(ThumbnailCallback);
  Bitmap myBitmap = new Bitmap("Climber.jpg");
  Image myThumbnail = myBitmap.GetThumbnailImage(40, 40, myCallback, IntPtr.Zero);
  e.Graphics.DrawImage(myThumbnail, 150, 75);
}

回答

GetThumbnailImage可以使用，但是如果我们想要更好的质量，则可以为BitMap类指定图像选项，然后将加载的图像保存到该位置。这是一些示例代码：

Image photo; // your uploaded image

Bitmap bmp = new Bitmap(resizeToWidth, resizeToHeight);
graphic = Graphics.FromImage(bmp);
graphic.InterpolationMode = InterpolationMode.HighQualityBicubic;
graphic.SmoothingMode = SmoothingMode.HighQuality;
graphic.PixelOffsetMode = PixelOffsetMode.HighQuality;
graphic.CompositingQuality = CompositingQuality.HighQuality;
graphic.DrawImage(photo, 0, 0, resizeToWidth, resizeToHeight);
imageToSave = bmp;

这提供了比GetImageThumbnail开箱即用的更好的质量

回答

避免使用GetThumbnailImage，它将提供非常不可预测的结果，因为即使嵌入式缩略图的大小完全错误，它也会尝试使用嵌入式JPEG缩略图(如果可用)。 DrawImage()是一个更好的解决方案。

将位图包装在using {}子句中，我们不希望泄漏的句柄在周围漂浮...

另外，我们需要将Jpeg编码质量设置为90，这是GDI +似乎最出色的地方：

System.Drawing.Imaging.ImageCodecInfo[] info = System.Drawing.Imaging.ImageCodecInfo.GetImageEncoders();
System.Drawing.Imaging.EncoderParameters encoderParameters;
encoderParameters = new System.Drawing.Imaging.EncoderParameters(1);
encoderParameters.Param[0] = new System.Drawing.Imaging.EncoderParameter(System.Drawing.Imaging.Encoder.Quality, 90L);

thumb.Save(ms, info[1], encoderParameters);

回答

使用上面的示例以及其他几个地方的示例，这里提供了一个简单的函数(感谢Nathanael Jones和其他示例)。

using System.Drawing;
using System.Drawing.Drawing2D;
using System.IO;

public static void ResizeImage(string FileNameInput, string FileNameOutput, double ResizeHeight, double ResizeWidth, ImageFormat OutputFormat)
{
    using (System.Drawing.Image photo = new Bitmap(FileNameInput))
    {
        double aspectRatio = (double)photo.Width / photo.Height;
        double boxRatio = ResizeWidth / ResizeHeight;
        double scaleFactor = 0;

        if (photo.Width < ResizeWidth && photo.Height < ResizeHeight)
        {
            // keep the image the same size since it is already smaller than our max width/height
            scaleFactor = 1.0;
        }
        else
        {
            if (boxRatio > aspectRatio)
                scaleFactor = ResizeHeight / photo.Height;
            else
                scaleFactor = ResizeWidth / photo.Width;
        }

        int newWidth = (int)(photo.Width * scaleFactor);
        int newHeight = (int)(photo.Height * scaleFactor);

        using (Bitmap bmp = new Bitmap(newWidth, newHeight))
        {
            using (Graphics g = Graphics.FromImage(bmp))
            {
                g.InterpolationMode = InterpolationMode.HighQualityBicubic;
                g.SmoothingMode = SmoothingMode.HighQuality;
                g.CompositingQuality = CompositingQuality.HighQuality;
                g.PixelOffsetMode = PixelOffsetMode.HighQuality;

                g.DrawImage(photo, 0, 0, newWidth, newHeight);

                if (ImageFormat.Png.Equals(OutputFormat))
                {
                    bmp.Save(FileNameOutput, OutputFormat);
                }
                else if (ImageFormat.Jpeg.Equals(OutputFormat))
                {
                    ImageCodecInfo[] info = ImageCodecInfo.GetImageEncoders();
                    EncoderParameters encoderParameters;
                    using (encoderParameters = new System.Drawing.Imaging.EncoderParameters(1))
                    {
                        // use jpeg info[1] and set quality to 90
                        encoderParameters.Param[0] = new System.Drawing.Imaging.EncoderParameter(System.Drawing.Imaging.Encoder.Quality, 90L);
                        bmp.Save(FileNameOutput, info[1], encoderParameters);
                    }
                }
            }
        }
    }
}