c#.net 產生清晰縮圖的解決方案

Bitmap類的GetThumbnailImage方法可以從指定的影像檔中產生縮圖，但是這種方式產生的縮圖清晰度不穩定，有時產生的縮圖的品質很差——模糊不清！

為什麼會這樣？為什麼產生的圖片是模糊的？

像JPEG這樣格式的映像可能把縮圖存在同一個檔案中。如果我們使用System.Drawing.Bitmap的GetThumbnailImage方法會檢測檔案中是否存在縮圖，如果找到該縮圖，它就會返回你所設定的寬高的縮圖版本。如果映像的縮圖的版本比你要求的大小要小，就會發生如下問題：產生的縮圖就會變得模糊，我們知道展開一個映像會導致映像品質的下降。

解決方案：

解決方案十分簡單！我們利用System.Drawing.Image對象來裝載源映像，  把映像縮放到你需要的比例，而又能保持高品質，接著儲存就搞定了！

下面通過ASP .NET C#來示範程式碼片段：

 

//你應該包含這些名稱空間

using System.Drawing; 

using System.Drawing.Drawing2D

//下面開始

//本例中假定了兩個變數：

String src="c:/myImages/a.jpg";   //源影像檔的絕對路徑

String dest="c:/myImages/a_th.jpg";    //產生的縮圖影像檔的絕對路徑

int thumbWidth=132;    //要產生的縮圖的寬度

System.Drawing.Image image = System.Drawing.Image.FromFile(src); //利用Image對象裝載源映像

//接著建立一個System.Drawing.Bitmap對象，並設定你希望的縮圖的寬度和高度。

int srcWidth=image.Width;
int srcHeight=image.Height;
int thumbHeight=(srcHeight/srcWidth)*thumbWidth;
Bitmap bmp = new Bitmap(thumbWidth, thumbHeight);

//從Bitmap建立一個System.Drawing.Graphics對象，用來繪製高品質的縮小圖。

System.Drawing.Graphics gr = System.Drawing.Graphics.FromImage(bmp);

//設定 System.Drawing.Graphics對象的SmoothingMode屬性為HighQuality

gr.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;

//下面這個也設成高品質

gr.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;

//下面這個設成High

gr.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High;

//把原始映像繪製成上面所設定寬高的縮小圖

System.Drawing.Rectangle rectDestination = new System.Drawing.Rectangle(0, 0, thumbWidth, thumbHeight);
gr.DrawImage(image, rectDestination, 0, 0, srcWidth, srcHeight, GraphicsUnit.Pixel);

//儲存映像，大功告成！

bmp.Save(dest);

//最後別忘了釋放資源
bmp.Dispose();
image.Dispose();

參考：

[記錄]高品質映像儲存代碼
 

[C#]

/**//// <param name="fileName">映像名</param>
/// <param name="quality">品質</param>
public static void SaveImage(string fileName, int quality)
{
   
    Bitmap  myBitmap= new Bitmap(fileName);

  
    System.Drawing.Imaging.EncoderParameters myEncoderParameters =
        new System.Drawing.Imaging.EncoderParameters(1);

    System.Drawing.Imaging.EncoderParameter myEncoderParameter =
        new System.Drawing.Imaging.EncoderParameter(
        System.Drawing.Imaging.Encoder.Quality, quality);
 
    myEncoderParameters.Param[0] = myEncoderParameter;

 
    System.Drawing.Imaging.ImageCodecInfo myImageCodecInfo;
    myImageCodecInfo = GetEncoderInfo("image/jpeg");

  
    string ext = myImageCodecInfo.FilenameExtension.Split(';')[0];
    ext = System.IO.Path.GetExtension(ext).ToLower();

    string saveName= System.IO.Path.ChangeExtension(fileName, ext);

    //儲存
    myBitmap.Save(saveName, myImageCodecInfo, myEncoderParameters);
}

//擷取MimeType
private static System.Drawing.Imaging.ImageCodecInfo
    GetEncoderInfo(string mineType)
{
   
    System.Drawing.Imaging.ImageCodecInfo [] myEncoders=
        System.Drawing.Imaging.ImageCodecInfo.GetImageEncoders();
   
    foreach (System.Drawing.Imaging.ImageCodecInfo myEncoder in myEncoders)
        if (myEncoder.MimeType == mineType)
            return myEncoder;
    return null;
}

[VB.NET]

Public Shared  Sub SaveImage()Sub SaveImage(ByVal fileName As String, _
        ByVal quality As Integer)
   
    Dim myBitmap As Bitmap =  New Bitmap(fileName)
 
   
    Dim myEncoderParameters As _
        New System.Drawing.Imaging.EncoderParameters(1)
   
    Dim myEncoderParameter As _
        New System.Drawing.Imaging.EncoderParameter( _
        System.Drawing.Imaging.Encoder.Quality,quality)
   
    myEncoderParameters.Param(0) = myEncoderParameter

   
    Dim myImageCodecInfo As System.Drawing.Imaging.ImageCodecInfo
    myImageCodecInfo = GetEncoderInfo("image/jpeg")
 
   
    Dim ext As String =  myImageCodecInfo.FilenameExtension.Split(";"c)(0)
    ext = System.IO.Path.GetExtension(ext).ToLower()
 
   
    Dim saveName As String = _
        System.IO.Path.ChangeExtension(fileName, ext)
 
    '儲存
    myBitmap.Save(saveName, myImageCodecInfo, myEncoderParameters)
End Sub
 
'擷取MimeType
Private Shared Function GetEncoderInfo()Function GetEncoderInfo(ByVal mineType As String) _
        As System.Drawing.Imaging.ImageCodecInfo
   
    Dim myEncoders() As System.Drawing.Imaging.ImageCodecInfo = _
        System.Drawing.Imaging.ImageCodecInfo.GetImageEncoders()
   
    Dim myEncoder As System.Drawing.Imaging.ImageCodecInfo
    For Each myEncoder In myEncoders
        If myEncoder.MimeType = mineType Then
            Return myEncoder
        End If
    Next
    Return Nothing
End Function

本文來自CSDN部落格，轉載請標明出處：http://blog.csdn.net/swazn_yj/archive/2009/08/23/4474589.aspx