c#.net 生成清晰缩略图的解决方案(转载)

Bitmap类的GetThumbnailImage方法可以从指定的图像文件中生成缩略图，但是这种方式生成的缩略图清晰度不稳定，有时生成的缩略图的质量很差——模糊不清！

为什么会这样？为什么生成的图片是模糊的？

像JPEG这样格式的图像可能把缩略图存在同一个文件中。如果我们使用 System.Drawing.Bitmap的GetThumbnailImage方法会检测文件中是否存在缩略图，如果找到该缩略图，它就会返回你所设定的宽高的缩略图版本。如果图像的缩略图的版本比你要求的大小要小，就会发生如下问题：生成的缩略图就会变得模糊，我们知道拉伸一个图像会导致图像质量的下降。

解决方案：

解决方法十分简单！我们利用System.Drawing.Image对象来装载源图像，把图像缩放到你需要的比例，而又能保持高质量，接着保存就搞定了！

下面通过ASP .NET C#来演示代码片段：

//你应该包含这些名称空间

using System.Drawing;

using System.Drawing.Drawing2D

//下面开始

//本例中假定了两个变量：

String src="c:/myImages/a.jpg";   //源图像文件的绝对路径

String dest="c:/myImages/a_th.jpg";    //生成的缩略图图像文件的绝对路径

int thumbWidth=132;    //要生成的缩略图的宽度

System.Drawing.Image image = System.Drawing.Image.FromFile(src); //利用Image对象装载源图像

//接着创建一个System.Drawing.Bitmap对象，并设置你希望的缩略图的宽度和高度。

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

//从Bitmap创建一个System.Drawing.Graphics对象，用来绘制高质量的缩小图。

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

//设置 System.Drawing.Graphics对象的SmoothingMode属性为HighQuality

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

//下面这个也设成高质量

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

//下面这个设成High

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

//把原始图像绘制成上面所设置宽高的缩小图

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

//保存图像，大功告成！

bmp.Save(dest);

//最后别忘了释放资源
bmp.Dispose();
image.Dispose();

参考：

[记录]高品质图像保存代码

[C#]

/**//// <param name="fileName">图像名</param>
/// <param name="quality">品质</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);

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

//获取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)

    '保存
    myBitmap.Save(saveName, myImageCodecInfo, myEncoderParameters)
End Sub

'获取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

转自:http://www.cnblogs.com/zhangsir/articles/1205862.html