如何：设置 JPEG 压缩级别
更新：2007 年 11 月
将图像保存到磁盘中时，可能需要修改图像的参数以最大限度地减小文件的大小或提高质量。可以通过修改 JPEG 图像的压缩级别来调整其质量。若要在保存 JPEG 图像时指定压缩级别，必须创建一个 EncoderParameters 对象，并且将此对象传递给 Image 类的 Save 方法。初始化 EncoderParameters 对象，以使其具有包含一个 EncoderParameter 的数组。创建 EncoderParameter 时，指定 Quality 编码器和所需的压缩级别。
示例
下面的示例代码创建一个 EncoderParameter 对象并保存三幅 JPEG 图像。通过修改传递给 EncoderParameter 构造函数的 long 值，使用不同的质量级别保存每个 JPEG 图像。质量级别 0 对应于最大压缩，而质量级别 100 对应于最小压缩。
Private Sub VaryQualityLevel()
' Get a bitmap.
Dim bmp1 As New Bitmap("c:\TestPhoto.jpg")
Dim jgpEncoder As ImageCodecInfo = GetEncoder(ImageFormat.Jpeg)
' Create an Encoder object based on the GUID
' for the Quality parameter category.
Dim myEncoder As System.Drawing.Imaging.Encoder = System.Drawing.Imaging.Encoder.Quality
' Create an EncoderParameters object.
' An EncoderParameters object has an array of EncoderParameter
' objects. In this case, there is only one
' EncoderParameter object in the array.
Dim myEncoderParameters As New EncoderParameters(1)
Dim myEncoderParameter As New EncoderParameter(myEncoder, 50&)
myEncoderParameters.Param(0) = myEncoderParameter
bmp1.Save("c:\TestPhotoQualityFifty.jpg", jgpEncoder, myEncoderParameters)
myEncoderParameter = New EncoderParameter(myEncoder, 100&)
myEncoderParameters.Param(0) = myEncoderParameter
bmp1.Save("c:\TestPhotoQualityHundred.jpg", jgpEncoder, myEncoderParameters)
' Save the bitmap as a JPG file with zero quality level compression.
myEncoderParameter = New EncoderParameter(myEncoder, 0&)
myEncoderParameters.Param(0) = myEncoderParameter
bmp1.Save("c:\TestPhotoQualityZero.jpg", jgpEncoder, myEncoderParameters)
End Sub 'VaryQualityLevel
...
Private Function GetEncoder(ByVal format As ImageFormat) As ImageCodecInfo
Dim codecs As ImageCodecInfo() = ImageCodecInfo.GetImageDecoders()
Dim codec As ImageCodecInfo
For Each codec In codecs
If codec.FormatID = format.Guid Then
Return codec
End If
Next codec
Return Nothing
End Function
private void VaryQualityLevel()
{
// Get a bitmap.
Bitmap bmp1 = new Bitmap(@"c:\TestPhoto.jpg");
ImageCodecInfo jgpEncoder = GetEncoder(ImageFormat.Jpeg);
// Create an Encoder object based on the GUID
// for the Quality parameter category.
System.Drawing.Imaging.Encoder myEncoder =
System.Drawing.Imaging.Encoder.Quality;
// Create an EncoderParameters object.
// An EncoderParameters object has an array of EncoderParameter
// objects. In this case, there is only one
// EncoderParameter object in the array.
EncoderParameters myEncoderParameters = new EncoderParameters(1);
EncoderParameter myEncoderParameter = new EncoderParameter(myEncoder, 50L);
myEncoderParameters.Param[0] = myEncoderParameter;
bmp1.Save(@"c:\TestPhotoQualityFifty.jpg", jgpEncoder, myEncoderParameters);
myEncoderParameter = new EncoderParameter(myEncoder, 100L);
myEncoderParameters.Param[0] = myEncoderParameter;
bmp1.Save(@"c:\TestPhotoQualityHundred.jpg", jgpEncoder, myEncoderParameters);
// Save the bitmap as a JPG file with zero quality level compression.
myEncoderParameter = new EncoderParameter(myEncoder, 0L);
myEncoderParameters.Param[0] = myEncoderParameter;
bmp1.Save(@"c:\TestPhotoQualityZero.jpg", jgpEncoder, myEncoderParameters);
}
...
private ImageCodecInfo GetEncoder(ImageFormat format)
{
ImageCodecInfo[] codecs = ImageCodecInfo.GetImageDecoders();
foreach (ImageCodecInfo codec in codecs)
{
if (codec.FormatID == format.Guid)
{
return codec;
}
}
return null;
}
编译代码
此示例需要：
Windows 窗体应用程序。
PaintEventArgs，它是 PaintEventHandler 的一个参数。
一个位于 c:\ 位置的名为 TestPhoto.jpg 的图像文件。