C# 将图像格式转换为 JPG

发件人： http://msdn.microsoft.com/en-us/library/system.drawing.imaging.encoderparameter.aspx

private void VaryQualityLevel()
{
// Get a bitmap.
Bitmap bmp1 = new Bitmap(@"c:'TestPhoto.jpg");
ImageCodecInfo jpgEncoder = 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", jpgEncoder, 
    myEncoderParameters);
myEncoderParameter = new EncoderParameter(myEncoder, 100L);
myEncoderParameters.Param[0] = myEncoderParameter;
bmp1.Save(@"c:'TestPhotoQualityHundred.jpg", jpgEncoder, 
    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", jpgEncoder, 
    myEncoderParameters);
}
...
private ImageCodecInfo GetEncoder(ImageFormat format)
{
    ImageCodecInfo[] codecs = ImageCodecInfo.GetImageDecoders();
    foreach (ImageCodecInfo codec in codecs)
    {
        if (codec.FormatID == format.Guid)
        {
            return codec;
        }
    }
    return null;
}

我使用这个简单的扩展来转换流，它所做的只是转换它，对质量没有任何影响。

public static Stream ConvertImage(this Stream originalStream, ImageFormat format)
{
                var image = Image.FromStream(originalStream);
                var stream = new MemoryStream();
                image.Save(stream, format);
                stream.Position = 0;
                return stream;
}

用法：

var outputStream = gifStream.ConvertImage(ImageFormat.Png);

Windows GDI+ 提供用于存储的 Image 类和 Bitmap 类 内存中的图像和操作内存中的图像。GDI+ 写入映像 在图像编码器的帮助下到磁盘文件，并从中加载图像 图像解码器的帮助下的磁盘文件。

检查 Image.Save 方法重载以实现您的功能。 支持各种图像格式类型的图像格式类。

按照Image.Save方法（字符串，图像编解码器信息，编码器参数）上的示例进行操作，这将使您可以使用编码等保存Jpg文件。

代码片段：

using System;
using System.Drawing;
using System.Drawing.Imaging;
class Example_SetJPEGQuality
{
    public static void Main()
    {
        Bitmap myBitmap;
        ImageCodecInfo myImageCodecInfo;
        Encoder myEncoder;
        EncoderParameter myEncoderParameter;
        EncoderParameters myEncoderParameters;
        // Create a Bitmap object based on a BMP file.
        myBitmap = new Bitmap("Shapes.bmp");
        // Get an ImageCodecInfo object that represents the JPEG codec.
        myImageCodecInfo = GetEncoderInfo("image/jpeg");
        // Create an Encoder object based on the GUID
        // for the Quality parameter category.
        myEncoder = 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.
        myEncoderParameters = new EncoderParameters(1);
        // Save the bitmap as a JPEG file with quality level 25.
        myEncoderParameter = new EncoderParameter(myEncoder, 25L);
        myEncoderParameters.Param[0] = myEncoderParameter;
        myBitmap.Save("Shapes025.jpg", myImageCodecInfo, myEncoderParameters);
        // Save the bitmap as a JPEG file with quality level 50.
        myEncoderParameter = new EncoderParameter(myEncoder, 50L);
        myEncoderParameters.Param[0] = myEncoderParameter;
        myBitmap.Save("Shapes050.jpg", myImageCodecInfo, myEncoderParameters);
        // Save the bitmap as a JPEG file with quality level 75.
        myEncoderParameter = new EncoderParameter(myEncoder, 75L);
        myEncoderParameters.Param[0] = myEncoderParameter;
        myBitmap.Save("Shapes075.jpg", myImageCodecInfo, myEncoderParameters);
    }

编解码器信息

  private static ImageCodecInfo GetEncoderInfo(String mimeType)
        {
            int j;
            ImageCodecInfo[] encoders;
            encoders = ImageCodecInfo.GetImageEncoders();
            for(j = 0; j < encoders.Length; ++j)
            {
                if(encoders[j].MimeType == mimeType)
                    return encoders[j];
            }
            return null;
        }
    }

希望这个帮助..

试试这个：

       public static class BitmapExtension
       {
          public static void Save(this Bitmap bitmap, String fileName, ImageFormat imageFormat, long quality = 75L)
          {
             using (var encoderParameters = new EncoderParameters(1))
             using (encoderParameters.Param[0] = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, quality))
             {
                ImageCodecInfo[] codecs = ImageCodecInfo.GetImageDecoders();
                bitmap.Save(fileName, codecs.Single(codec => codec.FormatID == imageFormat.Guid), encoderParameters);
             }
          }
       }
       ...
       Bitmap bitmap = new Bitmap("myImage.bmp");
       bitmap.Save("myImage.jpg", ImageFormat.Jpeg, 50L);