设置 JPEG 压缩级别

若要在保存 JPEG 图像时指定压缩级别，请初始化 EncoderParameters 对象，并将该对象的地址传递给 Image 类的 Save 方法。 初始化 EncoderParameters 对象，使其具有由一个 EncoderParameter 对象组成的数组。 初始化一个 EncoderParameter 对象，使其 Value 成员指向 0 到 100 之间的 ULONG 值。 将 EncoderParameter 对象的 Guid 成员设置为 EncoderQuality。

以下控制台应用程序保存三个 JPEG 映像，每个映像的质量级别不同。 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。

main函数依赖于帮助程序函数 GetEncoderClsid，该函数显示在检索编码器的类标识符中：

#include <windows.h>
#include <gdiplus.h>
#include <stdio.h>
using namespace Gdiplus;

INT GetEncoderClsid(const WCHAR* format, CLSID* pClsid);  // helper function

INT main()
{
   // Initialize GDI+.
   GdiplusStartupInput gdiplusStartupInput;
   ULONG_PTR gdiplusToken;
   GdiplusStartup(&gdiplusToken, &gdiplusStartupInput, NULL);

   CLSID             encoderClsid;
   EncoderParameters encoderParameters;
   ULONG             quality;
   Status            stat;

   // Get an image from the disk.
   Image* image = new Image(L"Shapes.bmp");

   // Get the CLSID of the JPEG encoder.
   GetEncoderClsid(L"image/jpeg", &encoderClsid);

   // Before we call Image::Save, we must initialize an
   // EncoderParameters object. The EncoderParameters object
   // has an array of EncoderParameter objects. In this
   // case, there is only one EncoderParameter object in the array.
   // The one EncoderParameter object has an array of values.
   // In this case, there is only one value (of type ULONG)
   // in the array. We will let this value vary from 0 to 100.

   encoderParameters.Count = 1;
   encoderParameters.Parameter[0].Guid = EncoderQuality;
   encoderParameters.Parameter[0].Type = EncoderParameterValueTypeLong;
   encoderParameters.Parameter[0].NumberOfValues = 1;

   // Save the image as a JPEG with quality level 0.
   quality = 0;
   encoderParameters.Parameter[0].Value = &quality;
   stat = image->Save(L"Shapes001.jpg", &encoderClsid, &encoderParameters);

   if(stat == Ok)
      wprintf(L"%s saved successfully.\n", L"Shapes001.jpg");
   else
      wprintf(L"%d  Attempt to save %s failed.\n", stat, L"Shapes001.jpg");

   // Save the image as a JPEG with quality level 50.
   quality = 50;
   encoderParameters.Parameter[0].Value = &quality;
   stat = image->Save(L"Shapes050.jpg", &encoderClsid, &encoderParameters);

   if(stat == Ok)
      wprintf(L"%s saved successfully.\n", L"Shapes050.jpg");
   else
      wprintf(L"%d  Attempt to save %s failed.\n", stat, L"Shapes050.jpg");

      // Save the image as a JPEG with quality level 100.
   quality = 100;
   encoderParameters.Parameter[0].Value = &quality;
   stat = image->Save(L"Shapes100.jpg", &encoderClsid, &encoderParameters);

   if(stat == Ok)
      wprintf(L"%s saved successfully.\n", L"Shapes100.jpg");
   else
      wprintf(L"%d  Attempt to save %s failed.\n", stat, L"Shapes100.jpg");

   delete image;
   GdiplusShutdown(gdiplusToken);
   return 0;
}