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Setting JPEG Compression Level

To specify the compression level when you save a JPEG image, initialize an EncoderParameters object and pass the address of that object to the Save method of the Image class. Initialize the EncoderParameters object so that it has an array consisting of one EncoderParameter object. Initialize that one EncoderParameter object so that its Value member points to a ULONG value from 0 through 100. Set the Guid member of the EncoderParameter object to EncoderQuality.

The following console application saves three JPEG images, each with a different quality level. A quality level of 0 corresponds to the greatest compression, and a quality level of 100 corresponds to the least compression.

The main function relies on the helper function GetEncoderClsid, which is shown in Retrieving the Class Identifier for an Encoder:


#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;
}

 

 

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Build date: 3/6/2012

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