Lossless transform of a JPEG image

When you compress a JPEG image, some of the information in the image is lost. If you open a JPEG file, alter the image, and save it to another JPEG file, the quality will decrease. If you repeat that process many times, you will see a substantial degradation in the image quality.

Because JPEG is one of the most popular image formats on the Web, and because people often like to modify JPEG images, GDI+ provides the following transformations that can be performed on JPEG images without loss of information:

• Rotate 90 degrees
• Rotate 180 degrees
• Rotate 270 degrees
• Flip horizontally
• Flip vertically

You can apply one of the transformations shown in the preceding list when you call the Save method of an Image object. If the following conditions are met, then the transformation will proceed without loss of information:

• The file used to construct the Image object is a JPEG file.
• The width and height of the image are both multiples of 16.

If the width and height of the image are not both multiples of 16, GDI+ will do its best to preserve the image quality when you apply one of the rotation or flipping transformations shown in the preceding list.

To transform 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 that consists of one EncoderParameter object. Initialize that one EncoderParameter object so that its Value member points to a ULONG variable that holds one of the following elements of the EncoderValue enumeration:

• EncoderValueTransformRotate90,
• EncoderValueTransformRotate180,
• EncoderValueTransformRotate270,
• EncoderValueTransformFlipHorizontal,
• EncoderValueTransformFlipVertical

Set the Guid member of the EncoderParameter object to EncoderTransformation.

The following console application creates an Image object from a JPEG file and then saves the image to a new file. During the save process, the image is rotated 90 degrees. If the width and height of the image are both multiples of 16, the process of rotating and saving the image causes no loss of information.

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             transformation;
   UINT              width;
   UINT              height;
   Status            stat;

   // Get a JPEG image from the disk.
   Image* image = new Image(L"Shapes.jpg");

   // Determine whether the width and height of the image 
   // are multiples of 16.
   width = image->GetWidth();
   height = image->GetHeight();

   printf("The width of the image is %u", width);
   if(width / 16.0 - width / 16 == 0)
      printf(", which is a multiple of 16.\n");
   else
      printf(", which is not a multiple of 16.\n");

   printf("The height of the image is %u", height);
   if(height / 16.0 - height / 16 == 0)
      printf(", which is a multiple of 16.\n");
   else
      printf(", which is not a multiple of 16.\n");

   // 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 set that value to EncoderValueTransformRotate90.

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

   // Rotate and save the image.
   transformation = EncoderValueTransformRotate90;
   encoderParameters.Parameter[0].Value = &transformation;
   stat = image->Save(L"ShapesR90.jpg", &encoderClsid, &encoderParameters);

   if(stat == Ok)
      wprintf(L"%s saved successfully.\n", L"ShapesR90.jpg");
   else
      wprintf(L"%d  Attempt to save %s failed.\n", stat, L"ShapesR90.jpg");

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