Implementing Exif Features

The Exif file format supports more complex comments than other formats. For a list of the comments, refer to Exif File Comments.

This example demonstrates how to handle Exif comments that are not stored as strings. (For a simpler example for saving strings, refer to the LFileSettings::SetComment function.)

This example does the following:

void CTt2View::TestFunction(HWND hWnd, L_TCHAR * szFilename)
{
   LBitmapBase LeadBitmap;

   L_CHAR * pCommentToGet; /* Comment string that we will get */
   LBuffer CommentToGet; /* Handle for memory management */
   L_INT CommentLength; /* Length of the string */
   L_CHAR CommentToSet[80]; /* Comment string that we will set */
   L_CHAR * pCommentToSet = CommentToSet;
   L_CHAR szUserComment[80]; /* User comment that we will add */
   L_CHAR * pUserComment = szUserComment;

   L_UINT32 * pDistance; /* Pointer for the Exif RATIONAL */
   L_UINT32 Numerator, Denominator; /* Components of the Exif RATIONAL */
   float fDistance; /* Result of the distance calculation */
   L_TCHAR szMessage[256]; /* MessageBox string */ 

   L_TCHAR szMessage2[256]; /* MessageBox string */

   L_INT i; /* Loop counter */
   /* Load the bitmap */
   LeadBitmap.Load(szFilename);
   /* Load the current comments from the file */
   for(i = 0 ; i < CMNT_LAST ; i++ )
   {
      /* Clear the comment */
      LFileSettings::SetComment(i,NULL,0);
      /* Use the return value to get the length of a comment in the file */
      CommentLength=LeadBitmap.File()->ReadComment(i,0);
      if(CommentLength>0)
      {
         /* Allocate and lock a zero-filled buffer for the comment */
         CommentToGet.Reallocate(CommentLength);
         /* Get the comment from the file */
         LeadBitmap.File()->ReadComment(i,&CommentToGet);
         /* Set the comment to be saved */
         LFileSettings::SetComment(i,(L_UCHAR *)CommentToGet.Lock(),CommentLength);
         /* Unlock and free memory */
         CommentToGet.Unlock();
      }
   }
   

   /* Reverse the image */
   LeadBitmap.Reverse();
   /* Create the CMNT_USERCOMMENT comment as a byte array */
   LFileSettings::SetComment(CMNT_USERCOMMENT, NULL, 0);
   pUserComment ="The image has been reversed.";
   pCommentToSet[0] = 'A';
   pCommentToSet[1] = 'S';
   pCommentToSet[2] = 'C';
   pCommentToSet[3] = 'I';
   pCommentToSet[4] = 'I';
   pCommentToSet[5] = '\0';
   pCommentToSet[6] = '\0';
   pCommentToSet[7] = '\0';
   for( i = 0; pUserComment[i] != '\0'; i++)
   {
      pCommentToSet[i+8] = pUserComment[i];
   }
   pCommentToSet[i+8] = '\0';

   /* Set the CMNT_USERCOMMENT comment */
   LFileSettings::SetComment(CMNT_USERCOMMENT,(L_UCHAR *)pCommentToSet,
   strlen(pUserComment) + 9);
   /* Save the image as an Exif file */
   LeadBitmap.Save(FILE_EXIF,24,0, NULL, NULL);
   /* Initialize the message string */
   szMessage[0] = '\0';
   /* Use the return value to get the length of the CMNT_SUBJECTDISTANCE comment */
   /* This shows how to translate the Exif RATIONAL data type */
   CommentLength=LeadBitmap.File()->ReadComment(CMNT_SUBJECTDISTANCE,0);

   if(CommentLength>0)
   {
      /* Allocate and lock a zero-filled buffer for the comment */
      CommentToGet.Reallocate(CommentLength);
      /* Get the actual comment from the file */
      LeadBitmap.File()->ReadComment(CMNT_SUBJECTDISTANCE,&CommentToGet);
      pDistance = (L_UINT32 *)CommentToGet.Lock();
      Numerator = pDistance[0];
      Denominator = pDistance[1];
      fDistance = (float)Numerator / (float)Denominator;
      wsprintf (szMessage,TEXT("Subject distance: %.6f\n"),fDistance );

      CommentToGet.Unlock();
   }
   /* Use the return value to get the length of the CMNT_USERCOMMENT comment */
   CommentLength=LeadBitmap.File()->ReadComment(CMNT_USERCOMMENT,0);
   if (CommentLength > 0)
   {
      /* Allocate and lock a zero-filled buffer for the comment */
      CommentToGet.Reallocate(CommentLength);
      /* Get the actual comment from the file */
      LeadBitmap.File()->ReadComment(CMNT_USERCOMMENT,&CommentToGet);
      /* If the prefix is ASCII, move the pointer and update the message string */
      pCommentToGet=(L_CHAR *)CommentToGet.Lock();
      if (strcmp(pCommentToGet,"ASCII")== 0)
      {
         pCommentToGet = pCommentToGet + 8;
      }

               wsprintf(szMessage2,TEXT("%s%hs"),szMessage, pCommentToGet);
               /* Unlock and free memory */
               CommentToGet.Unlock();
            }
            /* Show the two comments */
            ::MessageBox (NULL,szMessage2, TEXT("Notice"), MB_OK);
            /* Clear the comments */
            for(i = 0 ; i < CMNT_LAST ; i++ )
               LFileSettings::SetComment(i,NULL,0);
            /* Force a repaint */
            ::SendMessage(hWnd,WM_QUERYNEWPALETTE,0,0L);
            return;
         }