PRB: MetaFile Displays Incorrectly in Print Preview

• The Microsoft Foundation Classes (MFC) included with:

      - Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51 1.52
      - Microsoft Visual C++, 32-bit Edition, versions 1.0, 2.0, 2.1, 2.2,
        4.0
  

SYMPTOMS

When displaying a metafile within the Microsoft Foundation Classes (MFC) Print and Print Preview architecture, the metafile prints correctly but is only partially or incorrectly displayed in the Print Preview window.

CAUSE

To simulate a printed page in a window, CPreviewDC modifies the mapping mode for the Print Preview window by changing the parameters for such calls as SetWindowExt() and SetMapMode() before passing them on to the attribute and output device contexts (DCs).

However, CPreviewDC does not override CDC::PlayMetaFile(), which maps to the Windows application programming interface (API) ::PlayMetaFile(). If the metafile contains records for functions that affect the mapping mode, the CPreviewDC versions of these functions will not be called while the metafile is playing, causing the mapping mode in the Print Preview window to be set incorrectly (the attribute and output DCs lose synchronization).

RESOLUTION

Rather than PlayMetaFile(), use SaveDC(), ::EnumMetaFile(), and RestoreDC() to play the metafile. In the metafile enumeration callback procedure, trap the following functions and call the equivalent CDC functions:

   SetMapMode
   SetWindowExt, ScaleWindowExt
   SetViewportExt, ScaleViewportExt
   SetViewportOrg, OffsetViewportOrg

Bracket the call to ::EnumMetaFile() between calls to SaveDC() and RestoreDC() to ensure that the output and attribute DCs are correctly synchronized after playing the metafile.

MORE INFORMATION

The enumeration callback procedure should handle the above functions by calling the (virtual) CDC version of the function while passing everything else to PlayMetaFileRecord(). This allows CPreviewDC to keep the output DC and the attribute DC correctly synchronized with respect to mapping mode. For example:

Sample Code

   int CALLBACK __export EnumMFProc(HDC hdc,
                                    HANDLETABLE FAR *lpht,
                                    METARECORD  FAR *lpmr,
                                    int cObj,
                                    LPARAM lParam)
   {
     // lParam is available for passing application-specific data to the
     // enum proc (via the last parameter to EnumMetaFile). Let's make
     // things easier by passing in a pointer to the CDC object.
   
     CDC *pDC = (CDC *)lParam;
   
     switch(lpmr->rdFunction)
       {
       // SetWindowExt and SetMapMode are the most important ones to
       // look for.
   
       case META_SETWINDOWEXT:
           pDC->SetWindowExt(lpmr->rdParm[1], lpmr->rdParm[0]);
           break;
   
       case META_SETMAPMODE:
           pDC->SetMapMode(lpmr->rdParm[0]);
           break;
   
       case META_SETVIEWPORTEXT:
           pDC->SetViewportExt(lpmr->rdParm[1], lpmr->rdParm[0]);
           break;
   
       case META_SETVIEWPORTORG:
           pDC->SetViewportOrg(lpmr->rdParm[1], lpmr->rdParm[0]);
           break;
   
       // Add cases for the others if desired...
   
       default:
           PlayMetaFileRecord(hdc, lpht, lpmr, cObj);
       }
     return 1;
   }

Also, it is common for applications to set a different mapping mode before playing a metafile (depending on the metafile) and this mapping mode may conflict with the mapping mode used by the rest of the view class.

For these reasons, if any drawing will take place after playing the metafile (including playing another metafile, or even the same metafile again), the DC must be restored to the state it was in prior to playing the metafile. This can be accomplished by using SaveDC/RestoreDC, as in the following example: