Viewing PCX files

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This article was contributed by Pierre Fournier .

For my project, I needed to load PCX files and couldn’t find the source code to do it.
  I took a documentation about the file format of PCX files and decided to give it a
shot.  The effort was worth it!  If you would like to know how to load and
display single plane PCX files (like I did a couple of weeks ago), then this one’s for
you.

In my demo, the actual function that reads and decompresses the PCX file is located in
my derived CMDIChildWnd class.  1 function and 3 data members have been added:

public:
	void LoadPCX( LPCSTR lpcszFilename );

protected:
	CSize BitmapSize;
	BITMAPINFO *psBmpInfo;
	BYTE *pabRawBitmap;

The function that reads the PCX file goes like this:

void CChildFrame::LoadPCX( LPCSTR lpcszFilename )
{
   // Standard PCX header
   struct PCXHead {
     char   ID;
     char   Version;
     char   Encoding;
     char   BitPerPixel;
     short  X1;
     short  Y1;
     short  X2;
     short  Y2;
     short  HRes;
     short  VRes;
     char   ClrMap[16*3];
     char   Reserved1;
     char   NumPlanes;
     short  BPL;
     short  Pal_t;
     char   Filler[58];
   } sHeader;

   // Open the file and put its entire content in memory
   FILE *pFile = fopen( lpcszFilename, "rb" );
   if ( !pFile )
   {
      MessageBox("Unable to open the PCX file");
      return;
   }
   const long clFileSize = _filelength(_fileno(pFile));
   BYTE *pabFileData = (BYTE *)new BYTE[ clFileSize ];
   fread( pabFileData, clFileSize, 1, pFile );
   fclose( pFile );

   // Get the header
   memcpy( &sHeader, pabFileData, sizeof(sHeader) );

   // Each scan line MUST have a size that can be divided by a 'long' data type
   int iScanLineSize = sHeader.NumPlanes * sHeader.BPL;
   ldiv_t sDivResult = ldiv( iScanLineSize, sizeof(long) );
   if ( sDivResult.rem > 0 )
      iScanLineSize = (iScanLineSize/sizeof(long)+1) * sizeof(long);

   // Set the bitmap size data member
   BitmapSize = CSize( sHeader.X2-sHeader.X1+1, sHeader.Y2-sHeader.Y1+1 );
   const long clImageSize = iScanLineSize * BitmapSize.cy;

   // Set the bitmap information
   psBmpInfo = (BITMAPINFO *)new BYTE[ sizeof(BITMAPINFOHEADER) +
                                             (sizeof(RGBQUAD)*256) ];
   psBmpInfo->bmiHeader.biSize           = sizeof(BITMAPINFOHEADER);
   psBmpInfo->bmiHeader.biWidth          = BitmapSize.cx;
   psBmpInfo->bmiHeader.biHeight         = -BitmapSize.cy;
   psBmpInfo->bmiHeader.biPlanes         = sHeader.NumPlanes;
   psBmpInfo->bmiHeader.biBitCount       = sHeader.BitPerPixel;
   psBmpInfo->bmiHeader.biCompression    = BI_RGB;
   psBmpInfo->bmiHeader.biSizeImage      = 0;
   psBmpInfo->bmiHeader.biXPelsPerMeter  = 0;
   psBmpInfo->bmiHeader.biYPelsPerMeter  = 0;
   psBmpInfo->bmiHeader.biClrUsed        = 0;
   psBmpInfo->bmiHeader.biClrImportant   = 0;

   // Prepare a buffer large enough to hold the image
   pabRawBitmap = (BYTE *)new BYTE[ clImageSize ];
   if ( !pabRawBitmap )
   {
      MessageBox( "Can't allocate memory for the image" );
      delete [] pabFileData;
      return;
   }

   // Get the compressed image
   long lDataPos = 0;
   long lPos = 128;     // That's where the data begins

   for ( int iY=0; iY < BitmapSize.cy; iY++ )
   {
      // Decompress the scan line
      for ( int iX=0; iX < sHeader.BPL; )
      {
         UINT uiValue = pabFileData[lPos++];
         if ( uiValue > 192 ) {  // Two high bits are set = Repeat
            uiValue -= 192;                  // Repeat how many times?
            BYTE Color = pabFileData[lPos++];  // What color?

            if ( iX <= BitmapSize.cx )
            {  // Image data.  Place in the raw bitmap.
               for ( BYTE bRepeat=0; bRepeat < uiValue; bRepeat++ )
               {
                  pabRawBitmap[lDataPos++] = Color;
                  iX++;
               }
            }
            else
               iX += uiValue; // Outside the image.  Skip.
         }
         else
         {
            if ( iX <= BitmapSize.cx )
               pabRawBitmap[lDataPos++] = uiValue;
            iX++;
         }
      }

      // Pad the rest with zeros
      if ( iX < iScanLineSize )
      {
         for ( ;iX < iScanLineSize; iX++ )
            pabRawBitmap[lDataPos++] = 0;
      }
   }

   if ( pabFileData[lPos++] == 12 )          // Simple validation
      // Get the palette
      for ( short Entry=0; Entry < 256; Entry++ )
      {
         psBmpInfo->bmiColors[Entry].rgbRed       = pabFileData[lPos++];
         psBmpInfo->bmiColors[Entry].rgbGreen     = pabFileData[lPos++];
         psBmpInfo->bmiColors[Entry].rgbBlue      = pabFileData[lPos++];
         psBmpInfo->bmiColors[Entry].rgbReserved  = 0;
      }

   delete [] pabFileData;

   // Resize/Repaint the window
   const CSize BorderSize( GetSystemMetrics( SM_CXEDGE ) +
                           GetSystemMetrics( SM_CXFRAME ) - 1,
                           GetSystemMetrics( SM_CYEDGE ) +
                           GetSystemMetrics( SM_CYFRAME ) - 1 );
   SetWindowPos( NULL, 0, 0, BitmapSize.cx + (BorderSize.cx*2),
                 BitmapSize.cy + (BorderSize.cy*2)+GetSystemMetrics(SM_CYCAPTION),
                 SWP_NOMOVE | SWP_NOZORDER );
}

Now that we have the PCX loaded in memory, it is ready to be displayed.

BOOL CChildFrame::OnEraseBkgnd(CDC* pDC)
{
   if ( pabRawBitmap && psBmpInfo )
      // Display the image
      SetDIBitsToDevice( *pDC, 0, 0,
                         BitmapSize.cx, BitmapSize.cy, 0, 0,
                         0, BitmapSize.cy,
                         pabRawBitmap, psBmpInfo, DIB_RGB_COLORS);

   return CMDIChildWnd::OnEraseBkgnd(pDC);;
}

Downloads

Download source (MSVC++ 6.0) – 33 KB
Download demo project – 10 KB