C / C++ / MFC

fn=c:\test_00.bmp w=6400 h=4900
Canvas::SaveBmp bgn
got bitblt
got di bitmap
 got bm
 got bmi
 got size=94080000
 alloc'd n got bits
 writin file
 cleanup
Canvas::SaveBmp end

  HDC mCnv;
   if ((mCnv = ::CreateCompatibleDC     (_hdc))       == NULL)
      Die ("Canvas::SaveBmp  CreateCompatibleDC");
  HBITMAP mBmp, pBmp;
   if ((mBmp = ::CreateCompatibleBitmap (_hdc, w, h)) == NULL)
      Die ("Canvas::SaveBmp  CreateCompatibleBitmap");
   if ((pBmp = (HBITMAP) ::SelectObject (mCnv, mBmp)) == NULL)
      Die ("Canvas::SaveBmp  SelectObject");
   if (::BitBlt (mCnv, 0, 0, w, h, _hdc, 0, 0, SRCCOPY) == FALSE)
      Die ("Canvas::SaveBmp  BitBlt died");
// Get the actual bitmap struct
  BITMAP bm;
   ::GetObject (mBmp, sizeof (bm), (LPVOID) & bm);
DBG(" got bm");

// Initialize the bitmap infoheader
  BITMAPINFOHEADER bmi;
   MemSet (& bmi, 0,   sizeof (bmi));
   bmi.biSize        = sizeof (bmi);
   bmi.biWidth       = w;
   bmi.biHeight      = h;
   bmi.biPlanes      = 1;
   bmi.biBitCount    = bm.bmBitsPixel;
   bmi.biCompression = BI_RGB;

DBG(" got bmi");
// no bits param to get just the size
   if (! ::GetDIBits (mCnv, mBmp, 0, h, NULL, (BITMAPINFO *)(& bmi),
                      DIB_RGB_COLORS))
      Die ("Canvas::SaveBmp  GetDIBits died");
   if (bmi.biSizeImage == 0)
      Die ("Canvas::SaveBmp  GetDIBits - no size");

sprintf(dbg," got size=%d", bmi.biSizeImage);DBG(dbg);
  char *bits = new char [bmi.biSizeImage];
   if (! ::GetDIBits (mCnv, mBmp, 0, h, bits, (BITMAPINFO *)(& bmi),
                      DIB_RGB_COLORS))
      Die ("Canvas::SaveBmp  GetDIBits(2) died");

DBG(" alloc'd n got bits");
// write the BMP file
  BITMAPFILEHEADER bmf;
   MemSet (& bmf, 0, sizeof (bmf));
   bmf.bfType = ((WORD) 'B' | ('M' << 8));  // "BM"
   bmf.bfSize = sizeof (bmf) + sizeof (bmi) + bmi.biSizeImage;
   bmf.bfOffBits = sizeof (bmi);
  File f;
   if (f.Open (fn, "w")) {
DBG(" writin file");
      f.Put (& bmf, sizeof (bmf));
      f.Put (& bmi, sizeof (bmi));
      f.Put (bits,  bmi.biSizeImage);
      f.Shut ();
   }
// cleanup
DBG(" cleanup");
   delete [] bits;
   ::SelectObject (mCnv, pBmp);
   ::DeleteObject (      mBmp);
   ::DeleteDC     (mCnv);
DBG("Canvas::SaveBmp end");