TransparentBlt

[JohnF]

According to the SDK TransparentBlt() is not in Win95 and the Win98 implementation has a memory leak. Here is the code for DrawTransparentBitmap() supplied by Microsoft.

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void DrawTransparentBitmap(HDC hdc, HBITMAP hBitmap, short xStart,
                          short yStart, COLORREF cTransparentColor)
{
  BITMAP     bm;
  COLORREF   cColor;
  HBITMAP    bmAndBack, bmAndObject, bmAndMem, bmSave;
  HBITMAP    bmBackOld, bmObjectOld, bmMemOld, bmSaveOld;
  HDC        hdcMem, hdcBack, hdcObject, hdcTemp, hdcSave;
  POINT      ptSize;

  hdcTemp = CreateCompatibleDC(hdc);
  SelectObject(hdcTemp, hBitmap);   // Select the bitmap

  GetObject(hBitmap, sizeof(BITMAP), (LPSTR)&bm);
  ptSize.x = bm.bmWidth;            // Get width of bitmap
  ptSize.y = bm.bmHeight;           // Get height of bitmap
  DPtoLP(hdcTemp, &ptSize, 1);      // Convert from device
                                    // to logical points

  // Create some DCs to hold temporary data.
  hdcBack   = CreateCompatibleDC(hdc);
  hdcObject = CreateCompatibleDC(hdc);
  hdcMem    = CreateCompatibleDC(hdc);
  hdcSave   = CreateCompatibleDC(hdc);

  // Create a bitmap for each DC. DCs are required for a number of
  // GDI functions.

  // Monochrome DC
  bmAndBack   = CreateBitmap(ptSize.x, ptSize.y, 1, 1, NULL);

  // Monochrome DC
  bmAndObject = CreateBitmap(ptSize.x, ptSize.y, 1, 1, NULL);

  bmAndMem    = CreateCompatibleBitmap(hdc, ptSize.x, ptSize.y);
  bmSave      = CreateCompatibleBitmap(hdc, ptSize.x, ptSize.y);

  // Each DC must select a bitmap object to store pixel data.
  bmBackOld   = SelectObject(hdcBack, bmAndBack);
  bmObjectOld = SelectObject(hdcObject, bmAndObject);
  bmMemOld    = SelectObject(hdcMem, bmAndMem);
  bmSaveOld   = SelectObject(hdcSave, bmSave);

  // Set proper mapping mode.
  SetMapMode(hdcTemp, GetMapMode(hdc));

  // Save the bitmap sent here, because it will be overwritten.
  BitBlt(hdcSave, 0, 0, ptSize.x, ptSize.y, hdcTemp, 0, 0, SRCCOPY);

  // Set the background color of the source DC to the color.
  // contained in the parts of the bitmap that should be transparent
  cColor = SetBkColor(hdcTemp, cTransparentColor);

  // Create the object mask for the bitmap by performing a BitBlt
  // from the source bitmap to a monochrome bitmap.
  BitBlt(hdcObject, 0, 0, ptSize.x, ptSize.y, hdcTemp, 0, 0, SRCCOPY);

  // Set the background color of the source DC back to the original
  // color.
  SetBkColor(hdcTemp, cColor);

  // Create the inverse of the object mask.
  BitBlt(hdcBack, 0, 0, ptSize.x, ptSize.y, hdcObject, 0, 0, NOTSRCCOPY);

  // Copy the background of the main DC to the destination.
  BitBlt(hdcMem, 0, 0, ptSize.x, ptSize.y, hdc, xStart, yStart, SRCCOPY);

  // Mask out the places where the bitmap will be placed.
  BitBlt(hdcMem, 0, 0, ptSize.x, ptSize.y, hdcObject, 0, 0, SRCAND);

  // Mask out the transparent colored pixels on the bitmap.
  BitBlt(hdcTemp, 0, 0, ptSize.x, ptSize.y, hdcBack, 0, 0, SRCAND);

  // XOR the bitmap with the background on the destination DC.
  BitBlt(hdcMem, 0, 0, ptSize.x, ptSize.y, hdcTemp, 0, 0, SRCPAINT);

  // Copy the destination to the screen.
  BitBlt(hdc, xStart, yStart, ptSize.x, ptSize.y, hdcMem, 0, 0, SRCCOPY);

  // Place the original bitmap back into the bitmap sent here.
  BitBlt(hdcTemp, 0, 0, ptSize.x, ptSize.y, hdcSave, 0, 0, SRCCOPY);

  // Delete the memory bitmaps.
  DeleteObject(SelectObject(hdcBack, bmBackOld));
  DeleteObject(SelectObject(hdcObject, bmObjectOld));
  DeleteObject(SelectObject(hdcMem, bmMemOld));
  DeleteObject(SelectObject(hdcSave, bmSaveOld));

  // Delete the memory DCs.
  DeleteDC(hdcMem);
  DeleteDC(hdcBack);
  DeleteDC(hdcObject);
  DeleteDC(hdcSave);
  DeleteDC(hdcTemp);
}


John