PSS ID Number: Q79212
Article last modified on 10-20-1994

3.00

WINDOWS


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The information in this article applies to:

 - Microsoft Windows Software Development Kit for Windows version 3.0
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SUMMARY
=======

A portion of a graphic image that does not change the contents of the
screen is termed "transparent." The DrawIcon function can create an
image that contains transparent portions. It is also possible to
obtain this functionality using the BitBlt function; however, there
are some additional steps involved.

The first step is to obtain the contents of the area where the bitmap
will be drawn and to store this background image in a memory display
context (DC). Mask out the area of the background that corresponds to
the nontransparent portion of the image bitmap and mask out all
transparent pixels from the image bitmap. Use the XOR raster operation
to merge the image bitmap into the background bitmap. Finally, use the
BitBlt function to move the merged image to the destination DC.

MORE INFORMATION
================

The following nine steps describe a process used to draw transparent
bitmaps:

1. Create a DC to hold the image bitmap.

2. Select the image bitmap into the DC.

3. Create a memory DC to hold the final image. This is the destination
   DC.

4. Copy the portion of the screen that will be covered by the image
   into the destination DC.

5. Create an "AND mask" that contains the mask of the colors to draw
   (the nontransparent portions of the image). To do this, perform
   the following three steps:

   a. Set the background color of the image DC to the color that will
      be transparent in the image.

   b. Create a monochrome DC.

   c. BitBlt the image into the monochrome DC.

   This will create an AND mask of the bitmap by setting pixels that
   match the background color to white (1), and setting all other
   pixels to black (0).

6. Use BitBlt with the SRCAND raster operation code to copy the AND
   mask onto the destination DC.

7. Use BitBlt with the SRCAND raster operation code to copy the
   inverse of the AND mask onto the image DC.

8. Use BitBlt with the SRCPAINT raster operation code to copy the
   image DC onto the destination DC.

9. Use BitBlt to copy the contents of the destination DC to the
   appropriate portion of the screen.

The following code is a function that demonstrates the above steps:

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);
   }

The following is an example of how the DrawTransparentBitmap function
might be called:

   DrawTransparentBitmap(hdc,         // The destination DC.
                         hBitmap,     // The bitmap to be drawn.
                         xPos,        // X coordinate.
                         yPos,        // Y coordinate.
                         0x00FFFFFF); // The color for transparent
                                      // pixels (white, in this
                                      // example).

Additional reference words: 3.00
KBCategory: kbprg
KBSubcategory: GdiBmp

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Copyright Microsoft Corporation 1994.


 

