Transparent Blts in Windows NT (89375)

MORE INFORMATION

For this example, the source and target bitmaps contain 4 BPP. The call to the MaskBlt() function is as follows:

   MaskBlt(hdcTrg,  // handle of target DC
                0,  // x coord, upper-left corner of target rectangle
                0,  // y coord, upper-left corner of target rectangle
               15,  // width of source and target rectangles
               15,  // height of source and target rectangles
           hdcSrc,  // handle of source DC
                0,  // x coord, upper-left corner of source rectangle
                0,  // y coord, upper-left corner of source rectangle
          hbmMask,  // handle of monochrome bit-mask
                0,  // x coord, upper-left corner of mask rectangle
                0,  // y coord, upper-left corner of mask rectangle
       0xAACC0020   // raster-operation (ROP) code
                );

The legend is as follows

   '.' = 0,
   '@' = 1,
   '+' = 2,
   '*' = 3,
   '#' = 15

Source Bitmap      Mask Bitmap        Target Bitmap      Result
------------------------------------------------------------------------

***+++***+++***    .......@.......    ###############    #######*#######
***+++***+++***    ......@@@......    ###############    ######***######
***+++***+++***    .....@@@@@.....    ##...........##    ##...+***+...##
+++***+++***+++    ....@@@@@@@....    ##...........##    ##..**+++**..##
+++***+++***+++    ...@@@@@@@@@...    ##...........##    ##.***+++***.##
+++***+++***+++    ..@@@@@@@@@@@..    ##...........##    ##+***+++***+##
***+++***+++***    .@@@@@@@@@@@@@.    ##...........##    #**+++***+++**#
***+++***+++***    @@@@@@@@@@@@@@@    ##...........##    ***+++***+++***
***+++***+++***    .@@@@@@@@@@@@@.    ##...........##    #**+++***+++**#
+++***+++***+++    ..@@@@@@@@@@@..    ##...........##    ##+***+++***+##
+++***+++***+++    ...@@@@@@@@@...    ##...........##    ##.***+++***.##
+++***+++***+++    ....@@@@@@@....    ##...........##    ##..**+++**..##
***+++***+++***    .....@@@@@.....    ##...........##    ##...+***+...##
***+++***+++***    ......@@@......    ###############    ######***######
***+++***+++***    .......@.......    ###############    #######*#######
				

Note that the ROP "AA" is applied where 0 bits are in the mask and the ROP "CC" is applied where 1 bit is in the mask. This a transparency.

When creating a ROP4, you can use the following macro:

   #define ROP4(fore,back) ((((back) << 8) & 0xFF000000) | (fore))
				

This macro can be used to call the MaskBlt() function as follows:

   MaskBlt(hdcDest, xTrgt, yTrgt,
                          cx, cy,
              hdcSrc, xSrc, ySrc,
           hbmMask, xMask, yMask,
        ROP4(PATCOPY, NOTSRCCOPY)
                               );
				

This call would draw the selected brush where 1 bit appears in the mask and bitwise negation of the source bitmap where 0 bits appear in the mask.