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.
1 procedure DrawTransparentBitmap(DC: HDC; hBmp : HBITMAP ;
2 xStart: integer; yStart : integer; cTransparentColor : COLORREF);
3 var
4 bm: BITMAP;
5 cColor: COLORREF;
6 bmAndBack, bmAndObject, bmAndMem, bmSave: HBITMAP;
7 bmBackOld, bmObjectOld, bmMemOld, bmSaveOld: HBITMAP;
8 hdcMem, hdcBack, hdcObject, hdcTemp, hdcSave: HDC;
9 ptSize: TPOINT;
10
11 begin
12 hdcTemp := CreateCompatibleDC(dc);
13 SelectObject(hdcTemp, hBmp); // Select the bitmap
14
15 GetObject(hBmp, sizeof(BITMAP), @bm);
16 ptSize.x := bm.bmWidth; // Get width of bitmap
17 ptSize.y := bm.bmHeight; // Get height of bitmap
18 DPtoLP(hdcTemp, ptSize, 1); // Convert from device
19 // to logical points
20
21 // Create some DCs to hold temporary data.
22 hdcBack := CreateCompatibleDC(dc);
23 hdcObject := CreateCompatibleDC(dc);
24 hdcMem := CreateCompatibleDC(dc);
25 hdcSave := CreateCompatibleDC(dc);
26
27 // Create a bitmap for each DC. DCs are required for a number of
28 // GDI functions.
29
30 // Monochrome DC
31 bmAndBack := CreateBitmap(ptSize.x, ptSize.y, 1, 1, nil);
32
33 // Monochrome DC
34 bmAndObject := CreateBitmap(ptSize.x, ptSize.y, 1, 1, nil);
35
36 bmAndMem := CreateCompatibleBitmap(dc, ptSize.x, ptSize.y);
37 bmSave := CreateCompatibleBitmap(dc, ptSize.x, ptSize.y);
38
39 // Each DC must select a bitmap object to store pixel data.
40 bmBackOld := SelectObject(hdcBack, bmAndBack);
41 bmObjectOld := SelectObject(hdcObject, bmAndObject);
42 bmMemOld := SelectObject(hdcMem, bmAndMem);
43 bmSaveOld := SelectObject(hdcSave, bmSave);
44
45 // Set proper mapping mode.
46 SetMapMode(hdcTemp, GetMapMode(dc));
47
48 // Save the bitmap sent here, because it will be overwritten.
49 BitBlt(hdcSave, 0, 0, ptSize.x, ptSize.y, hdcTemp, 0, 0, SRCCOPY);
50
51 // Set the background color of the source DC to the color.
52 // contained in the parts of the bitmap that should be transparent
53 cColor := SetBkColor(hdcTemp, cTransparentColor);
54
55 // Create the object mask for the bitmap by performing a BitBlt
56 // from the source bitmap to a monochrome bitmap.
57 BitBlt(hdcObject, 0, 0, ptSize.x, ptSize.y, hdcTemp, 0, 0,
58 SRCCOPY);
59
60 // Set the background color of the source DC back to the original
61 // color.
62 SetBkColor(hdcTemp, cColor);
63
64 // Create the inverse of the object mask.
65 BitBlt(hdcBack, 0, 0, ptSize.x, ptSize.y, hdcObject, 0, 0,
66 NOTSRCCOPY);
67
68 // Copy the background of the main DC to the destination.
69 BitBlt(hdcMem, 0, 0, ptSize.x, ptSize.y, dc, xStart, yStart,
70 SRCCOPY);
71
72 // Mask out the places where the bitmap will be placed.
73 BitBlt(hdcMem, 0, 0, ptSize.x, ptSize.y, hdcObject, 0, 0, SRCAND);
74
75 // Mask out the transparent colored pixels on the bitmap.
76 BitBlt(hdcTemp, 0, 0, ptSize.x, ptSize.y, hdcBack, 0, 0, SRCAND);
77
78 // XOR the bitmap with the background on the destination DC.
79 BitBlt(hdcMem, 0, 0, ptSize.x, ptSize.y, hdcTemp, 0, 0, SRCPAINT);
80
81 // Copy the destination to the screen.
82 BitBlt(dc, xStart, yStart, ptSize.x, ptSize.y, hdcMem, 0, 0,
83 SRCCOPY);
84
85 // Place the original bitmap back into the bitmap sent here.
86 BitBlt(hdcTemp, 0, 0, ptSize.x, ptSize.y, hdcSave, 0, 0, SRCCOPY);
87
88 // Delete the memory bitmaps.
89 DeleteObject(SelectObject(hdcBack, bmBackOld));
90 DeleteObject(SelectObject(hdcObject, bmObjectOld));
91 DeleteObject(SelectObject(hdcMem, bmMemOld));
92 DeleteObject(SelectObject(hdcSave, bmSaveOld));
93
94 // Delete the memory DCs.
95 DeleteDC(hdcMem);
96 DeleteDC(hdcBack);
97 DeleteDC(hdcObject);
98 DeleteDC(hdcSave);
99 DeleteDC(hdcTemp);
100 end;
The following is an example of how the DrawTransparentBitmap function might be
called:
101 procedure TForm1.Button1Click(Sender: TObject);
102 begin
103 DrawTransparentBitmap(Form1.Canvas.Handle, Image1.Picture.Bitmap.Handle, 10, 10,
104 clWhite);
105 end;
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