Using AlphaBlend function in Delphi

Alpha blending is used to display an alpha bitmap, which is a bitmap that has transparent or semi-transparent pixels.
In addition to a red, green, and blue color channel, each pixel in an alpha bitmap has a transparency component known as its alpha channel. The alpha channel typically contains as many bits as a color channel. For example, an 8-bit alpha channel can represent 256 levels of transparency, from 0 (the entire bitmap is transparent) to 255 (the entire bitmap is opaque). Alpha blending mechanisms are invoked by calling AlphaBlend function. The AlphaBlend function displays bitmaps that have transparent or semitransparent pixels. It is not supported on Microsoft Windows 95 or on versions of Microsoft Windows NT prior to Microsoft Windows 2000.

The following code sample divides a window into three horizontal areas. Then it draws an alpha-blended bitmap in each of the window areas.

const
AC_SRC_ALPHA = $1;

procedure DrawAlphaBlend (hWnd : HWND;  hdcwnd : HDC);
var
    Ahdc : HDC;              // handle of the DC we will create
    bf : BLENDFUNCTION;      // structure for alpha blending
    Ahbitmap : HBITMAP;      // bitmap handle
    bmi : BITMAPINFO;        // bitmap header
    pvBits : pointer;        // pointer to DIB section
    ulWindowWidth,
    ulWindowHeight : ULONG;  // window width/height
    ulBitmapWidth,
    ulBitmapHeight : ULONG; // bitmap width/height
    rt : TRect;             // used for getting window dimensions
begin
    // get window dimensions
    GetClientRect(hWnd, rt);

    // calculate window width/height
    ulWindowWidth := rt.right - rt.left;
    ulWindowHeight := rt.bottom - rt.top;

    // make sure we have at least some window size
    if ((ulWindowWidth = 0 ) and  (ulWindowHeight=0)) then
        exit;

    // divide the window into 3 horizontal areas
    ulWindowHeight := trunc(ulWindowHeight / 3);

    // create a DC for our bitmap -- the source DC for AlphaBlend
    Ahdc := CreateCompatibleDC(hdcwnd);

    // zero the memory for the bitmap info
    ZeroMemory(@bmi, sizeof(BITMAPINFO));

    // setup bitmap info
    bmi.bmiHeader.biSize := sizeof(BITMAPINFOHEADER);
    bmi.bmiHeader.biWidth := trunc(ulWindowWidth - (ulWindowWidth/5)*2);
    ulBitmapWidth := trunc(ulWindowWidth - (ulWindowWidth/5)*2);
    bmi.bmiHeader.biHeight := trunc(ulWindowHeight - (ulWindowHeight/5)*2);
    ulBitmapHeight := trunc(ulWindowHeight - (ulWindowHeight/5)*2);
    bmi.bmiHeader.biPlanes := 1;
    bmi.bmiHeader.biBitCount := 32;         // four 8-bit components
    bmi.bmiHeader.biCompression := BI_RGB;
    bmi.bmiHeader.biSizeImage := ulBitmapWidth * ulBitmapHeight * 4;

    // create our DIB section and select the bitmap into the dc
    Ahbitmap := CreateDIBSection(Ahdc, bmi, DIB_RGB_COLORS, pvBits, 0, 0);
    SelectObject(Ahdc, Ahbitmap);

    bf.BlendOp := AC_SRC_OVER;
    bf.BlendFlags := 0;
    bf.SourceConstantAlpha := $7f;  // half of 0xff = 50% transparency
    bf.AlphaFormat := 0;             // ignore source alpha channel

    AlphaBlend(hdcwnd, trunc(ulWindowWidth/5), trunc(ulWindowHeight/5),
                    ulBitmapWidth, ulBitmapHeight,
                    Ahdc, 0, 0, ulBitmapWidth, ulBitmapHeight, bf);


    bf.BlendOp := AC_SRC_OVER;
    bf.BlendFlags := 0;
    bf.AlphaFormat := AC_SRC_ALPHA;  // use source alpha
    bf.SourceConstantAlpha := $ff;  // opaque (disable constant alpha)

    AlphaBlend(hdcwnd, trunc(ulWindowWidth/5),
     trunc(ulWindowHeight/5+ulWindowHeight), ulBitmapWidth, ulBitmapHeight,
      Ahdc, 0, 0, ulBitmapWidth, ulBitmapHeight, bf);

    bf.BlendOp := AC_SRC_OVER;
    bf.BlendFlags := 0;
    bf.AlphaFormat := 0;
    bf.SourceConstantAlpha := $3A;

    AlphaBlend(hdcwnd, trunc(ulWindowWidth/5),
               trunc(ulWindowHeight/5+2*ulWindowHeight), ulBitmapWidth,
               ulBitmapHeight, Ahdc, 0, 0, ulBitmapWidth,
               ulBitmapHeight, bf);

    // do cleanup
    DeleteObject(Ahbitmap);
    DeleteDC(Ahdc);

end;