Draw Alpha Blend Bitmap

const

 AC_SRC_ALPHA = $1;

procedure DrawAlphaBlend (Bitmap:TBitmap; hdcwnd : HDC; Alpha: Byte);

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

    canvas: TCanvas;

begin

    // calculate window width/height

    ulWindowWidth := Bitmap.Width;

    ulWindowHeight := Bitmap.Height;

    // 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 := ulWindowWidth;

    ulBitmapWidth := ulWindowWidth;

    bmi.bmiHeader.biHeight := ulWindowHeight;

    ulBitmapHeight := ulWindowHeight;

    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);

    canvas := TCanvas.Create;

    canvas.Handle := Ahdc;

    canvas.Draw(0,0,Bitmap);

    bf.BlendOp := AC_SRC_OVER;

    bf.BlendFlags := 0;

    bf.SourceConstantAlpha := Alpha;  // half of 0xff = 50% transparency

    bf.AlphaFormat := 0;             // ignore source alpha channel

    AlphaBlend(hdcwnd, 0, 0,

                    ulBitmapWidth, ulBitmapHeight,

                    Ahdc, 0, 0, ulBitmapWidth, ulBitmapHeight, bf);

    // do cleanup

    DeleteObject(Ahbitmap);

    DeleteDC(Ahdc);

end;