Combinação alfa de um bitmap

O exemplo de código a seguir divide uma janela em três áreas horizontais. Em seguida, ele desenha um bitmap combinado alfa em cada uma das áreas da janela da seguinte maneira:

• Na área superior, alfa constante = 50%, mas não há alfa de origem.
• Na área intermediária, alfa constante = 100% (desabilitado) e alfa de origem é 0 (transparente) no meio do bitmap e 0xff (opaco) em outro lugar.
• Na área inferior, alfa constante = 75% e alterações alfa de origem.

void DrawAlphaBlend (HWND hWnd, HDC hdcwnd)
{
    HDC hdc;               // handle of the DC we will create  
    BLENDFUNCTION bf;      // structure for alpha blending 
    HBITMAP hbitmap;       // bitmap handle 
    BITMAPINFO bmi;        // bitmap header 
    VOID *pvBits;          // pointer to DIB section 
    ULONG   ulWindowWidth, ulWindowHeight;      // window width/height 
    ULONG   ulBitmapWidth, ulBitmapHeight;      // bitmap width/height 
    RECT    rt;            // used for getting window dimensions 
    UINT32   x,y;          // stepping variables 
    UCHAR ubAlpha;         // used for doing transparent gradient 
    UCHAR ubRed;        
    UCHAR ubGreen;
    UCHAR ubBlue;
    float fAlphaFactor;    // used to do premultiply 
            
    // 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) || (!ulWindowHeight))
        return;

    // divide the window into 3 horizontal areas 
    ulWindowHeight = ulWindowHeight / 3;

    // create a DC for our bitmap -- the source DC for AlphaBlend  
    hdc = CreateCompatibleDC(hdcwnd);
    
    // zero the memory for the bitmap info 
    ZeroMemory(&bmi, sizeof(BITMAPINFO));

    // setup bitmap info  
    // set the bitmap width and height to 60% of the width and height of each of the three horizontal areas. Later on, the blending will occur in the center of each of the three areas. 
    bmi.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
    bmi.bmiHeader.biWidth = ulBitmapWidth = ulWindowWidth - (ulWindowWidth/5)*2;
    bmi.bmiHeader.biHeight = ulBitmapHeight = 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 
    hbitmap = CreateDIBSection(hdc, &bmi, DIB_RGB_COLORS, &pvBits, NULL, 0x0);
    SelectObject(hdc, hbitmap);

    // in top window area, constant alpha = 50%, but no source alpha 
    // the color format for each pixel is 0xaarrggbb  
    // set all pixels to blue and set source alpha to zero 
    for (y = 0; y < ulBitmapHeight; y++)
        for (x = 0; x < ulBitmapWidth; x++)
            ((UINT32 *)pvBits)[x + y * ulBitmapWidth] = 0x000000ff; 

    bf.BlendOp = AC_SRC_OVER;
    bf.BlendFlags = 0;
    bf.SourceConstantAlpha = 0x7f;  // half of 0xff = 50% transparency 
    bf.AlphaFormat = 0;             // ignore source alpha channel 

    if (!AlphaBlend(hdcwnd, ulWindowWidth/5, ulWindowHeight/5, 
                    ulBitmapWidth, ulBitmapHeight, 
                    hdc, 0, 0, ulBitmapWidth, ulBitmapHeight, bf))
        return;                     // alpha blend failed 
    
    // in middle window area, constant alpha = 100% (disabled), source  
    // alpha is 0 in middle of bitmap and opaque in rest of bitmap  
    for (y = 0; y < ulBitmapHeight; y++)
        for (x = 0; x < ulBitmapWidth; x++)
            if ((x > (int)(ulBitmapWidth/5)) && (x < (ulBitmapWidth-ulBitmapWidth/5)) &&
                (y > (int)(ulBitmapHeight/5)) && (y < (ulBitmapHeight-ulBitmapHeight/5)))
                //in middle of bitmap: source alpha = 0 (transparent). 
                // This means multiply each color component by 0x00. 
                // Thus, after AlphaBlend, we have a, 0x00 * r,  
                // 0x00 * g,and 0x00 * b (which is 0x00000000) 
                // for now, set all pixels to red 
                ((UINT32 *)pvBits)[x + y * ulBitmapWidth] = 0x00ff0000; 
            else
                // in the rest of bitmap, source alpha = 0xff (opaque)  
                // and set all pixels to blue  
                ((UINT32 *)pvBits)[x + y * ulBitmapWidth] = 0xff0000ff; 
            endif;
    
    bf.BlendOp = AC_SRC_OVER;
    bf.BlendFlags = 0;
    bf.AlphaFormat = AC_SRC_ALPHA;  // use source alpha  
    bf.SourceConstantAlpha = 0xff;  // opaque (disable constant alpha) 
   
    if (!AlphaBlend(hdcwnd, ulWindowWidth/5, ulWindowHeight/5+ulWindowHeight, ulBitmapWidth, ulBitmapHeight, hdc, 0, 0, ulBitmapWidth, ulBitmapHeight, bf))
        return;

    // bottom window area, use constant alpha = 75% and a changing 
    // source alpha. Create a gradient effect using source alpha, and  
    // then fade it even more with constant alpha 
    ubRed = 0x00;
    ubGreen = 0x00;
    ubBlue = 0xff;
    
    for (y = 0; y < ulBitmapHeight; y++)
        for (x = 0; x < ulBitmapWidth; x++)
        {
            // for a simple gradient, base the alpha value on the x  
            // value of the pixel  
            ubAlpha = (UCHAR)((float)x / (float)ulBitmapWidth * 255);
            //calculate the factor by which we multiply each component 
            fAlphaFactor = (float)ubAlpha / (float)0xff; 
            // multiply each pixel by fAlphaFactor, so each component  
            // is less than or equal to the alpha value. 
            ((UINT32 *)pvBits)[x + y * ulBitmapWidth] 
                = (ubAlpha << 24) |                       //0xaa000000 
                 ((UCHAR)(ubRed * fAlphaFactor) << 16) |  //0x00rr0000 
                 ((UCHAR)(ubGreen * fAlphaFactor) << 8) | //0x0000gg00 
                 ((UCHAR)(ubBlue   * fAlphaFactor));      //0x000000bb 
        }

    bf.BlendOp = AC_SRC_OVER;
    bf.BlendFlags = 0;
    bf.AlphaFormat = AC_SRC_ALPHA;   // use source alpha  
    bf.SourceConstantAlpha = 0xbf;   // use constant alpha, with  
                                     // 75% opaqueness 

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

    // do cleanup 
    DeleteObject(hbitmap);
    DeleteDC(hdc);
    
}