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);}