Visual Studio

void DrawAlphaBlend (HWND hWnd, HDC hdcwnd)
{
    HDC hdcDest;               // handle of the DC we will create 
    HDC hdcSource;               // handle of the DC we will create 
    BLENDFUNCTION bf;      // structure for alpha blending
    HBITMAP hbitmap;       // bitmap handle
    HBITMAP hbitmapSource;       // bitmap handle
    BITMAPINFO bmi;        // bitmap header
    BYTE *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
	BITMAPFILEHEADER	*pBmpFileHeadr = NULL;
	BITMAPFILEHEADER	*pBmpFileHeadrSource = NULL;
	BYTE *pvBitsSource; 
	BITMAPINFO *pbmi = NULL; 
	BLENDFUNCTION bfDest;      // structure for alpha blending
            
    // 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;
 
    // create a DC for our bitmap -- the source DC for AlphaBlend 
    hdcDest = CreateCompatibleDC(hdcwnd);
    
    // zero the memory for the bitmap info
    ZeroMemory(&bmi, sizeof(BITMAPINFO));
 
    // create our DIB section and select the bitmap into the dc
    pBmpFileHeadr = DibLoadImage(szDestinationFileName);
	if (pBmpFileHeadr)
	{
		pbmi = (BITMAPINFO *) (pBmpFileHeadr + 1) ;
		pvBits = (BYTE *) pBmpFileHeadr + pBmpFileHeadr->bfOffBits ;
		ulBitmapHeight = abs(pbmi->bmiHeader.biHeight);
		ulBitmapWidth = pbmi->bmiHeader.biWidth;
		hbitmap = CreateDIBSection(hdcDest, pbmi, DIB_RGB_COLORS, NULL, NULL, 0x0);
		SetDIBits(hdcDest,hbitmap,0,ulBitmapHeight,pvBits,pbmi,DIB_RGB_COLORS);
	}
 
    SelectObject(hdcDest, hbitmap);
 
    /* Now Load the Source Bitmap */
 

	hdcSource = CreateCompatibleDC(hdcwnd);
    
    // zero the memory for the bitmap info
    ZeroMemory(&bmi, sizeof(BITMAPINFO));
 
	pBmpFileHeadrSource = DibLoadImage(szSourceFileName);
	if (pBmpFileHeadrSource)
	{
		memcpy(&bmi,(BITMAPINFO *) (pBmpFileHeadrSource + 1), sizeof(BITMAPINFO));
		//pbmi = (BITMAPINFO *) (pBmpFileHeadrSource + 1) ;
		pvBitsSource = (BYTE *) pBmpFileHeadrSource + pBmpFileHeadrSource->bfOffBits ;
		ulBitmapHeight = abs(bmi.bmiHeader.biHeight);
		ulBitmapWidth = bmi.bmiHeader.biWidth;
		
		hbitmapSource = CreateDIBSection(hdcSource, &bmi, DIB_RGB_COLORS, NULL, NULL, 0x0);
		SetDIBits(hdcSource,hbitmapSource,0,ulBitmapHeight,pvBitsSource,&bmi,DIB_RGB_COLORS);
	}
 
    SelectObject(hdcSource, hbitmapSource);
 
    bf.BlendOp = AC_SRC_OVER;
    bf.BlendFlags = 0;
    bf.SourceConstantAlpha = 0xFF;  
    bf.AlphaFormat = AC_SRC_ALPHA;//--> Gives compilation error - undeclared identifier
 

    /* Blt the Destination Image */
    StretchBlt(hdcwnd,0,0,ulWindowWidth,ulWindowHeight,hdcDest,0,0,ulBitmapWidth,ulBitmapHeight,SRCCOPY);
 
    if (!AlphaBlend(hdcwnd, ulBitmapWidth, ulBitmapHeight, 
                    ulBitmapWidth, ulBitmapHeight, 
                    hdcSource, 0, 0, ulBitmapWidth, ulBitmapHeight, bf))
    // do cleanup
	if (pBmpFileHeadr)
		free(pBmpFileHeadr);
	if (pBmpFileHeadrSource)
		free(pBmpFileHeadrSource);
    DeleteObject(hbitmap);
    DeleteDC(hdcDest);
 
	DeleteObject(hbitmapSource);
    DeleteDC(hdcSource);
 
    
}