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A very basic alpha blend algorithm

, 19 Apr 2011 CPOL
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A simple implementation of alpha blending technique displaying the basic mechanism behind it
Looking over the internet, I didn't find any manual implementation of the AlphaBlend function supported by Windows. Due to some bindings of my project, I couldn't use the AlphaBlend provided by windows, so I had to implement it on my own.
 
I hope it (atleast the concept) becomes useful for some:
 
(Note: This will only work if the source and destination dc bitmaps are equal in size.)
 
void AlphaBlend(CDC* pDC, int xDest, int yDest, int nDestWidth, int nDestHeight,
   CDC* pSrcDC, int xSrc, int ySrc, BYTE uAlphaValue)
{
    BOOL bProceed = TRUE;
 
    CDC memDC;
    if(!memDC.CreateCompatibleDC(pDC))
    {
        bProceed = FALSE;
    }
 
    CBitmap memBmp;
    if(bProceed)
    {
        if(!memBmp.CreateCompatibleBitmap(pDC, nDestWidth, nDestHeight))
        {
            bProceed = FALSE;
        }
    }
 
    if(bProceed)
    {
        CBitmap* pOldBmp = memDC.SelectObject(&memBmp);
 
        memDC.BitBlt(0, 0, nDestWidth, nDestHeight, pDC, 0, 0, SRCCOPY);
 
        CDC* pDestDC = &memDC;
 
        ASSERT(pDestDC);
        ASSERT(pSrcDC);
 
        BYTE r1, r2, rDest;
        BYTE g1, g2, gDest;
        BYTE b1, b2, bDest;
 
        BYTE av = uAlphaValue; // Alpha value BYTE

        BYTE rem = 255 - av; // Remaining fraction

        COLORREF clrPixelDest, clrPixelSrc;
 
        for(int dy = yDest, sy = ySrc; dy < nDestHeight; dy++, sy++)
        {
            for(int dx = xDest, sx = xSrc; dx < nDestWidth; dx++, sx++)
            {
                clrPixelDest = pDestDC->GetPixel(dx, dy);
 
                r1 = GetRValue(clrPixelDest);
                g1 = GetGValue(clrPixelDest);
                b1 = GetBValue(clrPixelDest);
 
                clrPixelSrc = pSrcDC->GetPixel(sx, sy);
                r2 = GetRValue(clrPixelSrc);
                g2 = GetGValue(clrPixelSrc);
                b2 = GetBValue(clrPixelSrc);
 
                rDest = (r1*rem + r2*av) / 255;
                gDest = (g1*rem + g2*av) / 255;
                bDest = (b1*rem + b2*av) / 255;
 
                pDestDC->SetPixel(dx, dy, RGB(rDest, gDest, bDest));
            }
        }
 
        pDC->BitBlt(0, 0, nDestWidth, nDestHeight, &memDC, 0, 0, SRCCOPY);
 
        memDC.SelectObject(pOldBmp);
    }
}

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Mukit, Ataul
Chief Technology Officer Rational Technologies
Bangladesh Bangladesh
C++ is not C with classes, JQuery is not Javascript, Google Search is not Learning, Design Patterns are not fashion, A code written in 2005 is not backdated just because it's 2015
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Comments and Discussions

 
GeneralThank you. PinmemberConrad de Wet26-Aug-13 9:02 
GeneralReason for my vote of 5 OK! PinmvpManfred R. Bihy11-Apr-11 4:40 
GeneralMaybe sledgehammer, wud u be kind enough to post an improvem... PinmemberMukit, Ataul6-Apr-11 1:26 
GeneralReason for my vote of 1 *NEVER* use GetPixel / SetPixel in a... PinmemberSledgeHammer015-Apr-11 5:40 
GeneralRe: Acknowledged.. Although since GetPixel and SetPixel is done ... PinmemberMukit, Ataul5-Apr-11 19:15 
GeneralExcellent! [modified] Pinmemberprojectzombie25-Oct-11 18:34 
AnswerRe: Excellent! PinmemberMukit, Ataul25-Oct-11 19:14 
GeneralPathetic PinmemberMukit, Ataul15-May-11 1:28 

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