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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;
        bProceed = FALSE;
    CBitmap memBmp;
        if(!memBmp.CreateCompatibleBitmap(pDC, nDestWidth, nDestHeight))
            bProceed = FALSE;
        CBitmap* pOldBmp = memDC.SelectObject(&memBmp);
        memDC.BitBlt(0, 0, nDestWidth, nDestHeight, pDC, 0, 0, SRCCOPY);
        CDC* pDestDC = &memDC;
        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);


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


About the Author

Mukit, Ataul
Chief Technology Officer Rational Technologies
Bangladesh Bangladesh
Your Lord does not like people who waste by extravagance, and only the righteous C s, and does so in increments, and they are of those people who are mindful of what memory and time has in offer and they waste none of it.
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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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