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Posted 4 Apr 2011

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A very basic alpha blend algorithm

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3.12/5 (17 votes)
19 Apr 2011CPOL
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
Technical Lead Kotha Technologies
Bangladesh Bangladesh
If you are not in - you are out !
- Chapter 1

Comments and Discussions

GeneralThank you. Pin
Conrad de Wet26-Aug-13 8:02
MemberConrad de Wet26-Aug-13 8:02 
GeneralReason for my vote of 5 OK! Pin
Manfred Rudolf Bihy11-Apr-11 3:40
professionalManfred Rudolf Bihy11-Apr-11 3:40 
GeneralMaybe sledgehammer, wud u be kind enough to post an improvem... Pin
Mukit, Ataul6-Apr-11 0:26
MemberMukit, Ataul6-Apr-11 0:26 
GeneralReason for my vote of 1 *NEVER* use GetPixel / SetPixel in a... Pin
SledgeHammer015-Apr-11 4:40
MemberSledgeHammer015-Apr-11 4:40 
GeneralRe: Acknowledged.. Although since GetPixel and SetPixel is done ... Pin
Mukit, Ataul5-Apr-11 18:15
MemberMukit, Ataul5-Apr-11 18:15 
GeneralExcellent! Pin
projectzombie25-Oct-11 17:34
Memberprojectzombie25-Oct-11 17:34 
AnswerRe: Excellent! Pin
Mukit, Ataul25-Oct-11 18:14
MemberMukit, Ataul25-Oct-11 18:14 
GeneralPathetic Pin
Mukit, Ataul15-May-11 0:28
MemberMukit, Ataul15-May-11 0:28 

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