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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.)

C++
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
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 9:02
MemberConrad de Wet26-Aug-13 9:02 
GeneralReason for my vote of 5 OK! Pin
Manfred Rudolf Bihy11-Apr-11 4:40
professionalManfred Rudolf Bihy11-Apr-11 4:40 
GeneralMaybe sledgehammer, wud u be kind enough to post an improvem... Pin
Mukit, Ataul6-Apr-11 1:26
MemberMukit, Ataul6-Apr-11 1:26 
GeneralReason for my vote of 1 *NEVER* use GetPixel / SetPixel in a... Pin
SledgeHammer015-Apr-11 5:40
MemberSledgeHammer015-Apr-11 5:40 
GeneralRe: Acknowledged.. Although since GetPixel and SetPixel is done ... Pin
Mukit, Ataul5-Apr-11 19:15
MemberMukit, Ataul5-Apr-11 19:15 
GeneralExcellent! Pin
projectzombie25-Oct-11 18:34
Memberprojectzombie25-Oct-11 18:34 
AnswerRe: Excellent! Pin
Mukit, Ataul25-Oct-11 19:14
MemberMukit, Ataul25-Oct-11 19:14 
GeneralPathetic Pin
Mukit, Ataul15-May-11 1:28
MemberMukit, Ataul15-May-11 1:28 

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