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VC++ GDI Example: GDI AlphaBlend  and Bitmap Alpha



This article shows how the GDI function AlphaBlend is used to display and overlay 32 bit bitmaps with an alpha channel. It is mainly a hint, which would have saved me hours of research.

Code Snippet

Collapse Copy Code
// the blending of the bitmap is quite simple
// (in the OnPaint() method or the WM_PAINT message handler: 
GetObject(background, sizeof(BITMAP), &bm);
AlphaBlend(hdcMemWork, 0,0, bm.bmWidth, bm.bmHeight, hdcMem, 0, 0, 
           bm.bmWidth, bm.bmHeight, blendFkt)
SelectObject(hdcMem, hbmOld);

// The tricky thing is to pre multiply the alpha channel
// values after loading the 32 bit bitmaps
if(bmInfo.bmiHeader.biBitCount == 32 )
// Alpha Channel
    // pre-multiply rgb channels with alpha channel
    for (int y=0; y<bmInfo.bmiHeader.biHeight; ++y)
        BYTE *pPixel= (BYTE *) lpDIBBits + bmInfo.bmiHeader.biWidth * 4 * y;
        for (int x=0; x<bmInfo.bmiHeader.biWidth ; ++x)
            pPixel[0]= pPixel[0]*pPixel[3]/255;
            pPixel[1]= pPixel[1]*pPixel[3]/255;
            pPixel[2]= pPixel[2]*pPixel[3]/255;
            pPixel+= 4;

Points of interest

I played around a long time until AlphaBlend began working. At least the trick was to pre multiply the color values of the bitmap with the alpha value. Then, everything worked fine. Be careful with the width and height you provide to AlphaBlend(). If this exceeds the bounds of the bitmap, nothing is displayed. I prefer getting the width and height directly out of the bitmap data as shown.



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