Question

In the code snippet below I am trying to retrieve the RGB values of a pixel in Visual C++ and then set the same RGB values back to be the same pixel. i.e. this is just a test. However when I do it the resulting image is similar but the colors are wrong/off. The resulting image appears to be totally missing the yellow color pixels of the original image. Why am I not getting the same image back after this? Any help would be most appreciated.
Thanks

        BYTE *pbBitmap;
    HBITMAP hDestBitmap = CreateDIBSection(memDC1, &bi, DIB_RGB_COLORS,(void**)&pbBitmap, NULL, 0);

    if (hDestBitmap){
        SelectObject(DestDC2, hDestBitmap);
        BitBlt(DestDC2, 0, 0, nX, nY, memDC1, 0, 0, SRCCOPY);

        RGBQUAD *pRGB;
        LONG nPixels;
        for (pRGB = (RGBQUAD *)pbBitmap, nPixels = (nX * nY); nPixels > 0; ++pRGB, --nPixels){
            ULONG* pSrc =(ULONG*)pRGB;
            ULONG nRed = GetRValue(*pSrc);
            ULONG nGreen = GetGValue(*pSrc);
            ULONG nBlue = GetBValue(*pSrc);

            pRGB->rgbRed=nRed;
            pRGB->rgbGreen=nGreen;
            pRGB->rgbBlue=nBlue;
        }

        CRect rctPicture2;
        m_Picture2.GetWindowRect(&rctPicture2);
        ScreenToClient(&rctPicture2);
        dc.SetStretchBltMode(COLORONCOLOR);
        StretchBlt(dc,rctPicture2.left,rctPicture2.top,rctPicture2.Width(),rctPicture2.Height(),DestDC2,0,0,1152,864,SRCCOPY);
    }

Answer 1

The way you are accessing the colour values is wrong. GetRValue and other functions are meant to operate on a COLORREF value which is laid out in memory as red-green-blue-reserved, whereas the format of an RGBQUAD is blue-green-red-reserved. By using the Get?Value macros on an RGBQUAD, you’re swapping the red and blue channels.