Getting RGB data of a minimised window

Question

I want to get RGB data of a particular part of a running window which is minimised and I'm not quite sure how to do it. Here is my piece of code for getting bitmap info of a window for a specified rectangle.

BYTE* LoadBMPFromHandle(HWND hwnd, int leftTopX, int leftTopY, int width, int height)
{
BITMAPINFO bmi;
BITMAPINFOHEADER bmpiHeader;
HDC dc, memdc;
HBITMAP hbWnd;

bmpiHeader.biSize = sizeof(BITMAPINFOHEADER);
bmpiHeader.biPlanes = 1;
bmpiHeader.biBitCount = 24;
bmpiHeader.biCompression = BI_RGB;
bmpiHeader.biSizeImage = 0;
bmpiHeader.biXPelsPerMeter = 3780;
bmpiHeader.biYPelsPerMeter = 3780;
bmpiHeader.biClrUsed = 0;
bmpiHeader.biClrImportant = 0;
bmpiHeader.biWidth = width;
bmpiHeader.biHeight = height;
bmi.bmiHeader = bmpiHeader;

dc = GetDC(hwnd);
memdc = CreateCompatibleDC(dc);
hbWnd = CreateCompatibleBitmap(dc, width, height);
SelectObject(memdc, hbWnd);
BitBlt(memdc, 0, 0, width, height, dc, leftTopX, leftTopY, SRCCOPY);

int paddedWidth = width + (4-(width%4))%4;
BYTE* BGRwnd = new BYTE [paddedWidth*height*3];

if( 0 == GetDIBits(dc, hbWnd, 0, height, BGRwnd, &bmi, DIB_RGB_COLORS) )
{
    cout <<"GetDIBits function fail..." << endl;
    DeleteDC(memdc);
    DeleteObject(hbWnd);
    ReleaseDC(hwnd, dc);
    exit(0);
}

DeleteDC(memdc);
DeleteObject(hbWnd);
ReleaseDC(hwnd, dc);

return BGRwnd;
}

However, this function would only work if there is no another window overlapping with the one I have a handle to.

hwnd - handle to my window LoadBMPFromHandle has to retrieve BGR (blue green red) data of a rectangle with upper left coordinates (leftTopX, leftTopY) and dimesions width x height.

Thank You!