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finding bitmap in window

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i am attempting to load the calculator window image into an array of bits, load a small image which represents a portion of the calculator window, and check whether this small image resides within the larger one. i came up with some code which i thought did this, but it only works for some small images and not others. I was wondering whether some one could tell me why. I did think maybe that it was because i was comparing a DIB(the smaller image) to a DDB, but i am not entirely clear about this (i am new to bitmaps etc...) code:
	// small image
	HANDLE hBitMap1 = ::LoadImage(0, _T("c:\\tttt.bmp"), IMAGE_BITMAP, 0, 0, LR_LOADFROMFILE);    
	BITMAP bitmap1;
	::GetObject(hBitMap1, sizeof(BITMAP), &bitmap1);
	int SmallBps = bitmap1.bmBitsPixel / 8;
	int smallSize = bitmap1.bmHeight * bitmap1.bmWidth * SmallBps;    
	BYTE *smallBytes = new BYTE[smallSize];    
	::GetBitmapBits((HBITMAP)hBitMap1, smallSize, smallBytes);

	RECT rc;
	HWND hWnd = ::FindWindow(0, _T("Calculator"));
	if(!hWnd) return;
	::GetWindowRect (hWnd, &rc); 
	HDC hDC = ::GetDC(0);
	HDC memDC = ::CreateCompatibleDC(hDC);
	HBITMAP memBM = ::CreateCompatibleBitmap(hDC, rc.right-rc.left, rc.bottom-rc.top);
	HBITMAP OldBM = (HBITMAP)::SelectObject(memDC, memBM);
	::BitBlt(memDC, 0, 0, rc.right-rc.left, rc.bottom-rc.top, hDC, rc.left, rc.top, SRCCOPY);
	int BigBps = ::GetDeviceCaps(hDC, BITSPIXEL)/8;
	int Width  = rc.right-rc.left;
	int Height  = rc.bottom-rc.top;
	int size = BigBps * (Width * Height);
	BYTE *bigBytes = new BYTE[size];
	::GetBitmapBits(memBM, size, bigBytes);

	int NumOfFounds = 0;
	int j;
	for(int i = 0; i < Height; i ++)
		for(int hh = 0; hh < Width; hh ++)
			for(j = 0; j < bitmap1.bmHeight; j ++)
			  int PosOnSmall  = j * bitmap1.bmWidth * SmallBps; 
			  int PosOnBig  = (i*Width * BigBps) + (hh*BigBps)+ j * Width * BigBps; 
			  int SmallLineWidth = bitmap1.bmWidth * SmallBps;
			  if(memcmp(&bigBytes[PosOnBig], &smallBytes[PosOnSmall], SmallLineWidth) != 0)
			if ( j == bitmap1.bmHeight )
			  // Found

	printf("%d", NumOfFounds);
If anyone could shed any light on the situation i'd be very grateful, Thanks, David

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