Problem in Zooming an image in MFC

See more:

C++

MFC

Hi folks,
I've just started programming using MFC and still not familiar with its classes!
I have loaded a multispectral images using GDAL library. I want to add zooming function into my program.
First, I display my image into client area using this code:

::SetDIBitsToDevice(pDC->GetSafeHdc(), 0, 0,
        nXSize, nYSize,
        0, 0,
        0, nYSize,
        m_DispImage,
        &btminf,
        DIB_RGB_COLORS);

The image is larger than the client area. I want the user to push the zoomtofit button and the image would be fitted to the client area. I calculated the size of the image in new format and use this code to do the task:

HDC hdcMem=CreateCompatibleDC(pDC->GetSafeHdc());
	::StretchBlt (hdcMem,
		0, 0,
		XSize, YSize,
		pDC->GetSafeHdc(),
		0, 0,
		nXSize, nYSize,
		SRCCOPY ); 

But nothing is shown in the client area. Do you know what is the problem?