Setting the image property on the display control resets the zoom state

Programming Questions C-style API
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It’s true that setting the image handle will reset the zoom state - this is a design decision that has been made because you cannot be sure that with a new image the currently set zoom state is possible (e.g. if the zoom center would be outside the image) and on the other hand having a situation where the zoom state is altered only sometimes made no sense.

On top of that, setting a new image handle will trigger a repaint of the image, which is why @GaunterODim’s suggestion will lead to the flickering. To suppress that flicker, it’ll be necessary to suppress the repaints. How this is done differs between different GUI toolkits. MFC adds a member function SetRedraw() to each ActiveX control that will do the job:

...
long centerX, centerY;
centerX = centerY = 0;
double factor = 0.0;
m_cvDisp.GetDisplayZoomEx(&centerX, &centerY, &factor);
m_cvDisp.SetRedraw(FALSE);
m_cvDisp.SetImage(m_cvImg.GetImage());
m_cvDisp.SetRedraw(TRUE);
m_cvDisp.SetDisplayZoomEx(centerX, centerY, factor);
...

(this assumes that m_cvDisp is a Common Vision Blox display control and m_cvImg is a Common Vision Blox image control).

Other toolkits are not as forthcoming and the effect of SetRedraw needs to be triggered by sending the WM_SETREDRAW message to the display control. For example in C# this works as follows:

// import of SendMessage and definition of WM_SETREDRAW as found on http://www.pinvoke.net
[DllImport("user32.dll",EntryPoint="SendMessage")]
public extern static int SendMessage(IntPtr hwnd, uint msg, IntPtr wParam, IntPtr lParam);
const uint WM_SETREDRAW = 0x000B;
...
// this code goes where the image handle is updated
int centerX, centerY;
centerX = centerY = 0;
double factor;
axCVdisplay.GetDisplayZoomEx(ref centerX, ref centerY, ref factor);
SendMessage(axCVdisplay.Handle, WM_SETREDRAW, IntPtr.Zero, IntPtr.Zero);
axCVdisplay.Image = axCVImage.Image;
SendMessage(axCVdisplay.Handle, WM_SETREDRAW, new IntPtr(1), IntPtr.Zero);
axCVdisplay.SetDisplayZoomEx(centerX, centerY, factor);
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