How to programmatically disable the auto-focus of a webcam?

Question

I am trying to do computer vision using a webcam (the model is Hercules Dualpix). I know it is not the ideal camera to use, but I have no choice here.

The problem i

Answer 1

The Hercules cameras are UVC compliant, so they should work with the DirectShow Interface IAMCameraControl. You can set the focus to a specific value, and use the flags to set that you do not want it to be automatic. You can use IAMCameraControl::Get to poll the current state, because not all cameras do support turning off the focus.

IAMCameraControl *pCameraControl; 
HRESULT hr; 
hr = pFilter->QueryInterface(IID_IAMCameraControl, (void **)&pCameraControl); 
if (hr == S_OK) {
  long defaultFocusValue;
  hr = pCameraControl->GetRange(CameraControl_Focus,
                                NULL, // min
                                NULL, // max
                                NULL, // minstep
                                &defaultFocusValue, // default
                                NULL); // capflags
  hr = pCameraControl->Set(CameraControl_Focus, // property
                           defaultFocusValue, // value
                           CameraControl_Flags_Manual); 
} 

Focus has a range which is defined by each camera separately, so you should query it as shown to find the default value and the min, max if you want. In this example the pFilter is a pointer to the input filter that you have from DirectShow. You can get it by enumerating the devices and finding the one you want.