Determine whether image acquisition object is associated with image acquisition device
bool = isvalid(obj)
bool = isvalid(obj) returns true if the video input object obj is valid, otherwise false. An object is an invalid image acquisition object if it is no longer associated with any hardware; that is, the object was deleted using the delete function. If this is the case, obj should be cleared from the workspace.
If obj is an array of video input objects, bool is a logical array where each element in bool represents the corresponding element in obj. If an object in obj is valid, the isvalid function sets the corresponding element in bool to true, otherwise false.