I am extracting rooftops in an orthophoto using feature analyst. i prepared my training samples and runned the program to extract the rooftops. Initially after several trials i still could not get any results, no changes to the training samples at all. So i deleted everything and prepared a new training set. This time when i runned the program i received an error message "No training examples available for <class 0>" . The error message suggested to check masking settings. There are two files in the workspace, the masked area of interest and the training samples. I have tried selecting both of them and only the training sample but same error message. How do i resolve this?