A method and apparatus for classifying the state of a system are described. The state of the system can be in one of a plurality of different classes and the system can have at least one property represented by a set of data items. A current data item representing a property of the system is received. The system is classified as being in one of a plurality of different classes based on the probability that the system is in any one of the plurality of classes calculated using the current data item, a recursively calculated mean value for the set of data items representing the property of the system and at least one recursively calculated statistical parameter for the set of data items representing the property. It is determined whether to output a signal based on the class in which the system is classified.
Please help me write a proper abstract based on the patent claims. CLAIM: 1. A system, the system comprising: at least one operative part; at least one sensor which can output a set of data items representing a property of the operative part; and a data processing apparatus for classifying the state of the system and in communication with the sensor to receive data items from the sensor, wherein the data processing apparatus comprises a data processing device and a storage device in communication with the data processing device, the storage device storing computer program code executable by the data processing device to: receive a current data item representing a property of the system; classify the system as being in one of a plurality of different classes based on the probability that the system is in any one of the plurality of classes calculated using the current data item, a recursively calculated mean value for the set of data items representing the property of the system and at least one recursively calculated statistical parameter for the set of data items representing the property; and determine whether to output a signal based on the class in which the system is classified. 2. A system as claimed in claim 1, wherein the data processing apparatus has an output which is in communication with the system to output the signal to the system. 3. A system as claimed in claim 1, wherein the operative part comprises an image sensor. 4. A method for classifying the state of a system, wherein the state of the system can be in one of a plurality of different classes, the system having at least one property represented by a set of data items, the method comprising: receiving a current data item representing a property of the system; classifying the system as being in one of a plurality of different classes based on the probability that the system is in any one of the plurality of classes calculated using the current data item, a recursively calculated mean value for the set of data items representing the property of the system and at least one recursively calculated statistical parameter for the set of data items representing the property; and determining whether to output a signal based on the class in which the system is classified. 5. The method as claimed in claim 4, and further comprising: recursively calculating and storing an updated mean value for the data item representing the property of the system using the current data item; and recursively calculating and storing a plurality of updated statistical parameters for the set of data items representing the property. 6. The method as claimed in claim 4, and further comprising: receiving an input of an actual class of the system for the current data item. 7. The method as claimed in claim 6, and further comprising: setting the actual class of the system to the input actual class, or else to the class that the system was classified as being in. 8. The method as claimed in claim 4 and further comprising: maintaining a data structure which stores, for each of the plurality of classes, data items representing the recursively calculated mean, the recursively calculated statistical parameters and the associated class of the system. 9. The method as claimed in claim 4, and further comprising: creating a new data structure storing data items representing the recursively calculated mean, the recursively calculated statistical parameters and the associated class of the system when it is determined that the system is in a class which does not correspond to any previous class. 10. The method as claimed in claim 5, wherein the recursive calculating and storing is only carried out if either an actual class has been input or no actual class has been input and there is a low classification error associated with the class in which the system has been classified. 11. The method as claimed in claim 4, wherein each recursive calculation uses a previously stored value representing all of the data items received previously to the current data item. 12. The method as claimed in claim 4, wherein the statistical parameters include the covariance matrix, the inverse of the covariance matrix and the determinant of the covariance matrix. 13. The method as claimed in claim 12, wherein a normalised outer product of all previously received data items and a measure of the average of the current data item is used to calculate the statistical parameters. 14. The method as claimed in claim 4, wherein the system has a plurality of properties each represented by a set of data items and wherein: a current data item representing each property of the system is received and wherein classifying the system as being in one of a plurality of different classes is based on the probability that the system is in any one of the plurality of classes calculated using the current data item representing each property of the system. 15. A method as claimed in claim 4, wherein the system includes one or more sensors for outputting one or more sets of data items representing one or more properties of the system. 16. The method of claim 4, wherein the signal is selected from: a control signal; a data signal; a feedback signal; an alarm signal; a command signal; a warning signal; an alert signal; a servo signal; a trigger signal; a data capture signal; and a data acquisition signal. 17. The system of claim 1, wherein the system is an electrical or electro-mechanical system. 18. The method of claim 16, wherein the system is a video system, the sensor is an image sensor and the set of data items comprises image data. 19. The method of claim 18, wherein the property relates to a sub-region of a frame of image data. 20. The method of claim 18 and further comprising: processing the image data to extract one or more image features. 21. The method as claimed in claim 4, wherein the method includes recursively calculating a covariance matrix, and further comprising: occasionally regularising the covariance matrix to avoid singularity of the covariance matrix. 22. The method of claim 4, wherein the method is a real-time method. 23. A data processing apparatus for classifying the state of a system, comprising: a data processing device; and a storage device in communication with the data processing device, the storage device storing computer program code executable by the data processing device to carry out the method of claim 4. 24. A computer readable medium storing computer program code executable by a data processing device to carry out the method of claim 4.