ROC curve with classification problems I am working with face anti-spoofing. For classification, the Euclidean distance measure is used. The test image vector is compared with all real image vectors , and the minimum distance is chosen (min.1) . The same vector is compared with all fake image vectors and the minimum value is chosen (min. 2). The minimum rule is applied again to choose the minimum of both min.1 and min.2 to identify the correct class of the test image. My problem is in plotting the ROC curve. I do not have a threshold which can be changed. I am using hard decision rules. How can I plot the curve in this case
Yes. You can. 
ROC curve is a Pd vs Pfa plot where Pd and Pfa are the probability of detection and probability of false alarm respectively.