To evaluate the classification capability of the proposed
system, we firstly divided the testing data set to two parts:
high quality (positive) part and low quality (negative) part. If
the actual N-WER of the testing sample was bigger than a predefined
threshold T, it was labeled as a low quality (negative)
image. Otherwise, it was labeled as a high quality (positive)
image. During the classification, a test image was classified
as positive or negative according to its predict N-WER and
a threshold T0 which traversed from 0 to 1. Fig 4 illustrated
ROC curves of the classification results where the pre-defined
threshold T for testing sets were 0.1, 0.3, 0.5 and 0.7 respectively.
It can be observed that the proposed system achieved
better classification performance when threshold T was set to
be 0.3 and 0.5 for the testing data set.