featureEvaluator->setImage();

Hello,at cascadeclassifier.cpp file,method fillPassedSamples() there is this line featureEvaluator->setImage(img, isPositive ? 1 : 0, i); Can anybody provide a detailed description about what this line does?And after this line there is the below line,the point i fail to understand is;why isn't the predict() method not taking the img as argument? if(predict(i) == 1.0F) { some code here }