在roc函数(R中的包pROC)中强制输入“方向”参数具体做什么?

I want to create a roc object by using the function 'roc' of pROC package in R, and plot the roc object. However, I am uncertain what the 'direction' argument does. the median predictor values of my controls is smaller than the median predictor value of the cases. so I think the correct direction should be '<'. But if I plot it with direction argument '>'. it just flipped the ROC curve across the diagonal line as a mirror image. I am wondering in this situation that, data telling you 1 thing, and the argument is coercing a different direction, what is being compared to what, and how is the comparison made? I read through the pROC manual on this function for the argument 'direction', and the explanation is very brief and unclear.I want to create a roc object by using the func