What is confidence in OpenCV's FaceRecognizer?

Question

I've been working on a face recognition project using OpenCV's FaceRecognizer, doing gender differentiation. The algorithm works pretty well, but I wanted to implement some extra features into my program like the confidence of the prediction.

The predict function can output a confidence level, but I'm not sure what it means. What does this confidence actually measure, and can I convert it into a percentage?

int predictedLabel = -1;
double confidence = 0.0;
model->predict(face_resized, predictedLabel, confidence);
string result_message = format("Predicted class = %d / Confidence = %d.", predictedLabel, confidence);
cout << result_message << endl;

Here's what the output looks like. https://www.dropbox.com/s/65h1n5180ulz3hl/facerecConfidence%20.jpg

Solution

There's a brief discussion on what this distance actually is on the OpenCV-users list here.

To summarise, the function they use is:

distance = 1.0f - sqrt( distSq / (float)(nTrainFaces * nEigens) ) / 255.0f 

However, the author of the function says that it is a very rough guide and not a full proof guide. See the link to the users list discussion for a reference to the paper and a suggestion for an alternative metric.