How to obtain probability estimates from RTrees classifier in OpenCV 3.x?

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I am using OpenCV's implementation of Random Forests to classify some data. It appears to be working well, but I would like to obtain confidence values for the different classes. The output of RTrees::predict gives me a class number but I can't find a confidence or probability.

For example, scikit-learn provides this information in is predict_proba method (docs):

The predicted class probabilities of an input sample is computed as the mean predicted class probabilities of the trees in the forest. The class probability of a single tree is the fraction of samples of the same class in a leaf.