cv::ml::RTrees variable importance

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Hi

I am working with a random forest (cv::ml::rtrees) and would like a ranking of the features and, if possible, some weight to indicate their relative importance. I have seen that rtrees contains a getVarImportance method which returns a cv::mat with some values. Doing tests with a dataset, I have noticed that when I call this method I get a matrix with a value per feature, as expected. However, the values are all zero but one. I kind of expected a little more information. Is this what this method does or am I missing something here?

(Note: Yes, I have done setCalculateVarImportance(true) :))