OpenCV change keypoint or descriptor parameters after creation - opencv

Question

In recent versions, OpenCV allows easy creation of keypoint detectors, descriptors or matchers using the create function, e.g.
cv::Ptr<cv::FeatureDetector> featureDetector = cv::FeatureDetector::create("FAST")
This call does NOT support parameters. E.g. SURF, FAST, etc. all have many parameters.
How can I change them now?
I already figured out parts of it, e.g. I can get the list (list of strings) of parameters via
std::vector<std::string> parameters;
featureDetector->getParams(parameters);
and apparently I need to get somehow to the cv::Algorithm* object to call set(char*, bool/int/float/... value), but I don't know how.

Answer 1

Actually as it turns out, featureDetector already is an Algorithm object, i.e. you can simply set parameters on it directly, e.g.
featureDetector->set("someParam", someValue)
If you want to know about the parameters of a feature detector, you can use this function which prints them for you:
void ClassificationUtilities::printParams( cv::Algorithm* algorithm ) {
std::vector<std::string> parameters;
algorithm->getParams(parameters);
for (int i = 0; i < (int) parameters.size(); i++) {
std::string param = parameters[i];
int type = algorithm->paramType(param);
std::string helpText = algorithm->paramHelp(param);
std::string typeText;
switch (type) {
case cv::Param::BOOLEAN:
typeText = "bool";
break;
case cv::Param::INT:
typeText = "int";
break;
case cv::Param::REAL:
typeText = "real (double)";
break;
case cv::Param::STRING:
typeText = "string";
break;
case cv::Param::MAT:
typeText = "Mat";
break;
case cv::Param::ALGORITHM:
typeText = "Algorithm";
break;
case cv::Param::MAT_VECTOR:
typeText = "Mat vector";
break;
}
std::cout << "Parameter '" << param << "' type=" << typeText << " help=" << helpText << std::endl;
}
}