Using Opencv to calculate the haar feature - c++

Question

I try to calculate Haar feature using opencv (Given an image).
Input: an image
output: haar feature
For that, I am using the FeatureEvaluator from OpenCV.
But I got an exception when I try to calculate one feature.
Here is how I am doing:
Ptr<FeatureEvaluator> ptrHaar = FeatureEvaluator::create(FeatureEvaluator::HAAR);
Mat img = imread(image_path); // image of size 2048*1536 correctly loaded
ptrHaar->setImage(img, Size(100, 100));
ptrHaar->setWindow(Point(0, 0));
double res = ptrHaar->calcOrd(0); // get the exception here

Answer 1

I think that you need to load/create some type of Haar feature, not just create an object. Try to load some Haar cascade classifier using load method and than try to use calcOrd method.

Answer 2

Your code is almost right. Only is missing is to read the CascadeClassifier previously trained. You can do this as follow:
FileStorage fs( "cascade.xml", FileStorage::READ );
//2) Then, create a FileNode to access the features:
FileNode featuresNode = fs["cascade"]["features"];
//3) Create the FeatureEvaluator, as you did in your first line
//4) Read the FileNode you have created:
ptrHaar->read(featuresNode);
And continue accordingly your code.
Note that ptrHaar->calcOrd(0) will read just the first feature rectangle, if you have more to read, you will need a loop, like this:
FileNodeIterator it = featuresNode.begin(), it_end = featuresNode.end();
int idx = 0;
for( ; it != it_end; ==it, idx++ )
{
res = ptrHaar.calcOrd(idx);
}