HOGDescriptor HOG

you can use hog class in opencv as follows

HOGDescriptor hog;
vector<float> ders;
vector<Point>locs;

//This function computes the hog features for you

hog.compute(grayImg,ders,Size(32,32),Size(0,0),locs);

//The HOG features computed for grayImg are stored in ders vecotor to make it into a matrix which can be used for traing later use the following for loop

Hogfeat.create(ders.size(),1,CV_32FC1);

for(int i=0;i<ders.size();i++)
{
  Hogfeat.at<float>(i,0)=ders.at(i);

}

//Now your HOG features are stored in Hogfeat matrix

you can also set the window size, cell size and block size by using object hog as follows

hog.blockSize=16;
hog.cellSize=4;
hog.blockStride=8;

//This is for comparing the HOG features of two images without using any SVM 
//(It is not an efficent way but useful when you want to compare only few or two images)
//Simple distance
//Consider you have two hog feature vectors for two images Hogfeat1 and Hogfeat2 and those are same size.
double distance=0;
for(int i=0;i<Hogfeat.rows;i++)
{
   distance+ = abs(Hogfeat.at<float>(i,0) - Hogfeat.at<float>(i,0));
}
if(distance < Threshold)
cout<<"Two images are of same class"<<endl;
else
cout<<"Two images are of different class"<<endl;

Hope it is usefull :)