What is the need of re-sampling the image for HOG features? I read Dalal and Triggs paper for HOG description and a blog Chris McCormick HOG regarding the same. The blog says that the image needs to be re-sampled at different scales to recognize different person. 

My question is: Already we have a window which we place on the image having a size of 64*128 and which slides over the image. Then why re-sampling instead of sliding the whole window over the image which can detect the persons instead. ?

Please rectify if I am wrong, thanks in advance !!
You are indeed sliding window over the image which has fixed size, but the picture may have different resolution or simply the person in the photo is in some distance from the camera (so the face size in the picture will be related to distance) but your classifier should work anyway, so checking at different sizes is one of the ways to detect people at different depth.