OpenCV HOGDescripter Python

Question

I was wondering if anyone knew why there is no documentation for HOGDescriptors in the Python bindings of OpenCV.

Maybe I\'ve just missed them, but the only code I\'

Answer 1

1. Get Inbuilt Documentation: Following command on your python console will help you know the structure of class HOGDescriptor:

import cv2 help(cv2.HOGDescriptor())

2. Example code: Here is a snippet of code to initialize an cv2.HOGDescriptor with different parameters (The terms I used here are standard terms which are well defined in OpenCV documentation here):

import cv2
image = cv2.imread("test.jpg",0)
winSize = (64,64)
blockSize = (16,16)
blockStride = (8,8)
cellSize = (8,8)
nbins = 9
derivAperture = 1
winSigma = 4.
histogramNormType = 0
L2HysThreshold = 2.0000000000000001e-01
gammaCorrection = 0
nlevels = 64
hog = cv2.HOGDescriptor(winSize,blockSize,blockStride,cellSize,nbins,derivAperture,winSigma,
                        histogramNormType,L2HysThreshold,gammaCorrection,nlevels)
#compute(img[, winStride[, padding[, locations]]]) -> descriptors
winStride = (8,8)
padding = (8,8)
locations = ((10,20),)
hist = hog.compute(image,winStride,padding,locations)

3. Reasoning: The resultant hog descriptor will have dimension as: 9 orientations X (4 corner blocks that get 1 normalization + 6x4 blocks on the edges that get 2 normalizations + 6x6 blocks that get 4 normalizations) = 1764. as I have given only one location for hog.compute().

4. Different way to initialize HOGDescriptor:
One more way to initialize is from xml file which contains all parameter values:

hog = cv2.HOGDescriptor("hog.xml")

To get an xml file one can do following:

hog = cv2.HOGDescriptor()
hog.save("hog.xml")

and edit the respective parameter values in xml file.