使用Python在OpenCv中设置ORB参数

Question

I've been using OpenCV 2.4 in Python to match features between two images, but I want to change one of the parameters of the "ORB" detector (the number of features it extracts "nfeatures") and there seems to be no way to do so in Python.

For C++ you can load a parameter yml/xml file by the 'read' (or 'load' for java?) methods of FeatureDetector/DescriptorExtractor. However the Python binding is missing this function/method.

It's also missing the binding to create an ORB object directly, so I can't pass the parameters there (Python binding seems to requires you to use cv2.DescriptorExtractor_create by string name -- which will segfault if you pass a bad string name or the parameters along with it... Additionally that function cannot take any other arguments it seems to pass onto the constructor.

My only hope seemed to be loading the complete object from xml with cv2.cv.Load(filename), but that seems to expect an object instance and not an Algorithm definition, for which I can't find any Python bindings in new or old syntax. I tried several variations on the file loading step, including mimicking the style of saved xml files from OpenCV with no luck.

Has anyone has success in one of the steps I tried above to pass parameters onto a detector (SURF or ORB, or any generic algorithm) in OpenCV?

Here is the code I am using to extract features:

def findFeatures(greyimg, detector="ORB", descriptor="ORB"):
    nfeatures = 2000 # No way to pass to detector...?
    detector = cv2.FeatureDetector_create(detector)
    descriptorExtractor = cv2.DescriptorExtractor_create(descriptor)
    keypoints = detector.detect(greyimg)
    (keypoints, descriptors) = descriptorExtractor.compute(greyimg, keypoints)
    return keypoints, descriptors

EDIT

Changing detector settings seems to only segfault on windows implementation -- waiting for a patch or fix to appear on OpenCV's site.

Answer 1

import cv2

# to see all ORB parameters and their values
detector = cv2.FeatureDetector_create("ORB")    
print "ORB parameters (dict):", detector.getParams()
for param in detector.getParams():
    ptype = detector.paramType(param)
    if ptype == 0:
        print param, "=", detector.getInt(param)
    elif ptype == 2:
        print param, "=", detector.getDouble(param)

# to set the nFeatures
print "nFeatures before:", detector.getInt("nFeatures")
detector.setInt("nFeatures", 1000)
print "nFeatures after:", detector.getInt("nFeatures")

with output:

ORB parameters (dict): ['WTA_K', 'edgeThreshold', 'firstLevel', 'nFeatures', 'nLevels', 'patchSize', 'scaleFactor', 'scoreType']
WTA_K = 2
edgeThreshold = 31
firstLevel = 0
nFeatures = 500
nLevels = 8
patchSize = 31
scaleFactor = 1.20000004768
scoreType = 0
nFeatures before: 500
nFeatures after: 1000

EDIT: To do the same with OpenCV 3.0 is now easier

import cv2

detector = cv2.ORB_create()
for attribute in dir(new_detector):
    if not attribute.startswith("get"):
        continue
    param = attribute.replace("get", "")
    get_param = getattr(new_backend, attribute)
    val = get_param()
    print param, '=', val

and analogically with a setter.