Cannot use sift and other nonfree modules

[simbaforrest]

Expected behaviour

I was using opencv-contrib-python. I was hoping I can use:

sift = cv2.xfeatures2d_SURF.create()
kp = sift.detect(gray, None)

Actual behaviour

But when I run sift, there is the following error:

cv2.error: OpenCV(3.4.3) C:\projects\opencv-python\opencv_contrib\modules\xfeatures2d\src\surf.cpp:1016: error: (-213:The function/feature is not implemented) This algorithm is patented and is excluded in this configuration; Set OPENCV_ENABLE_NONFREE CMake option and rebuild the library in function 'cv::xfeatures2d::SURF::create'

Steps to reproduce

• example code

import numpy as np
import cv2

gray = np.zeros((480,640), np.uint8)

sift = cv2.xfeatures2d_SURF.create()
kp = sift.detect(gray, None)

img = cv2.drawKeypoints(gray, kp)

cv2.imwrite('sift.jpg', img)

• operating system: Win 10
• architecture: x64
• opencv-python version: 3.4.3

[simbaforrest]

I suspect it is due to the compiler flag OPENCV_ENABLE_NONFREE has not been set during compilation.

[waveform80]

Duplicate of #126