enable nonfree feature ?

[mh-cbon]

How can i enable non free feature ? https://github.com/hybridgroup/gocv/blob/a897c364055ad1328c2162830747da556a8a9aa2/contrib/README.md

It is weird, the build stage appears to include contrib stuff, but this is my output

$ make run
docker run -it \
	-v /home/mh-cbon/gow/src/test/opencv:/go/src/opencv\
	gocv
2019/10/29 19:37:24 [370 265]
2019/10/29 19:37:24 [195 320]
2019/10/29 19:37:24 0.527027027027027 1 0.527027027027027
2019/10/29 19:37:24 [103 169]
terminate called after throwing an instance of 'cv::Exception'
  what():  OpenCV(4.0.1) /tmp/opencv/opencv_contrib-4.0.1/modules/xfeatures2d/src/sift.cpp:1207: 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 'create'

the related code is

	si := contrib.NewSIFT()
	kp := si.Detect(matTemplate)
	gocv.DrawKeyPoints(matTemplate, kp,
		&matTemplate,
		color.RGBA{R: 0, B: 0, G: 0},
		gocv.DrawDefault)

[mh-cbon]

i cooked a docker file then build it, it worked. Would be great to have env variable to enable it.

FROM denismakogon/gocv-alpine:4.0.1-buildstage as build-stage

ENV OPENCV_VERSION=4.0.1
ENV BUILD="ca-certificates \
         git \
         build-base \
         musl-dev \
         alpine-sdk \
         make \
         gcc \
         g++ \
         libc-dev \
         linux-headers \
         libjpeg-turbo \
         libpng \
         libwebp \
         libwebp-dev \
         tiff \
         libavc1394 \
         jasper-libs \
         openblas \
         libgphoto2 \
         gstreamer \
         gst-plugins-base"

ENV DEV="clang clang-dev cmake pkgconf \
         openblas-dev gstreamer-dev gst-plugins-base-dev \
         libgphoto2-dev libjpeg-turbo-dev libpng-dev \
         tiff-dev jasper-dev libavc1394-dev"


RUN apk update && \
    apk add --no-cache ${BUILD} ${DEV}

RUN mkdir /tmp/opencv && \
    cd /tmp/opencv && \
    wget -O opencv.zip https://github.com/opencv/opencv/archive/${OPENCV_VERSION}.zip && \
    unzip opencv.zip && \
    wget -O opencv_contrib.zip https://github.com/opencv/opencv_contrib/archive/${OPENCV_VERSION}.zip && \
    unzip opencv_contrib.zip && \
    mkdir /tmp/opencv/opencv-${OPENCV_VERSION}/build && cd /tmp/opencv/opencv-${OPENCV_VERSION}/build && \
    cmake \
    -D CMAKE_BUILD_TYPE=RELEASE \
    -D CMAKE_INSTALL_PREFIX=/usr/local \
    -D OPENCV_EXTRA_MODULES_PATH=/tmp/opencv/opencv_contrib-${OPENCV_VERSION}/modules \
    -D WITH_FFMPEG=YES \
    -D OPENCV_ENABLE_NONFREE=YES \
    -D INSTALL_C_EXAMPLES=NO \
    -D INSTALL_PYTHON_EXAMPLES=NO \
    -D BUILD_ANDROID_EXAMPLES=NO \
    -D BUILD_DOCS=NO \
    -D BUILD_TESTS=NO \
    -D BUILD_PERF_TESTS=NO \
    -D BUILD_EXAMPLES=NO \
    -D BUILD_opencv_java=NO \
    -D BUILD_opencv_python=NO \
    -D BUILD_opencv_python2=NO \
    -D BUILD_opencv_python3=NO \
    -D OPENCV_GENERATE_PKGCONFIG=YES .. && \
    make -j4 && \
    make install && \
    cd && rm -rf /tmp/opencv

RUN apk del ${DEV_DEPS} && \
    rm -rf /var/cache/apk/*

ENV PKG_CONFIG_PATH /usr/local/lib64/pkgconfig
ENV LD_LIBRARY_PATH /usr/local/lib64
ENV CGO_CPPFLAGS -I/usr/local/include
ENV CGO_CXXFLAGS "--std=c++1z"
ENV CGO_LDFLAGS "-L/usr/local/lib -lopencv_core -lopencv_face -lopencv_videoio -lopencv_imgproc -lopencv_highgui -lopencv_imgcodecs -lopencv_objdetect -lopencv_features2d -lopencv_video -lopencv_dnn -lopencv_xfeatures2d -lopencv_plot -lopencv_tracking"

FROM build-stage

RUN go get -u -d gocv.io/x/gocv
RUN cd $GOPATH/src/gocv.io/x/gocv && go build -o $GOPATH/bin/gocv-version ./cmd/version/main.go

RUN go env

RUN mkdir -p /go/src/opencv
WORKDIR /go/src/opencv
COPY . .

ENTRYPOINT ["go", "run", "main.go"]