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I've made some progress in resolving this issue. Making changes as described here leads to generation of java wrappers for SIFT and SURF - opencv-310.jar has org.opencv.xfeatures2d package now. Some fix was required in /opencv/modules/java/generator/gen_java.py. Namely inserted 2 lines as shown below:

    def addImports(self, ctype):
    if ctype.startswith('vector_vector'):
        self.imports.add("org.opencv.core.Mat")
        self.imports.add("org.opencv.utils.Converters")
        self.imports.add("java.util.List")
        self.imports.add("java.util.ArrayList")
        self.addImports(ctype.replace('vector_vector', 'vector'))
    elif ctype.startswith('Feature2D'):                                           #added
        self.imports.add("org.opencv.features2d.Feature2D")                       #added
    elif ctype.startswith('vector'):
        self.imports.add("org.opencv.core.Mat")
        self.imports.add('java.util.ArrayList')
        if type_dict[ctype]['j_type'].startswith('MatOf'):
            self.imports.add("org.opencv.core." + type_dict[ctype]['j_type'])
        else:
            self.imports.add("java.util.List")
            self.imports.add("org.opencv.utils.Converters")
            self.addImports(ctype.replace('vector_', ''))

After these changes wrappers are generated with no issues. However the main problem still remains, how to use these wrappers from Java )). For example SIFT.create() gives the pointer to a new SIFT class but calling any class method (for example detect()) crashes Java. I also noticed that using MSER.create() directly from Java also leads to the same issue. So now I'm thinking of adding some wrappers classes, which will work similarly to org.opencv.features2d.FeatureDetector but not sure how to do it properly. Any hints would be highly appreciated.

I've made some progress in resolving this issue. Making changes as described here leads to generation of java wrappers for SIFT and SURF - opencv-310.jar has org.opencv.xfeatures2d package now. Some fix was required in /opencv/modules/java/generator/gen_java.py. Namely inserted 2 lines as shown below:

    def addImports(self, ctype):
    if ctype.startswith('vector_vector'):
        self.imports.add("org.opencv.core.Mat")
        self.imports.add("org.opencv.utils.Converters")
        self.imports.add("java.util.List")
        self.imports.add("java.util.ArrayList")
        self.addImports(ctype.replace('vector_vector', 'vector'))
    elif ctype.startswith('Feature2D'):                                           #added
        self.imports.add("org.opencv.features2d.Feature2D")                       #added
    elif ctype.startswith('vector'):
        self.imports.add("org.opencv.core.Mat")
        self.imports.add('java.util.ArrayList')
        if type_dict[ctype]['j_type'].startswith('MatOf'):
            self.imports.add("org.opencv.core." + type_dict[ctype]['j_type'])
        else:
            self.imports.add("java.util.List")
            self.imports.add("org.opencv.utils.Converters")
            self.addImports(ctype.replace('vector_', ''))

After these changes wrappers are generated with no issues. successfully. However the main problem still remains, how to use these wrappers from Java )). For example SIFT.create() gives the pointer to a new SIFT class but calling any class method (for example instance detect()) crashes Java. I also noticed that using MSER.create() directly from Java also leads to the same issue. crash.

So now it looks like the problem is isolated to the way how Feature2D.create() methods are wrapped in Java. The solution seems to be the following (again, changing /opencv/modules/java/generator/gen_java.py):

Find the string:

ret = "%(ctype)s* curval = new %(ctype)s(_retval_);return (jlong)curval->get();" % { 'ctype':fi.ctype }

Replace it with the following:

ret = "%(ctype)s* curval = new %(ctype)s(_retval_);return (jlong)curval;" % { 'ctype':fi.ctype }

Rebuild the opencv. That is it, all create() methods will start working properly for all children of Feature2D class including experimental and non-free methods. FeatureDescriptor/DescriptorExtractor wrappers can be deprecated I think as Feature2D is much easier to use.

BUT! I'm thinking of adding some wrappers classes, which will work similarly to org.opencv.features2d.FeatureDetector but not sure how to do it properly. Any hints would if the suggested fix is safe for other OpenCV modules. Is there a scenario when (jlong)curval needs to be highly appreciated.

I've made some progress in resolving this issue. Making changes as described here leads to generation of java wrappers for SIFT and SURF - opencv-310.jar has org.opencv.xfeatures2d package now. Some fix was required in /opencv/modules/java/generator/gen_java.py. Namely inserted 2 lines as shown below:

    def addImports(self, ctype):
    if ctype.startswith('vector_vector'):
        self.imports.add("org.opencv.core.Mat")
        self.imports.add("org.opencv.utils.Converters")
        self.imports.add("java.util.List")
        self.imports.add("java.util.ArrayList")
        self.addImports(ctype.replace('vector_vector', 'vector'))
    elif ctype.startswith('Feature2D'):                                           #added
        self.imports.add("org.opencv.features2d.Feature2D")                       #added
    elif ctype.startswith('vector'):
        self.imports.add("org.opencv.core.Mat")
        self.imports.add('java.util.ArrayList')
        if type_dict[ctype]['j_type'].startswith('MatOf'):
            self.imports.add("org.opencv.core." + type_dict[ctype]['j_type'])
        else:
            self.imports.add("java.util.List")
            self.imports.add("org.opencv.utils.Converters")
            self.addImports(ctype.replace('vector_', ''))

After these changes wrappers are generated successfully. However the main problem still remains, how to use these wrappers from Java )). For example SIFT.create() gives the pointer to a new SIFT class but calling any class method (for instance detect()) crashes Java. I also noticed that using MSER.create() directly from Java also leads to the same crash.

So it looks like the problem is isolated to the way how Feature2D.create() methods are wrapped in Java. The solution seems to be the following (again, changing /opencv/modules/java/generator/gen_java.py):

Find the string:

ret = "%(ctype)s* curval = new %(ctype)s(_retval_);return (jlong)curval->get();" % { 'ctype':fi.ctype }

Replace it with the following:

ret = "%(ctype)s* curval = new %(ctype)s(_retval_);return (jlong)curval;" % { 'ctype':fi.ctype }

Rebuild the opencv. That is it, all create() methods will start working properly for all children of Feature2D class including experimental and non-free methods. FeatureDescriptor/DescriptorExtractor wrappers can be deprecated I think as Feature2D is much easier to use.

BUT! I'm not sure if the suggested fix is safe for other OpenCV modules. Is there a scenario when (jlong)curval needs to be dereferenced? It looks like the same fix was suggested already here.