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I want to have the descriptor for keypoints with color information. so, i want to use "OpponentColorDescriptorExtractor" as mentioned here:

http://docs.opencv.org/modules/features2d/doc/common_interfaces_of_descriptor_extractors.html#opponentcolordescriptorextractor

I am using the following code but i am getting an error:

The following code is working:

SiftDescriptorExtractor detector;   
Ptr<DescriptorExtractor> oppDescExtractor;

OpponentColorDescriptorExtractor opponentDescExtractor(oppDescExtractor);

The error is:

OpenCV Error: Assertion failed (!descriptorExtractor.empty()) in OpponentColorDescriptorExtractor,

So, how should i declare "OpponentColorDescriptorExtractor" ?