Undefined reference to algorithm

edit

First, I'd include

#include <opencv2/nonfree/nonfree.hpp>

Second you need to initialize the non-free module:

cv::initModule_nonfree();

edit Maybe the following CMakeLists.txt helps you building it (afaik the pkg-config file is still broken in the current opencv-version):

project(Test)
cmake_minimum_required(VERSION 2.8)
set(CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}/cmake")
include_directories(${CMAKE_CURRENT_SOURCE_DIR})
find_package( OpenCV REQUIRED )
if( OpenCV_FOUND )
list( APPEND ThirdParty_LIBS ${OpenCV_LIBS} )
    include_directories( ${OpenCV_INCLUDE_DIRS} )
endif( OpenCV_FOUND )

set(${PROJECT_NAME}_SRC test.cpp)
add_executable(${PROJECT_NAME} ${${PROJECT_NAME}_SRC})
target_link_libraries(${PROJECT_NAME} ${OpenCV_LIBS})

Many times the problem is related to the order of the libraries:

LIBS += -lopencv_imgcodecs -lopencv_objdetect -lopencv_core -lopencv_highgui -lopencv_imgproc -lopencv_video
LIBS += -L$${_OPENCV_PATH_ANDROID}/sdk/native/3rdparty/libs/armeabi-v7a -ltbb

I need to put "-lopencv_objdetect" before "-lopencv_core"

cv::Algorithm class is implemented in core module. You need to add opencv_core to linker library list, i.e. -lopencv_core.