This header file contains code snippet for easy mapping Eigen types to OpenCV and back with minimal overhead.

Eigen2CV.h

/**
 * @brief   Mapping functions from Eigen data types to OpenCV
 * @author  Eugene Khvedchenya <ekhvedchenya@gmail.com>
 * @details This header file contains code snippet for easy mapping Eigen types to OpenCV and back with minimal overhead.
 * @more    computer-vision.talks.com/articles/mapping-eigen-to-opencv/
 * Features:
 *  - Mapping plain data types with no overhead (read/write access)
 *  - Mapping expressions via evaluation (read only acess)
 *
 * Known issues:
 *  - Does not support .transpose()
 */

#pragma once

#include <opencv2/opencv.hpp>
#include <Eigen/Core>

namespace octane {
    
    namespace details
    {
        struct Const   {};
        struct Mutable {};
    }
    
    
    template<typename T>
    struct opencv_matrix;
    
    template<>
    struct opencv_matrix<double>{ static const int type = CV_64FC1; };
    template<>
    struct opencv_matrix<float> { static const int type = CV_32FC1; };
    template<>
    struct opencv_matrix<int>   { static const int type = CV_32SC1; };
    template<>
    struct opencv_matrix<unsigned char> { static const int type = CV_8UC1; };
    template<>
    struct opencv_matrix<bool>
    {
        static_assert(sizeof(bool) == 1, "Requires bool to be 1 byte");
        static const int type = CV_8UC1;
    };
    
    /**
     * PlainObjectBase<D> - map directly (read/write)
     * ArrayWrapper/MatrixWrapper on PlainObjectBase - map directly (read/write)
     * Block<D> - recursively instantiate. Can be read-only (derived is expression) or read/write (derived is planar)
     * Expression - evaluate (read-only)
     */
    template<
    typename Derived,
    typename Base,
    typename ConstPolicy,
    typename StorageKind = typename Eigen::internal::traits<Derived>::StorageKind
    >
    class Eigen2CV;
    
    class Eigen2CVBase
    {
    public:
        
        operator cv::Mat() const
        {
            return mBody;
        }
        
        operator cv::_InputArray() const
        {
            return cv::_InputArray(mBody);
        }
        
    protected:
        
        template<typename Derived>
        void mapPlaneMemory(const Derived& src)
        {
            const bool isRowMajor = int(Derived::Flags) & Eigen::RowMajorBit;
            const int stride = src.outerStride() * sizeof(typename Derived::Scalar);
            
            if (isRowMajor)
                this->mapPlaneMemoryRowMajor(src.data(),
                                             src.rows(),
                                             src.cols(),
                                             stride);
            else
                this->mapPlaneMemoryColMajor(src.data(),
                                             src.rows(),
                                             src.cols(),
                                             stride);
        }
        
        template <typename Scalar>
        void mapPlaneMemoryRowMajor(const Scalar* planeData, int rows, int cols, int stride)
        {
            this->mBody = cv::Mat(rows, cols, opencv_matrix<Scalar>::type, const_cast<Scalar*>(planeData), stride);
        }
        
        template <typename Scalar>
        void mapPlaneMemoryColMajor(const Scalar* planeData, int rows, int cols, int stride)
        {
            this->mBody = cv::Mat(cols, rows, opencv_matrix<Scalar>::type, const_cast<Scalar*>(planeData), stride);
        }
        
        template <typename Derived, typename T>
        void assignMatrix(Eigen::DenseBase<Derived>& dst, const cv::Mat_<T>& src)
        {
            typedef Eigen::Matrix<T, Eigen::Dynamic, Eigen::Dynamic, Eigen::RowMajor> PlainMatrixType;
            
            dst = Eigen::Map<PlainMatrixType>((T*)src.data, src.rows, src.cols).template cast<typename Derived::Scalar>();
        }
        
        cv::Mat mBody;
    };
    
    template<typename Derived>
    class Eigen2CV<Derived, Eigen::PlainObjectBase<Derived>, details::Const> : public Eigen2CVBase
    {
    public:
        
        typedef typename Derived::Scalar Scalar;
        typedef Eigen2CV<Derived, Eigen::PlainObjectBase<Derived>, details::Mutable> Self;
        
        Eigen2CV(const Eigen::PlainObjectBase<Derived>& src)
        : mMappedView(src)
        {
            this->mapPlaneMemory(mMappedView);
        }
        
    private:
        const Eigen::PlainObjectBase<Derived>& mMappedView;
    };
    
    template<typename Derived>
    class Eigen2CV<Derived, Eigen::PlainObjectBase<Derived>, details::Mutable> : public Eigen2CVBase
    {
    public:
        
        typedef typename Derived::Scalar Scalar;
        typedef Eigen2CV<Derived, Eigen::PlainObjectBase<Derived>, details::Mutable> Self;
        
        Eigen2CV(Eigen::PlainObjectBase<Derived>& src)
        : mMappedView(src)
        {
            this->mapPlaneMemory(mMappedView);
        }
        
        template <typename T>
        Self& operator=(const cv::Mat_<T>& src)
        {
            assignMatrix<Derived, T>(mMappedView, src);
            return *this;
        }
        
        /**
         * @brief Assignment operator to copy OpenCV Mat data to mapped Eigen object.
         */
        Self& operator= (const cv::Mat& m)
        {
            switch (m.type())
            {
                case CV_8U:  return *this = (cv::Mat_<uint8_t>)m;
                case CV_16U: return *this = (cv::Mat_<uint16_t>)m;
                case CV_16S: return *this = (cv::Mat_<int16_t>)m;
                case CV_32S: return *this = (cv::Mat_<int32_t>)m;
                case CV_32F: return *this = (cv::Mat_<float>)m;
                case CV_64F: return *this = (cv::Mat_<double>)m;
                default:
                    throw std::runtime_error("Unsupported OpenCV matrix type");
            };
        }
        
        operator cv::_InputOutputArray()
        {
            return cv::_InputOutputArray(this->mBody);
        }
        
    protected:
        
    private:
        Eigen::PlainObjectBase<Derived>& mMappedView;
    };
    
    
    
    
    template<typename Derived>
    class Eigen2CV<Derived,
    Eigen::Block<Derived>,
    details::Mutable,
    Eigen::Dense> : public Eigen2CVBase
    {
    public:
        typedef typename Derived::Scalar Scalar;
        typedef Eigen2CV<Derived, Eigen::Block<Derived>, details::Mutable> Self;
        
        Eigen2CV(const Eigen::Block<Derived>& src)
        : mMappedView(src)
        {
            this->mapPlaneMemory(mMappedView);
        }
        
        operator cv::_OutputArray()
        {
            return cv::_OutputArray(this->mBody);
        }
        
        template <typename T>
        Self& operator=(const cv::Mat_<T>& src)
        {
            assignMatrix<Derived, T>(mMappedView, src);
            return *this;
        }
        
        /**
         * @brief Assignment operator to copy OpenCV Mat data to mapped Eigen object.
         */
        Self& operator= (const cv::Mat& m) throw ()
        {
            switch (m.type())
            {
                case CV_8U:  return *this = (cv::Mat_<uint8_t>)m;
                case CV_16U: return *this = (cv::Mat_<uint16_t>)m;
                case CV_16S: return *this = (cv::Mat_<int16_t>)m;
                case CV_32S: return *this = (cv::Mat_<int32_t>)m;
                case CV_32F: return *this = (cv::Mat_<float>)m;
                case CV_64F: return *this = (cv::Mat_<double>)m;
                default:
                    throw std::runtime_error("Unsupported OpenCV matrix type");
            };
        }
        
    private:
        const Eigen::Block<Derived>& mMappedView;
    };
    
    
    /**
     * @brief Most generic and most inefficient mapper - it evaluates input object into a local copy.
     */
    template<typename Derived>
    class Eigen2CV<Derived, Eigen::EigenBase<Derived>, details::Const> : public Eigen2CVBase
    {
    public:
        typedef typename Derived::Scalar Scalar;
        typedef typename Eigen::internal::plain_matrix_type<Derived>::type StorageType;
        
        Eigen2CV(const Eigen::EigenBase<Derived>& src)
        {
            mStorage = src;
            this->mapPlaneMemory(mStorage);
        }
        
    private:
        StorageType mStorage;
    };
    
    
    template<typename E>
    Eigen2CV<E, Eigen::PlainObjectBase<E>, details::Mutable>
    eigen2cv(Eigen::PlainObjectBase<E>& src)
    {
        return Eigen2CV<E, Eigen::PlainObjectBase<E>, details::Mutable>(src);
    }
    
    template<typename E>
    Eigen2CV<E, Eigen::PlainObjectBase<E>, details::Const>
    eigen2cv(const Eigen::PlainObjectBase<E>& src)
    {
        return Eigen2CV<E, Eigen::PlainObjectBase<E>, details::Const>(src);
    }
    
    template<typename E>
    Eigen2CV<E, Eigen::Block<E>, details::Mutable>
    eigen2cv(const Eigen::Block<E>& src)
    {
        return Eigen2CV<E, Eigen::Block<E>, details::Mutable>(src);
    }
    
    template<typename E>
    auto eigen2cv(const Eigen::ArrayWrapper<E>& src) ->
    decltype(eigen2cv(src.nestedExpression().eval()))
    {
        return eigen2cv(src.nestedExpression().eval());
    }
    
    template<typename E>
    auto eigen2cv(const Eigen::MatrixWrapper<E>& src) ->
    decltype(eigen2cv(src.nestedExpression().eval()))
    {
        return eigen2cv(src.nestedExpression().eval());
    }
    
    template<typename E>
    Eigen2CV<E, Eigen::EigenBase<E>, details::Const>
    eigen2cv(const Eigen::EigenBase<E>& src)
    {
        return Eigen2CV<E, Eigen::EigenBase<E>, details::Const>(src);
    }
}