OpenCV MAT基本图像容器

参考博客：

OpenCv中cv::Mat和IplImage，CvMat之间的转换

Mat - 基本图像容器

Mat类型较CvMat和IplImage有更强的矩阵运算能力，支持常见的矩阵运算（参照Matlab中的各种矩阵运算），所以将IplImage类型和CvMat类型转换为Mat类型更易于数据处理。

关于 Mat ，首先要知道的是你不必再手动地（1）为其开辟空间（2）在不需要时立即将空间释放。但手动地做还是可以的：大多数OpenCV函数仍会手动地为输出数据开辟空间。当传递一个已经存在的 Mat 对象时，开辟好的矩阵空间会被重用。也就是说，我们每次都使用大小正好的内存来完成任务。

基本上讲 Mat 是一个类，由两个数据部分组成：矩阵头（包含矩阵尺寸，存储方法，存储地址等信息）和一个指向存储所有像素值的矩阵（根据所选存储方法的不同矩阵可以是不同的维数）的指针。矩阵头的尺寸是常数值，但矩阵本身的尺寸会依图像的不同而不同，通常比矩阵头的尺寸大数个数量级。因此，当在程序中传递图像并创建拷贝时，大的开销是由矩阵造成的，而不是信息头。

为了解决图像传输拖程序速度,OpenCV使用引用计数机制。其思路是让每个 Mat 对象有自己的信息头，但共享同一个矩阵。这通过让矩阵指针指向同一地址而实现。而拷贝构造函数则 只拷贝信息头和矩阵指针 ，而不拷贝矩阵。

Mat对象操作：

1、创建Mat类对象

Mat A, C;                                 // 只创建信息头部分
A = imread(argv[1], CV_LOAD_IMAGE_COLOR); // 这里为矩阵开辟内存

Mat B(A);                                 // 使用拷贝构造函数
C = A;                                    // 赋值运算符

2、创建ROI的信息头

Mat D (A, Rect(10, 10, 100, 100) ); // using a rectangle
Mat E = A(Range:all(), Range(1,3)); // using row and column boundaries

3、矩阵的拷贝

Mat F = A.clone();
Mat G;
A.copyTo(G);

总结：

• OpenCV函数中输出图像的内存分配是自动完成的（如果不特别指定的话）。
• 使用OpenCV的C++接口时不需要考虑内存释放问题。
• 赋值运算符和拷贝构造函数（ ctor ）只拷贝信息头。
• 使用函数 clone() 或者 copyTo() 来拷贝一副图像的矩阵。

　

像素值的存储

颜色空间：

指对一个给定的颜色，如何组合颜色元素以对其编码。最简单的颜色空间要属灰度级空间，只处理黑色和白色，对它们进行组合可以产生不同程度的灰色。

对于 彩色 方式则有更多种类的颜色空间，但不论哪种方式都是把颜色分成三个或者四个基元素，通过组合基元素可以产生所有的颜色。RGB颜色空间是最常用的一种颜色空间，这归功于它也是人眼内部构成颜色的方式。它的基色是红色、绿色和蓝色，有时为了表示透明颜色也会加入第四个元素 alpha (A)。

• RGB是最常见的，这是因为人眼采用相似的工作机制，它也被显示设备所采用。
• HSV和HLS把颜色分解成色调、饱和度和亮度/明度。这是描述颜色更自然的方式，比如可以通过抛弃最后一个元素，使算法对输入图像的光照条件不敏感。
• YCrCb在JPEG图像格式中广泛使用。
• CIE L*a*b*是一种在感知上均匀的颜色空间，它适合用来度量两个颜色之间的 距离 。

存储类型：

每个组成元素都有其自己的定义域，取决于其数据类型。如何存储一个元素决定了我们在其定义域上能够控制的精度。最小的数据类型是char ，占一个字节或者8位，可以是有符号型（0到255之间）或无符号型（-127到+127之间）。尽管使用三个 char 型元素已经可以表示1600万种可能的颜色（使用RGB颜色空间），但若使用float（4字节，32位）或double（8字节，64位）则能给出更加精细的颜色分辨能力。但同时也要切记增加元素的尺寸也会增加了图像所占的内存空间。

int flags;　　

int dims;　　　　矩阵的维度（>=2）

int rows, cols; 　 矩阵行列值（ 对二维矩阵而言，超过二维赋值为(-1,-1) ）

uchar *data; 　　数据指针

int* refcount;　　引用计数，当矩阵指针指向用户分配的数据，指针为NULL(？)

uchar* datastart; 设定ROI区域空间的三个指针
uchar* dataend;
uchar* datalimit;

MatAllocator* allocator; 　　custom allocator(?)

MSize size;　　　　
MStep step;　　每行数据字节数

class CV_EXPORTS Mat
{
public:
    //! default constructor
    Mat();
    //! constructs 2D matrix of the specified size and type
    // (_type is CV_8UC1, CV_64FC3, CV_32SC(12) etc.)
    Mat(int rows, int cols, int type);
    Mat(Size size, int type);
    //! constucts 2D matrix and fills it with the specified value _s.
    Mat(int rows, int cols, int type, const Scalar& s);
    Mat(Size size, int type, const Scalar& s);

    //! constructs n-dimensional matrix
    Mat(int ndims, const int* sizes, int type);
    Mat(int ndims, const int* sizes, int type, const Scalar& s);

    //! copy constructor
    Mat(const Mat& m);
    //! constructor for matrix headers pointing to user-allocated data
    Mat(int rows, int cols, int type, void* data, size_t step=AUTO_STEP);
    Mat(Size size, int type, void* data, size_t step=AUTO_STEP);
    Mat(int ndims, const int* sizes, int type, void* data, const size_t* steps=0);

    //! creates a matrix header for a part of the bigger matrix
    Mat(const Mat& m, const Range& rowRange, const Range& colRange=Range::all());
    Mat(const Mat& m, const Rect& roi);
    Mat(const Mat& m, const Range* ranges);
    //! converts old-style CvMat to the new matrix; the data is not copied by default
    Mat(const CvMat* m, bool copyData=false);
    //! converts old-style CvMatND to the new matrix; the data is not copied by default
    Mat(const CvMatND* m, bool copyData=false);
    //! converts old-style IplImage to the new matrix; the data is not copied by default
    Mat(const IplImage* img, bool copyData=false);
    //! builds matrix from std::vector with or without copying the data
    template<typename _Tp> explicit Mat(const vector<_Tp>& vec, bool copyData=false);
    //! builds matrix from cv::Vec; the data is copied by default
    template<typename _Tp, int n> explicit Mat(const Vec<_Tp, n>& vec, bool copyData=true);
    //! builds matrix from cv::Matx; the data is copied by default
    template<typename _Tp, int m, int n> explicit Mat(const Matx<_Tp, m, n>& mtx, bool copyData=true);
    //! builds matrix from a 2D point
    template<typename _Tp> explicit Mat(const Point_<_Tp>& pt, bool copyData=true);
    //! builds matrix from a 3D point
    template<typename _Tp> explicit Mat(const Point3_<_Tp>& pt, bool copyData=true);
    //! builds matrix from comma initializer
    template<typename _Tp> explicit Mat(const MatCommaInitializer_<_Tp>& commaInitializer);

    //! download data from GpuMat
    explicit Mat(const gpu::GpuMat& m);

    //! destructor - calls release()
    ~Mat();
    //! assignment operators
    Mat& operator = (const Mat& m);
    Mat& operator = (const MatExpr& expr);

    //! returns a new matrix header for the specified row
    Mat row(int y) const;
    //! returns a new matrix header for the specified column
    Mat col(int x) const;
    //! ... for the specified row span
    Mat rowRange(int startrow, int endrow) const;
    Mat rowRange(const Range& r) const;
    //! ... for the specified column span
    Mat colRange(int startcol, int endcol) const;
    Mat colRange(const Range& r) const;
    //! ... for the specified diagonal
    // (d=0 - the main diagonal,
    //  >0 - a diagonal from the lower half,
    //  <0 - a diagonal from the upper half)
    Mat diag(int d=0) const;
    //! constructs a square diagonal matrix which main diagonal is vector "d"
    static Mat diag(const Mat& d);

    //! returns deep copy of the matrix, i.e. the data is copied
    Mat clone() const;
    //! copies the matrix content to "m".
    // It calls m.create(this->size(), this->type()).
    void copyTo( OutputArray m ) const;
    //! copies those matrix elements to "m" that are marked with non-zero mask elements.
    void copyTo( OutputArray m, InputArray mask ) const;
    //! converts matrix to another datatype with optional scalng. See cvConvertScale.
    void convertTo( OutputArray m, int rtype, double alpha=1, double beta=0 ) const;

    void assignTo( Mat& m, int type=-1 ) const;

    //! sets every matrix element to s
    Mat& operator = (const Scalar& s);
    //! sets some of the matrix elements to s, according to the mask
    Mat& setTo(InputArray value, InputArray mask=noArray());
    //! creates alternative matrix header for the same data, with different
    // number of channels and/or different number of rows. see cvReshape.
    Mat reshape(int cn, int rows=0) const;
    Mat reshape(int cn, int newndims, const int* newsz) const;

    //! matrix transposition by means of matrix expressions
    MatExpr t() const;
    //! matrix inversion by means of matrix expressions
    MatExpr inv(int method=DECOMP_LU) const;
    //! per-element matrix multiplication by means of matrix expressions
    MatExpr mul(InputArray m, double scale=1) const;

    //! computes cross-product of 2 3D vectors
    Mat cross(InputArray m) const;
    //! computes dot-product
    double dot(InputArray m) const;

    //! Matlab-style matrix initialization
    static MatExpr zeros(int rows, int cols, int type);
    static MatExpr zeros(Size size, int type);
    static MatExpr zeros(int ndims, const int* sz, int type);
    static MatExpr ones(int rows, int cols, int type);
    static MatExpr ones(Size size, int type);
    static MatExpr ones(int ndims, const int* sz, int type);
    static MatExpr eye(int rows, int cols, int type);
    static MatExpr eye(Size size, int type);

    //! allocates new matrix data unless the matrix already has specified size and type.
    // previous data is unreferenced if needed.
    void create(int rows, int cols, int type);
    void create(Size size, int type);
    void create(int ndims, const int* sizes, int type);

    //! increases the reference counter; use with care to avoid memleaks
    void addref();
    //! decreases reference counter;
    // deallocates the data when reference counter reaches 0.
    void release();

    //! deallocates the matrix data
    void deallocate();
    //! internal use function; properly re-allocates _size, _step arrays
    void copySize(const Mat& m);

    //! reserves enough space to fit sz hyper-planes
    void reserve(size_t sz);
    //! resizes matrix to the specified number of hyper-planes
    void resize(size_t sz);
    //! resizes matrix to the specified number of hyper-planes; initializes the newly added elements
    void resize(size_t sz, const Scalar& s);
    //! internal function
    void push_back_(const void* elem);
    //! adds element to the end of 1d matrix (or possibly multiple elements when _Tp=Mat)
    template<typename _Tp> void push_back(const _Tp& elem);
    template<typename _Tp> void push_back(const Mat_<_Tp>& elem);
    void push_back(const Mat& m);
    //! removes several hyper-planes from bottom of the matrix
    void pop_back(size_t nelems=1);

    //! locates matrix header within a parent matrix. See below
    void locateROI( Size& wholeSize, Point& ofs ) const;
    //! moves/resizes the current matrix ROI inside the parent matrix.
    Mat& adjustROI( int dtop, int dbottom, int dleft, int dright );
    //! extracts a rectangular sub-matrix
    // (this is a generalized form of row, rowRange etc.)
    Mat operator()( Range rowRange, Range colRange ) const;
    Mat operator()( const Rect& roi ) const;
    Mat operator()( const Range* ranges ) const;

    //! converts header to CvMat; no data is copied
    operator CvMat() const;
    //! converts header to CvMatND; no data is copied
    operator CvMatND() const;
    //! converts header to IplImage; no data is copied
    operator IplImage() const;

    template<typename _Tp> operator vector<_Tp>() const;
    template<typename _Tp, int n> operator Vec<_Tp, n>() const;
    template<typename _Tp, int m, int n> operator Matx<_Tp, m, n>() const;

    //! returns true iff the matrix data is continuous
    // (i.e. when there are no gaps between successive rows).
    // similar to CV_IS_MAT_CONT(cvmat->type)
    bool isContinuous() const;

    //! returns true if the matrix is a submatrix of another matrix
    bool isSubmatrix() const;

    //! returns element size in bytes,
    // similar to CV_ELEM_SIZE(cvmat->type)
    size_t elemSize() const;
    //! returns the size of element channel in bytes.
    size_t elemSize1() const;
    //! returns element type, similar to CV_MAT_TYPE(cvmat->type)
    int type() const;
    //! returns element type, similar to CV_MAT_DEPTH(cvmat->type)
    int depth() const;
    //! returns element type, similar to CV_MAT_CN(cvmat->type)
    int channels() const;
    //! returns step/elemSize1()
    size_t step1(int i=0) const;
    //! returns true if matrix data is NULL
    bool empty() const;
    //! returns the total number of matrix elements
    size_t total() const;

    //! returns N if the matrix is 1-channel (N x ptdim) or ptdim-channel (1 x N) or (N x 1); negative number otherwise
    int checkVector(int elemChannels, int depth=-1, bool requireContinuous=true) const;

    //! returns pointer to i0-th submatrix along the dimension #0
    uchar* ptr(int i0=0);
    const uchar* ptr(int i0=0) const;

    //! returns pointer to (i0,i1) submatrix along the dimensions #0 and #1
    uchar* ptr(int i0, int i1);
    const uchar* ptr(int i0, int i1) const;

    //! returns pointer to (i0,i1,i3) submatrix along the dimensions #0, #1, #2
    uchar* ptr(int i0, int i1, int i2);
    const uchar* ptr(int i0, int i1, int i2) const;

    //! returns pointer to the matrix element
    uchar* ptr(const int* idx);
    //! returns read-only pointer to the matrix element
    const uchar* ptr(const int* idx) const;

    template<int n> uchar* ptr(const Vec<int, n>& idx);
    template<int n> const uchar* ptr(const Vec<int, n>& idx) const;

    //! template version of the above method
    template<typename _Tp> _Tp* ptr(int i0=0);
    template<typename _Tp> const _Tp* ptr(int i0=0) const;

    template<typename _Tp> _Tp* ptr(int i0, int i1);
    template<typename _Tp> const _Tp* ptr(int i0, int i1) const;

    template<typename _Tp> _Tp* ptr(int i0, int i1, int i2);
    template<typename _Tp> const _Tp* ptr(int i0, int i1, int i2) const;

    template<typename _Tp> _Tp* ptr(const int* idx);
    template<typename _Tp> const _Tp* ptr(const int* idx) const;

    template<typename _Tp, int n> _Tp* ptr(const Vec<int, n>& idx);
    template<typename _Tp, int n> const _Tp* ptr(const Vec<int, n>& idx) const;

    //! the same as above, with the pointer dereferencing
    template<typename _Tp> _Tp& at(int i0=0);
    template<typename _Tp> const _Tp& at(int i0=0) const;

    template<typename _Tp> _Tp& at(int i0, int i1);
    template<typename _Tp> const _Tp& at(int i0, int i1) const;

    template<typename _Tp> _Tp& at(int i0, int i1, int i2);
    template<typename _Tp> const _Tp& at(int i0, int i1, int i2) const;

    template<typename _Tp> _Tp& at(const int* idx);
    template<typename _Tp> const _Tp& at(const int* idx) const;

    template<typename _Tp, int n> _Tp& at(const Vec<int, n>& idx);
    template<typename _Tp, int n> const _Tp& at(const Vec<int, n>& idx) const;

    //! special versions for 2D arrays (especially convenient for referencing image pixels)
    template<typename _Tp> _Tp& at(Point pt);
    template<typename _Tp> const _Tp& at(Point pt) const;

    //! template methods for iteration over matrix elements.
    // the iterators take care of skipping gaps in the end of rows (if any)
    template<typename _Tp> MatIterator_<_Tp> begin();
    template<typename _Tp> MatIterator_<_Tp> end();
    template<typename _Tp> MatConstIterator_<_Tp> begin() const;
    template<typename _Tp> MatConstIterator_<_Tp> end() const;

    enum { MAGIC_VAL=0x42FF0000, AUTO_STEP=0, CONTINUOUS_FLAG=CV_MAT_CONT_FLAG, SUBMATRIX_FLAG=CV_SUBMAT_FLAG };

    /*! includes several bit-fields:
         - the magic signature
         - continuity flag
         - depth
         - number of channels
     */
    int flags;
    //! the matrix dimensionality, >= 2
    int dims;
    //! the number of rows and columns or (-1, -1) when the matrix has more than 2 dimensions
    int rows, cols;
    //! pointer to the data
    uchar* data;

    //! pointer to the reference counter;
    // when matrix points to user-allocated data, the pointer is NULL
    int* refcount;

    //! helper fields used in locateROI and adjustROI
    uchar* datastart;
    uchar* dataend;
    uchar* datalimit;

    //! custom allocator
    MatAllocator* allocator;

    struct CV_EXPORTS MSize
    {
        MSize(int* _p);
        Size operator()() const;
        const int& operator[](int i) const;
        int& operator[](int i);
        operator const int*() const;
        bool operator == (const MSize& sz) const;
        bool operator != (const MSize& sz) const;

        int* p;
    };

    struct CV_EXPORTS MStep
    {
        MStep();
        MStep(size_t s);
        const size_t& operator[](int i) const;
        size_t& operator[](int i);
        operator size_t() const;
        MStep& operator = (size_t s);

        size_t* p;
        size_t buf[2];
    protected:
        MStep& operator = (const MStep&);
    };

    MSize size;
    MStep step;

protected:
    void initEmpty();
};