OpenCV笔记（一）——Mat的引用计数机制

Mat是Opencv2里面主要的类。Mat的对象常常用来表示一副图像的信息。

Mat的基本操作十分简单，不多说了。下面这段代码能够读七八分明白，应该就算对Mat有大体了解。

/*  For description look into the help() function. */

#include "opencv2/core/core.hpp"
#include <iostream>

using namespace std;
using namespace cv;

static void help()
{
    cout
    << "\n--------------------------------------------------------------------------" << endl
    << "This program shows how to create matrices(cv::Mat) in OpenCV and its serial"
    << " out capabilities"                                                            << endl
    << "That is, cv::Mat M(...); M.create and cout << M. "                            << endl
    << "Shows how output can be formated to OpenCV, python, numpy, csv and C styles." << endl
    << "Usage:"                                                                       << endl
    << "./cvout_sample"                                                               << endl
    << "--------------------------------------------------------------------------"   << endl
    << endl;
}

int main(int,char**)
{
    help();
    // create by using the constructor
    Mat M(2,2, CV_8UC3, Scalar(0,0,255));
    cout << "M = " << endl << " " << M << endl << endl;

    // create by using the create function()
    M.create(4,4, CV_8UC(2));
    cout << "M = "<< endl << " "  << M << endl << endl;

    // create multidimensional matrices
    int sz[3] = {2,2,2};
    Mat L(3,sz, CV_8UC(1), Scalar::all(0));
    // Cannot print via operator <<

    // Create using MATLAB style eye, ones or zero matrix
    Mat E = Mat::eye(4, 4, CV_64F);
    cout << "E = " << endl << " " << E << endl << endl;

    Mat O = Mat::ones(2, 2, CV_32F);
    cout << "O = " << endl << " " << O << endl << endl;

    Mat Z = Mat::zeros(3,3, CV_8UC1);
    cout << "Z = " << endl << " " << Z << endl << endl;

    // create a 3x3 double-precision identity matrix
    Mat C = (Mat_<double>(3,3) << 0, -1, 0, -1, 5, -1, 0, -1, 0);
    cout << "C = " << endl << " " << C << endl << endl;

    Mat RowClone = C.row(1).clone();
    cout << "RowClone = " << endl << " " << RowClone << endl << endl;

    // Fill a matrix with random values
    Mat R = Mat(3, 2, CV_8UC3);
    randu(R, Scalar::all(0), Scalar::all(255));

    // Demonstrate the output formating options
    cout << "R (default) = " << endl <<        R           << endl << endl;
    cout << "R (python)  = " << endl << format(R,"python") << endl << endl;
    cout << "R (numpy)   = " << endl << format(R,"numpy" ) << endl << endl;
    cout << "R (csv)     = " << endl << format(R,"csv"   ) << endl << endl;
    cout << "R (c)       = " << endl << format(R,"C"     ) << endl << endl;

    Point2f P(5, 1);
    cout << "Point (2D) = " << P << endl << endl;

    Point3f P3f(2, 6, 7);
    cout << "Point (3D) = " << P3f << endl << endl;


    vector<float> v;
    v.push_back( (float)CV_PI);   v.push_back(2);    v.push_back(3.01f);

    cout << "Vector of floats via Mat = " << Mat(v) << endl << endl;

    vector<Point2f> vPoints(20);
    for (size_t i = 0; i < vPoints.size(); ++i)
        vPoints[i] = Point2f((float)(i * 5), (float)(i % 7));

    cout << "A vector of 2D Points = " << vPoints << endl << endl;
    return 0;
}

 

文档上边说，Mat的数据成员由头部和数据组成。我们打开core.hpp，看看Mat类的声明中的数据成员部分：

    /*! 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;

Mat的头部信息：flags，dims，rows，cols，data指针，refcount指针。

Mat的数据部分：显然，意味着data指针指向的空间。

而refcount指针，就是起引用计数的作用。有了引用计数机制，使得我们的Mat能够很好地支持浅拷贝的操作：使用等号操作符给Mat赋值（或者使用拷贝构造的时候），仅拷贝头部信息，如此能够节约许多的空间和时间。而深拷贝的工作，就交给成员方法clone()和copyTo()。、

 

现在假设我们有Mat的对象A，当我们对A进行初始化工作的时候，调用create方法，引用计数为1（对相关的代码有疑问，所以此处不给出。）以下引用官方文档。

1. If the current array shape and the type match the new ones, return immediately. Otherwise, de-reference the previous data by calling Mat::release().
2. Initialize the new header.
3. Allocate the new data of total()*elemSize() bytes.
4. Allocate the new, associated with the data, reference counter and set it to 1.

2014年11月14日补充：使用了Source Insight来阅读代码，找到了OpenCV中Mat类的create方法，主要代码摘录如下：

// 初始化头部信息及数据信息
flags = (_type & CV_MAT_TYPE_MASK) | MAGIC_VAL;

size_t totalsize = alignSize(step.p[0]*size.p[0], (int)sizeof(*refcount));
data = datastart = (uchar*)fastMalloc(totalsize + (int)sizeof(*refcount));
refcount = (int*)(data + totalsize);
*refcount = 1;

令Mat的对象B=A，此时将引用计数增到2。

inline Mat& Mat::operator = (const Mat& m)
{
    if( this != &m )
    {
        if( m.refcount )
            CV_XADD(m.refcount, 1);
        release();
        flags = m.flags;
        if( dims <= 2 && m.dims <= 2 )
        {
            dims = m.dims;
            rows = m.rows;
            cols = m.cols;
            step[0] = m.step[0];
            step[1] = m.step[1];
        }
        else
            copySize(m);
        data = m.data;
        datastart = m.datastart;
        dataend = m.dataend;
        datalimit = m.datalimit;
        refcount = m.refcount;
        allocator = m.allocator;
    }
    return *this;
}

static inline int CV_XADD(int* addr, int delta)
 {
     int tmp = *addr;
     *addr += delta; 
     return tmp;
 }

当B析构的时候，将引用计数减1，当前引用计数为1:

inline Mat::~Mat()
{
    release();
    if( step.p != step.buf )
        fastFree(step.p);
}

inline void Mat::release()
{
    if( refcount && CV_XADD(refcount, -1) == 1 )
        deallocate();
    data = datastart = dataend = datalimit = 0;
    size.p[0] = 0;
    refcount = 0;
}

当A析构的时候，引用计数为0，于是调用deallocate()，数据信息和头部信息一起清空。