将cv :: Mat的行转换为std :: vector [英] Converting a row of cv::Mat to std::vector

问题描述

我有一个相当简单的问题：如何取一行 cv :: Mat 并获取 std :: vector ？ cv :: Mat 包含 doubles （为了问题的目的，它可以是任何简单的数据类型） p>

通过OpenCV文档只是很混乱，除非我的书签页面我找不到一个文档页面两次Googling，只有很多，不容易导航。

我发现 cv :: Mat :: at（..）可以访问Matrix元素，我记得从 C OpenCV ，有至少3种不同的方式来访问元素，所有他们用于不同的目的...不记得什么用于： /

因此，复制Matrix元素一个元素一定会起作用，我正在寻找一种效率更高的方法，

解决方案

OpenCV中的数据矩阵以行为主的顺序布置，使得每行被保证是连续的。这意味着您可以将行中的数据解释为纯C数组。以下示例直接来自文档：

  //计算正矩阵元素的总和
 //（假设M是双精度矩阵）
 double sum = 0; 
 for（int i = 0; i  {
 const double * Mi = M.ptr double（i）; 
 for（int j = 0; j  sum + = std :: max（Mi [j]，0。 
} 
  

因此，最有效的方法是将普通指针传递给 std :: vector ：

  //指向第i行的指针
 const double * p = mat.ptr double（i）; 
 
 //将数据复制到向量。注意（p + mat.cols）指向行的
 //结尾。 
 std :: vector< double> vec（p，p + mat.cols）; 
  

这肯定比使用 begin code>和 end（），因为那些涉及额外的计算以支持行之间的间隙。

I have a fairly simple question: how to take one row of cv::Mat and get all the data in std::vector? The cv::Mat contains doubles (it can be any simple datatype for the purpose of the question).

Going through OpenCV documentation is just very confusing, unless I bookmark the page I can not find a documentation page twice by Googling, there's just to much of it and not easy to navigate.

I have found the cv::Mat::at(..) to access the Matrix element, but I remember from C OpenCV that there were at least 3 different ways to access elements, all of them used for different purposes... Can't remember what was used for which :/

So, while copying the Matrix element-by-element will surely work, I am looking for a way that is more efficient and, if possible, a bit more elegant than a for loop for each row.

解决方案

Data in OpenCV matrices is laid out in row-major order, so that each row is guaranteed to be contiguous. That means that you can interpret the data in a row as a plain C array. The following example comes directly from the documentation:

// compute sum of positive matrix elements
// (assuming that M is double-precision matrix)
double sum=0;
for(int i = 0; i < M.rows; i++)
{
    const double* Mi = M.ptr<double>(i);
    for(int j = 0; j < M.cols; j++)
        sum += std::max(Mi[j], 0.);
}

Therefore the most efficient way is to pass the plain pointer to std::vector:

// Pointer to the i-th row
const double* p = mat.ptr<double>(i);

// Copy data to a vector.  Note that (p + mat.cols) points to the
// end of the row.
std::vector<double> vec(p, p + mat.cols);

This is certainly faster than using the iterators returned by begin() and end(), since those involve extra computation to support gaps between rows.

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