为另一个分配矩阵中的特定范围创建 cv::Mat header

Question

我希望给定的 ColumnVector 的值是 right-shifted 并复制到另一个比 ColumnVector 大一个元素的 RowVector。我想通过将 columnVector 复制到 RowVector 中的某个范围来完成此操作。

// The given columnVector ( this code snippet is added to make the error reproducable)
int yDimLen = 26; // arbitrary value
cv::Mat columnVector(yDimLen, 1, CV_32SC1, cv::Scalar(13)); // one column AND yDimLen rows
for (int r = 0; r < yDimLen; r++)   // chech the values
{
    printf("%d\t", *(columnVector.ptr<int>(r)));
}

// Copy elemnets of columnVector to rowVector starting from column number 1
cv::Mat rowVector(1, yDimLen + 1, CV_32SC1, cv::Scalar(0));
cv::Mat rowVector_rangeHeader = rowVector.colRange(cv::Range(1, yDimLen + 1)); // header matrix, 1 in included AND (yDimLen + 1) is excluded
columnVector.copyTo(rowVector_rangeHeader);

// check the values
printf("\n---------------------------------\n");
for (int c = 0; c < yDimLen; c++)
{
    printf("%d\t", rowVector_rangeHeader.ptr<int>(0)[c]); // displays 13
}

printf("\n---------------------------------\n");
for (int c = 0; c < yDimLen + 1; c++)
{
    printf("%d\t", rowVector.ptr<int>(0)[c]); // displays 0 !!!
}

header矩阵不是指向包含rowVector的同一个内存地址吗？如果不是，如何将数据复制到一行的特定部分？

Answer 1

来自 the docs of cv::Mat::copyTo:

Before copying the data, the method invokes:
m.create(this->size(), this->type());
so that the destination matrix is reallocated if needed.

只要目标的形状或数据类型与源不匹配，就需要重新分配。在你的情况下，它的形状。为了证明这一点，让我们在 copyTo 调用周围添加一些跟踪：

std::cout << "Shapes involved in `copyTo`:\n";
std::cout << "Source: " << columnVector.size() << "\n";
std::cout << "Destination: " << rowVector_rangeHeader.size() << "\n";

columnVector.copyTo(rowVector_rangeHeader);

std::cout << "After the `copyTo`:\n";
std::cout << "Destination: " << rowVector_rangeHeader.size() << "\n";

输出：

Shapes involved in `copyTo`:
Source: [1 x 26]
Destination: [26 x 1]
After the `copyTo`:
Destination: [1 x 26]

解决方案很简单 -- reshape 源与目标匹配。

columnVector.reshape(1, 1).copyTo(rowVector_rangeHeader);

现在我们得到以下输出：

Shapes involved in `copyTo`:
Source: [1 x 26]
Destination: [26 x 1]
After the `copyTo`:
Destination: [26 x 1]

和rowVector内容如下：

[0, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13]

作为替代方案，您可以将 std::copy 与 MatIterator 一起使用。

std::copy(columnVector.begin<int>()
    , columnVector.end<int>()
    , rowVector_rangeHeader.begin<int>());

rowVector内容同上：

[0, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13]

为另一个分配矩阵中的特定范围创建 cv::Mat header

create cv::Mat header for a certain range within another allocated matrix

c++

opencv

opencv-mat