6 XML/YAML file storages. Writing to a file storage.
7 --------------------------------------------------
9 You can store and then restore various OpenCV data structures to/from XML (http://www.w3c.org/XML) or YAML
10 (http://www.yaml.org) formats. Also, it is possible store and load arbitrarily complex data structures, which include OpenCV data structures, as well as primitive data types (integer and floating-point numbers and text strings) as their elements.
12 Use the following procedure to write something to XML or YAML:
13 #. Create new :ocv:class:`FileStorage` and open it for writing. It can be done with a single call to :ocv:func:`FileStorage::FileStorage` constructor that takes a filename, or you can use the default constructor and then call :ocv:func:`FileStorage::open`. Format of the file (XML or YAML) is determined from the filename extension (".xml" and ".yml"/".yaml", respectively)
14 #. Write all the data you want using the streaming operator ``<<``, just like in the case of STL streams.
15 #. Close the file using :ocv:func:`FileStorage::release`. ``FileStorage`` destructor also closes the file.
17 Here is an example: ::
19 #include "opencv2/opencv.hpp"
24 int main(int, char** argv)
26 FileStorage fs("test.yml", FileStorage::WRITE);
28 fs << "frameCount" << 5;
29 time_t rawtime; time(&rawtime);
30 fs << "calibrationDate" << asctime(localtime(&rawtime));
31 Mat cameraMatrix = (Mat_<double>(3,3) << 1000, 0, 320, 0, 1000, 240, 0, 0, 1);
32 Mat distCoeffs = (Mat_<double>(5,1) << 0.1, 0.01, -0.001, 0, 0);
33 fs << "cameraMatrix" << cameraMatrix << "distCoeffs" << distCoeffs;
34 fs << "features" << "[";
35 for( int i = 0; i < 3; i++ )
39 uchar lbp = rand() % 256;
41 fs << "{:" << "x" << x << "y" << y << "lbp" << "[:";
42 for( int j = 0; j < 8; j++ )
43 fs << ((lbp >> j) & 1);
51 The sample above stores to XML and integer, text string (calibration date), 2 matrices, and a custom structure "feature", which includes feature coordinates and LBP (local binary pattern) value. Here is output of the sample:
57 calibrationDate: "Fri Jun 17 14:09:29 2011\n"
58 cameraMatrix: !!opencv-matrix
62 data: [ 1000., 0., 320., 0., 1000., 240., 0., 0., 1. ]
63 distCoeffs: !!opencv-matrix
67 data: [ 1.0000000000000001e-01, 1.0000000000000000e-02,
68 -1.0000000000000000e-03, 0., 0. ]
70 - { x:167, y:49, lbp:[ 1, 0, 0, 1, 1, 0, 1, 1 ] }
71 - { x:298, y:130, lbp:[ 0, 0, 0, 1, 0, 0, 1, 1 ] }
72 - { x:344, y:158, lbp:[ 1, 1, 0, 0, 0, 0, 1, 0 ] }
74 As an exercise, you can replace ".yml" with ".xml" in the sample above and see, how the corresponding XML file will look like.
76 Several things can be noted by looking at the sample code and the output:
78 The produced YAML (and XML) consists of heterogeneous collections that can be nested. There are 2 types of collections: named collections (mappings) and unnamed collections (sequences). In mappings each element has a name and is accessed by name. This is similar to structures and ``std::map`` in C/C++ and dictionaries in Python. In sequences elements do not have names, they are accessed by indices. This is similar to arrays and ``std::vector`` in C/C++ and lists, tuples in Python. "Heterogeneous" means that elements of each single collection can have different types.
80 Top-level collection in YAML/XML is a mapping. Each matrix is stored as a mapping, and the matrix elements are stored as a sequence. Then, there is a sequence of features, where each feature is represented a mapping, and lbp value in a nested sequence.
83 When you write to a mapping (a structure), you write element name followed by its value. When you write to a sequence, you simply write the elements one by one. OpenCV data structures (such as cv::Mat) are written in absolutely the same way as simple C data structures - using **``<<``** operator.
86 To write a mapping, you first write the special string **"{"** to the storage, then write the elements as pairs (``fs << <element_name> << <element_value>``) and then write the closing **"}"**.
89 To write a sequence, you first write the special string **"["**, then write the elements, then write the closing **"]"**.
92 In YAML (but not XML), mappings and sequences can be written in a compact Python-like inline form. In the sample above matrix elements, as well as each feature, including its lbp value, is stored in such inline form. To store a mapping/sequence in a compact form, put ":" after the opening character, e.g. use **"{:"** instead of **"{"** and **"[:"** instead of **"["**. When the data is written to XML, those extra ":" are ignored.
96 * A complete example using the FileStorage interface can be found at opencv_source_code/samples/cpp/filestorage.cpp
99 Reading data from a file storage.
100 ---------------------------------
102 To read the previously written XML or YAML file, do the following:
105 Open the file storage using :ocv:func:`FileStorage::FileStorage` constructor or :ocv:func:`FileStorage::open` method. In the current implementation the whole file is parsed and the whole representation of file storage is built in memory as a hierarchy of file nodes (see :ocv:class:`FileNode`)
108 Read the data you are interested in. Use :ocv:func:`FileStorage::operator []`, :ocv:func:`FileNode::operator []` and/or :ocv:class:`FileNodeIterator`.
111 Close the storage using :ocv:func:`FileStorage::release`.
113 Here is how to read the file created by the code sample above: ::
115 FileStorage fs2("test.yml", FileStorage::READ);
117 // first method: use (type) operator on FileNode.
118 int frameCount = (int)fs2["frameCount"];
121 // second method: use FileNode::operator >>
122 fs2["calibrationDate"] >> date;
124 Mat cameraMatrix2, distCoeffs2;
125 fs2["cameraMatrix"] >> cameraMatrix2;
126 fs2["distCoeffs"] >> distCoeffs2;
128 cout << "frameCount: " << frameCount << endl
129 << "calibration date: " << date << endl
130 << "camera matrix: " << cameraMatrix2 << endl
131 << "distortion coeffs: " << distCoeffs2 << endl;
133 FileNode features = fs2["features"];
134 FileNodeIterator it = features.begin(), it_end = features.end();
136 std::vector<uchar> lbpval;
138 // iterate through a sequence using FileNodeIterator
139 for( ; it != it_end; ++it, idx++ )
141 cout << "feature #" << idx << ": ";
142 cout << "x=" << (int)(*it)["x"] << ", y=" << (int)(*it)["y"] << ", lbp: (";
143 // you can also easily read numerical arrays using FileNode >> std::vector operator.
144 (*it)["lbp"] >> lbpval;
145 for( int i = 0; i < (int)lbpval.size(); i++ )
146 cout << " " << (int)lbpval[i];
153 .. ocv:class:: FileStorage
155 XML/YAML file storage class that encapsulates all the information necessary for writing or reading data to/from a file.
157 FileStorage::FileStorage
158 ------------------------
161 .. ocv:function:: FileStorage::FileStorage()
163 .. ocv:function:: FileStorage::FileStorage(const string& source, int flags, const string& encoding=string())
165 :param source: Name of the file to open or the text string to read the data from. Extension of the file (``.xml`` or ``.yml``/``.yaml``) determines its format (XML or YAML respectively). Also you can append ``.gz`` to work with compressed files, for example ``myHugeMatrix.xml.gz``. If both ``FileStorage::WRITE`` and ``FileStorage::MEMORY`` flags are specified, ``source`` is used just to specify the output file format (e.g. ``mydata.xml``, ``.yml`` etc.).
167 :param flags: Mode of operation. Possible values are:
169 * **FileStorage::READ** Open the file for reading.
171 * **FileStorage::WRITE** Open the file for writing.
173 * **FileStorage::APPEND** Open the file for appending.
175 * **FileStorage::MEMORY** Read data from ``source`` or write data to the internal buffer (which is returned by ``FileStorage::release``)
177 :param encoding: Encoding of the file. Note that UTF-16 XML encoding is not supported currently and you should use 8-bit encoding instead of it.
179 The full constructor opens the file. Alternatively you can use the default constructor and then call :ocv:func:`FileStorage::open`.
186 .. ocv:function:: bool FileStorage::open(const string& filename, int flags, const string& encoding=string())
188 :param filename: Name of the file to open or the text string to read the data from.
189 Extension of the file (``.xml`` or ``.yml``/``.yaml``) determines its format (XML or YAML respectively).
190 Also you can append ``.gz`` to work with compressed files, for example ``myHugeMatrix.xml.gz``.
191 If both ``FileStorage::WRITE`` and ``FileStorage::MEMORY`` flags are specified, ``source``
192 is used just to specify the output file format (e.g. ``mydata.xml``, ``.yml`` etc.).
194 :param flags: Mode of operation. See FileStorage constructor for more details.
196 :param encoding: Encoding of the file. Note that UTF-16 XML encoding is not supported currently and you should use 8-bit encoding instead of it.
199 See description of parameters in :ocv:func:`FileStorage::FileStorage`. The method calls :ocv:func:`FileStorage::release` before opening the file.
202 FileStorage::isOpened
203 ---------------------
204 Checks whether the file is opened.
206 .. ocv:function:: bool FileStorage::isOpened() const
208 :returns: ``true`` if the object is associated with the current file and ``false`` otherwise.
210 It is a good practice to call this method after you tried to open a file.
215 Closes the file and releases all the memory buffers.
217 .. ocv:function:: void FileStorage::release()
219 Call this method after all I/O operations with the storage are finished.
222 FileStorage::releaseAndGetString
223 --------------------------------
224 Closes the file and releases all the memory buffers.
226 .. ocv:function:: string FileStorage::releaseAndGetString()
228 Call this method after all I/O operations with the storage are finished. If the storage was opened for writing data and ``FileStorage::WRITE`` was specified
231 FileStorage::getFirstTopLevelNode
232 ---------------------------------
233 Returns the first element of the top-level mapping.
235 .. ocv:function:: FileNode FileStorage::getFirstTopLevelNode() const
237 :returns: The first element of the top-level mapping.
242 Returns the top-level mapping
244 .. ocv:function:: FileNode FileStorage::root(int streamidx=0) const
246 :param streamidx: Zero-based index of the stream. In most cases there is only one stream in the file. However, YAML supports multiple streams and so there can be several.
248 :returns: The top-level mapping.
251 FileStorage::operator[]
252 -----------------------
253 Returns the specified element of the top-level mapping.
255 .. ocv:function:: FileNode FileStorage::operator[](const string& nodename) const
257 .. ocv:function:: FileNode FileStorage::operator[](const char* nodename) const
259 :param nodename: Name of the file node.
261 :returns: Node with the given name.
264 FileStorage::operator*
265 ----------------------
266 Returns the obsolete C FileStorage structure.
268 .. ocv:function:: CvFileStorage* FileStorage::operator *()
270 .. ocv:function:: const CvFileStorage* FileStorage::operator *() const
272 :returns: Pointer to the underlying C FileStorage structure
275 FileStorage::writeRaw
276 ---------------------
277 Writes multiple numbers.
279 .. ocv:function:: void FileStorage::writeRaw( const string& fmt, const uchar* vec, size_t len )
281 :param fmt: Specification of each array element that has the following format ``([count]{'u'|'c'|'w'|'s'|'i'|'f'|'d'})...`` where the characters correspond to fundamental C++ types:
283 * **u** 8-bit unsigned number
285 * **c** 8-bit signed number
287 * **w** 16-bit unsigned number
289 * **s** 16-bit signed number
291 * **i** 32-bit signed number
293 * **f** single precision floating-point number
295 * **d** double precision floating-point number
297 * **r** pointer, 32 lower bits of which are written as a signed integer. The type can be used to store structures with links between the elements.
299 ``count`` is the optional counter of values of a given type. For example, ``2if`` means that each array element is a structure of 2 integers, followed by a single-precision floating-point number. The equivalent notations of the above specification are ' ``iif`` ', ' ``2i1f`` ' and so forth. Other examples: ``u`` means that the array consists of bytes, and ``2d`` means the array consists of pairs of doubles.
301 :param vec: Pointer to the written array.
303 :param len: Number of the ``uchar`` elements to write.
305 Writes one or more numbers of the specified format to the currently written structure. Usually it is more convenient to use :ocv:func:`operator <<` instead of this method.
307 FileStorage::writeObj
308 ---------------------
309 Writes the registered C structure (CvMat, CvMatND, CvSeq).
311 .. ocv:function:: void FileStorage::writeObj( const string& name, const void* obj )
313 :param name: Name of the written object.
315 :param obj: Pointer to the object.
317 See :ocv:cfunc:`Write` for details.
320 FileStorage::getDefaultObjectName
321 ---------------------------------
322 Returns the normalized object name for the specified name of a file.
324 .. ocv:function:: static string FileStorage::getDefaultObjectName(const string& filename)
326 :param filename: Name of a file
328 :returns: The normalized object name.
333 Writes data to a file storage.
335 .. ocv:function:: template<typename _Tp> FileStorage& operator << (FileStorage& fs, const _Tp& value)
337 .. ocv:function:: template<typename _Tp> FileStorage& operator << ( FileStorage& fs, const vector<_Tp>& vec )
339 :param fs: Opened file storage to write data.
341 :param value: Value to be written to the file storage.
343 :param vec: Vector of values to be written to the file storage.
345 It is the main function to write data to a file storage. See an example of its usage at the beginning of the section.
350 Reads data from a file storage.
352 .. ocv:function:: template<typename _Tp> void operator >> (const FileNode& n, _Tp& value)
354 .. ocv:function:: template<typename _Tp> void operator >> (const FileNode& n, vector<_Tp>& vec)
356 .. ocv:function:: template<typename _Tp> FileNodeIterator& operator >> (FileNodeIterator& it, _Tp& value)
358 .. ocv:function:: template<typename _Tp> FileNodeIterator& operator >> (FileNodeIterator& it, vector<_Tp>& vec)
360 :param n: Node from which data will be read.
362 :param it: Iterator from which data will be read.
364 :param value: Value to be read from the file storage.
366 :param vec: Vector of values to be read from the file storage.
368 It is the main function to read data from a file storage. See an example of its usage at the beginning of the section.
373 .. ocv:class:: FileNode
375 File Storage Node class. The node is used to store each and every element of the file storage opened for reading. When XML/YAML file is read, it is first parsed and stored in the memory as a hierarchical collection of nodes. Each node can be a “leaf” that is contain a single number or a string, or be a collection of other nodes. There can be named collections (mappings) where each element has a name and it is accessed by a name, and ordered collections (sequences) where elements do not have names but rather accessed by index. Type of the file node can be determined using :ocv:func:`FileNode::type` method.
377 Note that file nodes are only used for navigating file storages opened for reading. When a file storage is opened for writing, no data is stored in memory after it is written.
384 .. ocv:function:: FileNode::FileNode()
386 .. ocv:function:: FileNode::FileNode(const CvFileStorage* fs, const CvFileNode* node)
388 .. ocv:function:: FileNode::FileNode(const FileNode& node)
390 :param fs: Pointer to the obsolete file storage structure.
392 :param node: File node to be used as initialization for the created file node.
394 These constructors are used to create a default file node, construct it from obsolete structures or from the another file node.
399 Returns element of a mapping node or a sequence node.
401 .. ocv:function:: FileNode FileNode::operator[](const string& nodename) const
403 .. ocv:function:: FileNode FileNode::operator[](const char* nodename) const
405 .. ocv:function:: FileNode FileNode::operator[](int i) const
407 :param nodename: Name of an element in the mapping node.
409 :param i: Index of an element in the sequence node.
411 :returns: Returns the element with the given identifier.
416 Returns type of the node.
418 .. ocv:function:: int FileNode::type() const
420 :returns: Type of the node. Possible values are:
422 * **FileNode::NONE** Empty node.
424 * **FileNode::INT** Integer.
426 * **FileNode::REAL** Floating-point number.
428 * **FileNode::FLOAT** Synonym or ``REAL``.
430 * **FileNode::STR** Text string in UTF-8 encoding.
432 * **FileNode::STRING** Synonym for ``STR``.
434 * **FileNode::REF** Integer of type ``size_t``. Typically used for storing complex dynamic structures where some elements reference the others.
436 * **FileNode::SEQ** Sequence.
438 * **FileNode::MAP** Mapping.
440 * **FileNode::FLOW** Compact representation of a sequence or mapping. Used only by the YAML writer.
442 * **FileNode::USER** Registered object (e.g. a matrix).
444 * **FileNode::EMPTY** Empty structure (sequence or mapping).
446 * **FileNode::NAMED** The node has a name (i.e. it is an element of a mapping).
451 Checks whether the node is empty.
453 .. ocv:function:: bool FileNode::empty() const
455 :returns: ``true`` if the node is empty.
460 Checks whether the node is a "none" object
462 .. ocv:function:: bool FileNode::isNone() const
464 :returns: ``true`` if the node is a "none" object.
469 Checks whether the node is a sequence.
471 .. ocv:function:: bool FileNode::isSeq() const
473 :returns: ``true`` if the node is a sequence.
478 Checks whether the node is a mapping.
480 .. ocv:function:: bool FileNode::isMap() const
482 :returns: ``true`` if the node is a mapping.
487 Checks whether the node is an integer.
489 .. ocv:function:: bool FileNode::isInt() const
491 :returns: ``true`` if the node is an integer.
496 Checks whether the node is a floating-point number.
498 .. ocv:function:: bool FileNode::isReal() const
500 :returns: ``true`` if the node is a floating-point number.
505 Checks whether the node is a text string.
507 .. ocv:function:: bool FileNode::isString() const
509 :returns: ``true`` if the node is a text string.
514 Checks whether the node has a name.
516 .. ocv:function:: bool FileNode::isNamed() const
518 :returns: ``true`` if the node has a name.
523 Returns the node name.
525 .. ocv:function:: string FileNode::name() const
527 :returns: The node name or an empty string if the node is nameless.
532 Returns the number of elements in the node.
534 .. ocv:function:: size_t FileNode::size() const
536 :returns: The number of elements in the node, if it is a sequence or mapping, or 1 otherwise.
539 FileNode::operator int
540 ----------------------
541 Returns the node content as an integer.
543 .. ocv:function:: FileNode::operator int() const
545 :returns: The node content as an integer. If the node stores a floating-point number, it is rounded.
548 FileNode::operator float
549 ------------------------
550 Returns the node content as float.
552 .. ocv:function:: FileNode::operator float() const
554 :returns: The node content as float.
557 FileNode::operator double
558 -------------------------
559 Returns the node content as double.
561 .. ocv:function:: FileNode::operator double() const
563 :returns: The node content as double.
566 FileNode::operator string
567 -------------------------
568 Returns the node content as text string.
570 .. ocv:function:: FileNode::operator string() const
572 :returns: The node content as a text string.
577 Returns pointer to the underlying obsolete file node structure.
579 .. ocv:function:: CvFileNode* FileNode::operator *()
581 :returns: Pointer to the underlying obsolete file node structure.
586 Returns the iterator pointing to the first node element.
588 .. ocv:function:: FileNodeIterator FileNode::begin() const
590 :returns: Iterator pointing to the first node element.
595 Returns the iterator pointing to the element following the last node element.
597 .. ocv:function:: FileNodeIterator FileNode::end() const
599 :returns: Iterator pointing to the element following the last node element.
604 Reads node elements to the buffer with the specified format.
606 .. ocv:function:: void FileNode::readRaw( const string& fmt, uchar* vec, size_t len ) const
608 :param fmt: Specification of each array element. It has the same format as in :ocv:func:`FileStorage::writeRaw`.
610 :param vec: Pointer to the destination array.
612 :param len: Number of elements to read. If it is greater than number of remaining elements then all of them will be read.
614 Usually it is more convenient to use :ocv:func:`operator >>` instead of this method.
618 Reads the registered object.
620 .. ocv:function:: void* FileNode::readObj() const
622 :returns: Pointer to the read object.
624 See :ocv:cfunc:`Read` for details.
628 .. ocv:class:: FileNodeIterator
630 The class ``FileNodeIterator`` is used to iterate through sequences and mappings. A standard STL notation, with ``node.begin()``, ``node.end()`` denoting the beginning and the end of a sequence, stored in ``node``. See the data reading sample in the beginning of the section.
633 FileNodeIterator::FileNodeIterator
634 ----------------------------------
637 .. ocv:function:: FileNodeIterator::FileNodeIterator()
639 .. ocv:function:: FileNodeIterator::FileNodeIterator(const CvFileStorage* fs, const CvFileNode* node, size_t ofs=0)
641 .. ocv:function:: FileNodeIterator::FileNodeIterator(const FileNodeIterator& it)
643 :param fs: File storage for the iterator.
645 :param node: File node for the iterator.
647 :param ofs: Index of the element in the node. The created iterator will point to this element.
649 :param it: Iterator to be used as initialization for the created iterator.
651 These constructors are used to create a default iterator, set it to specific element in a file node or construct it from another iterator.
654 FileNodeIterator::operator*
655 ---------------------------
656 Returns the currently observed element.
658 .. ocv:function:: FileNode FileNodeIterator::operator *() const
660 :returns: Currently observed element.
663 FileNodeIterator::operator->
664 ----------------------------
665 Accesses methods of the currently observed element.
667 .. ocv:function:: FileNode FileNodeIterator::operator ->() const
670 FileNodeIterator::operator ++
671 -----------------------------
672 Moves iterator to the next node.
674 .. ocv:function:: FileNodeIterator& FileNodeIterator::operator ++ ()
676 .. ocv:function:: FileNodeIterator FileNodeIterator::operator ++ (int)
679 FileNodeIterator::operator --
680 -----------------------------
681 Moves iterator to the previous node.
683 .. ocv:function:: FileNodeIterator& FileNodeIterator::operator -- ()
685 .. ocv:function:: FileNodeIterator FileNodeIterator::operator -- (int)
688 FileNodeIterator::operator +=
689 -----------------------------
690 Moves iterator forward by the specified offset.
692 .. ocv:function:: FileNodeIterator& FileNodeIterator::operator +=( int ofs )
694 :param ofs: Offset (possibly negative) to move the iterator.
697 FileNodeIterator::operator -=
698 -----------------------------
699 Moves iterator backward by the specified offset (possibly negative).
701 .. ocv:function:: FileNodeIterator& FileNodeIterator::operator -=( int ofs )
703 :param ofs: Offset (possibly negative) to move the iterator.
706 FileNodeIterator::readRaw
707 -------------------------
708 Reads node elements to the buffer with the specified format.
710 .. ocv:function:: FileNodeIterator& FileNodeIterator::readRaw( const string& fmt, uchar* vec, size_t maxCount=(size_t)INT_MAX )
712 :param fmt: Specification of each array element. It has the same format as in :ocv:func:`FileStorage::writeRaw`.
714 :param vec: Pointer to the destination array.
716 :param maxCount: Number of elements to read. If it is greater than number of remaining elements then all of them will be read.
718 Usually it is more convenient to use :ocv:func:`operator >>` instead of this method.