Add C++ and Python tutorials about how to use the NumPy NPZ format to…

… easily save arrays of data.

Add PlotRGBIrDepthData.py script to illustrate how to read NPZ data saved from example/device/framegrabber/saveRealSenseData.cpp program and easy interoperability.
Add tutorial code into tutorial/misc/npz/ folder.

doc/tutorial/misc/tutorial-npz.dox

@@ -0,0 +1,124 @@
+/**
+\page tutorial-npz Tutorial: Read / Save arrays of data from / to NPZ file format
+
+\tableofcontents
+
+\section tuto-npz-intro Introduction
+
+\note
+Please refer to the <a href="tutorial-read-write-NPZ-format.html">Python tutorial</a> for a short overview of the NPZ
+format from a Python point of view.
+
+
+The NPY / NPZ ("a zip file containing multiple NPY files") file format is a "standard binary file format in NumPy",
+appropriate for binary serialization of large chunks of data.
+A description of the NPY format is available
+<a href="https://numpy.org/doc/stable/reference/generated/numpy.lib.format.html">here</a>.
+
+The C++ implementation of this binary format relies on the <a href="https://github.com/rogersce/cnpy">rogersce/cnpy</a>
+library, available under the MIT license. Additional example code can be found directly from the
+<a href="https://github.com/rogersce/cnpy/blob/master/example1.cpp">rogersce/cnpy repository</a>.
+
+
+\subsection tuto-npz-intro-comparison Comparison with some other file formats
+
+The NPZ binary format is intended to provide a quick and efficient mean to read/save large arrays of data, mostly for
+debugging purpose. While the first and direct option for saving data would be to use file text, the choice of the NPZ
+format presents the following advantages:
+  - it is a binary format, that is the resulting file size will be smaller compared to a plain text file (especially
+with floating-point numbers),
+  - it provides exact floating-point representation, that is there is no need to bother with floating-point precision
+(see for instance the <a href="https://en.cppreference.com/w/cpp/io/manip/setprecision">setprecision</a> or
+<a href="https://en.cppreference.com/w/cpp/io/manip/fixed">std::hexfloat</a> functions),
+  - it provides some basic compatibility with the NumPy NPZ format
+(<a href="https://numpy.org/doc/stable/reference/generated/numpy.load.html">numpy.load</a> and
+<a href="https://numpy.org/doc/stable/reference/generated/numpy.savez.html">numpy.savez</a>),
+  - large arrays of data can be easily appended, with support for multi-dimensional arrays.
+
+On the other hand, the main disadvantages are:
+  - it is a non-human readable format, suitable for saving large arrays of data, but not for easy debugging,
+  - saving `string` data is not direct, since it must be treated as vector of `char` data,
+  - the current implementation only works on little-endian platform (which is the major endianness nowadays).
+
+You can refer to this Wikipedia page for an exhaustive comparison of
+<a href="https://en.wikipedia.org/wiki/Comparison_of_data-serialization_formats">data-serialization formats</a>.
+
+
+\section tuto-npz-hands-on Hands-on
+
+\subsection tuto-npz-hands-on-save-string How to save/read string data
+
+Saving C++ `std::string` data can be achieved the following way:
+  - create a `string` object and convert it to a `vector<char>` object:
+  \snippet tutorial-npz.cpp Save_string_init
+  - add and save the data to the `.npz` file, the identifier is the variable name and the `"w"` means `write`
+(`"a"` means `append` to the archive):
+  \snippet tutorial-npz.cpp Save_string_save
+
+
+Reading back the data can be done easily:
+  - load the data:
+  \snippet tutorial-npz.cpp Read_string_load
+  - the identifier is then needed,
+  - a conversion from `vector<char>` to `std::string` object is required:
+  \snippet tutorial-npz.cpp Read_string
+
+
+\note
+In the previous example, there is no need to save a "null-terminated" character since it is handled at reading using
+a specific constructor which uses iterators to the begenning and ending of the `string` data.
+Additional information can be found <a href="https://stackoverflow.com/a/45491652">here</a>.
+The other approach would consist to
+  - append the null character "\0" to the vector: "vec_save_string.push_back(`\0`);"
+  - and uses the constructor that accepts a pointer of data: "std::string read_string(arr_string_data.data<char>());"
+
+
+\subsection tuto-npz-hands-on-save-basic How to save basic data types
+
+Saving C++ basic data type such as `int32_t`, `float` or even `std::complex<double>` is straightforward:
+
+\snippet tutorial-npz.cpp Save_basic_types
+
+Reading back the data can be done easily:
+
+\snippet tutorial-npz.cpp Read_basic_types
+
+
+\subsection tuto-npz-hands-on-save-img How to save a vpImage
+
+Finally, one of the advantages of the `NPZ` is the possibility to save multi-dimensional arrays easily.
+As an example, we will save first a `vpImage<vpRGBa>`.
+
+Following code shows how to read an image:
+
+\snippet tutorial-npz.cpp Save_image_read
+
+Then, saving a color image can be achieved as easily as:
+
+\snippet tutorial-npz.cpp Save_image
+
+We have passed the address to the bitmap array, that is a vector of `vpRGBa`. The shape of the array is thus
+"height x width" since all basic elements of the bitmap are already of `vpRGBa` type (4 `unsigned char` elements).
+
+Reading back the image is done with:
+
+\snippet tutorial-npz.cpp Read_image
+
+The `vpImage` constructor accepting a `vpRGBa` pointer is used, with the appropriate image height and width values.
+
+Finally, the image is displayed.
+
+
+\subsection tuto-npz-hands-on-save-multi How to save a multi-dimensional array
+
+Similarly, the following code shows how to save a multi-dimensional array with a shape corresponding to `{H x W x 3}`:
+
+\snippet tutorial-npz.cpp Save_multi_array
+
+Finally, the image can be read back and displayed with:
+
+\snippet tutorial-npz.cpp Read_multi_array
+
+A specific conversion from `RGB` to `RGBa` must be done for compatibility with the ViSP `vpRGBa` format.
+
+*/

doc/tutorial/python/tutorial-read-write-NPZ-format.dox

@@ -0,0 +1,111 @@
+/**
+
+\page tutorial-read-write-NPZ-format Tutorial: NumPy NPY/NPZ file format for reading/writing large arrays of data
+
+\tableofcontents
+
+\section tutorial-read-write-NPZ-format-intro Introduction
+
+\note
+Please refer to the <a href="tutorial-npz.html">C++ tutorial</a> for an overview of the NPZ format and a quick usage
+from a C++ point of view.
+
+
+NumPy offers the possibility to save and read arrays of data in binary format. This is an alternative to the NumPy
+<a href="https://numpy.org/doc/stable/reference/generated/numpy.savetxt.html">`numpy.savetxt`</a> function, which
+allows the user to save 1D/2D arrays in plain text.
+
+The NPY format, and the NPZ format which is a collection of NPY data zipped into a single file, offer the following
+advantages:
+  - easy usage with <a href="https://numpy.org/doc/stable/reference/generated/numpy.savez.html">`numpy.savez`</a>
+and <a href="https://numpy.org/doc/stable/reference/generated/numpy.load.html">`numpy.load`</a> for saving and reading
+arrays of data,
+  - binary format contrary to using a plain text file, which reduces the file size,
+  - no data loss when saving, this can be problematic when dealing with floating-point numbers,
+  - easy access to the different saved variables since the returned loaded object is a dictionnary.
+
+In contrary, the main disadvantages are:
+  - it is a non-human readable format,
+  - it is meant to be use with arrays of basic data type, hierarchical structures of data are not suitables for instance.
+
+\note
+You can refer to this Wikipedia page for an exhaustive comparison of
+<a href="https://en.wikipedia.org/wiki/Comparison_of_data-serialization_formats">data-serialization formats</a>.
+\n You can refer to the following page for a more thorough description of the
+<a href="https://numpy.org/doc/stable/reference/generated/numpy.lib.format.html">NPY format</a>.
+
+
+\section tutorial-read-write-NPZ-format-examples Examples
+
+\subsection tutorial-read-write-NPZ-format-examples-quick Quick overview in Python
+
+The following code snippet illustrates how to save 1-D vector, multi-dimensional array and append data to file.
+
+\code{.py}
+
+#! python3
+# -*- coding: utf-8 -*-
+import numpy as np
+from tempfile import TemporaryFile
+import matplotlib.pyplot as plt
+
+def main():
+    # https://numpy.org/doc/stable/reference/generated/numpy.savez.html
+    outfile = TemporaryFile()
+    x_vec = np.arange(10)
+    sin_x = np.sin(x_vec)
+    np.savez(outfile, x=x_vec, y=sin_x)
+
+    _ = outfile.seek(0) # Only needed here to simulate closing & reopening file
+    npzfile = np.load(outfile)
+    print(f"npzfile.filesz: {npzfile.files}")
+
+    # append data to the file: https://stackoverflow.com/a/71183105
+    img = np.random.randint(low=0, high=256, size=(48, 64, 3), dtype=np.uint8)
+    print(f"img: {img.shape}")
+    data_dict = dict(npzfile)
+    data_dict["img"] = img
+    np.savez(outfile, **data_dict)
+
+    _ = outfile.seek(0) # Only needed here to simulate closing & reopening file
+    npzfile = np.load(outfile)
+    print(f"npzfile.filesz: {npzfile.files}")
+
+    plt.imshow(npzfile["img"])
+    plt.show()
+
+if __name__ == '__main__':
+    main()
+
+\endcode
+
+
+\subsection tutorial-read-write-NPZ-format-examples-realsense Demo: read and display data from RealSense sensors
+
+In this demo, we will first use \ref example/device/framegrabber/saveRealSenseData.cpp "saveRealSenseData.cpp" to
+save data on disk:
+  - save "[-s]" color "[-c]" infrared "[-i]" depth "[-d]" and pointcloud "[-p]" data on disk:
+    - \code ./saveRealSenseData -s -c -i -d -p \endcode
+  - use "[-e <pattern>]" to specify the filename pattern:
+    - \code ./saveRealSenseData -s -c -i -d -p -e %06d \endcode
+  - use "[-o <output folder>]" to specify the output folder, a folder with the current timestamp will be
+automatically created inside it:
+    - \code ./saveRealSenseData -s -c -i -d -p -o output_dir \endcode
+  - use "[-C]" to save data on user click:
+    - \code ./saveRealSenseData -s -c -i -d -p -C \endcode
+  - use "[-f <fps>]" to specify the acquisition framerate:
+    - \code ./saveRealSenseData -s -c -i -d -p -f 60 \endcode
+  - use "[-b]" to force depth and pointcloud data to be saved in little-endian binary format:
+    - \code ./saveRealSenseData -s -c -i -d -p -b \endcode
+  - use "[-z]" to save pointcloud data in NumPy NPZ format (if this option is not passed and ViSP is not built with
+the PCL library as dependency, the NPZ format is used by default, unless the "[-b]" option is passed):
+    - \code ./saveRealSenseData -s -c -i -d -p -z \endcode
+
+\note
+Saving pointcloud data is very time consuming. If you need acquisition data to be as close as possible to the camera
+framerate, you can save instead the depth data and compute the 3D pointcloud later using the stereo-camera parameters.
+
+Then, you can use the PlotRGBIrDepthData.py Python script to display the data:
+  - \code python3 PlotRGBIrDepthData.py -i <folder> \endcode
+
+*/

doc/tutorial/tutorial-users.dox

@@ -186,6 +186,7 @@ This page introduces the user to other tools that may be useful.
 - \subpage tutorial-json <br>This tutorial explains how to read and save data in the portable JSON format. It focuses on saving the data generated by a visual servoing experiment and exporting it to Python in order to generate plots.
 - \subpage tutorial-synthetic-blenderproc <br> This tutorial shows you how to easily generate synthetic data from the 3D model of an object and obtain various modalities. This data can then be used to train a neural network for your own task.
 - \subpage tutorial-spc <br> This tutorial shows you how to monitor if a signal is "in control" using Statistical Process Control methods.
+- \subpage tutorial-npz <br> This tutorial shows you how to read / save arrays of data from / to NPZ file format, a binary format compatible with the NumPy library.
 */
 
 /*! \page tutorial_munkres Munkres Assignment Algorithm

doc/tutorial/tutorial_python.dox

@@ -3,4 +3,7 @@
 This page introduces the user to the way to exploit ViSP with Python.
 
 - \subpage tutorial-install-python-bindings <br>In this tutorial you will learn how to install ViSP Python bindings.
+
+- \subpage tutorial-read-write-NPZ-format <br>In this tutorial you will learn how to use NumPy NPY/NPZ file format
+to read and save large arrays of data.
 */