Demo code: in-memory image encoding and npz data encoding

[s-trinh]

Why

• on another project, I have discovered the .npy or .npz file formats used in NumPy
• for a ROS project I have used cnpy for my needs: for a sequence of data, save some basic datatypes (int, bool, ...) and save some vector of double data
• it should allow exchanging data easily between C++ and Python/NumPy
• maybe this file format can also be useful here

But

• the ViSP library is already quite big and only if it is beneficial to other people this code can be added
• it requires C++11 and lots of works are still needed to clean the code
• I have used cnpy but I have not searched if better options exist or if there are already in ViSP the code to achieve the same thing

What

• the demo code is:
  • in tutorial-mb-generic-tracker-save.cpp, save the images + some data to be displayed into a npz file
  • in tutorial-mb-generic-tracker-read.cpp, open the npz file, display the images + the model of the teabox

How

• code from the cnpy library are directly included into the vpIoTools class for facility
• readPNGfromMem and writePNGtoMem functions are added to be able to save compressed image data using code from the stb library, the same can be done for jpg encoding
• warning: Java binding must be disable to be able to build the library correctly

[codecov]

Codecov Report

Attention: 164 lines in your changes are missing coverage. Please review.

Comparison is base (d70f9cd) 53.58% compared to head (4cfa003) 52.47%.
Report is 18 commits behind head on master.

Files	Patch %	Lines
modules/core/src/tools/file/vpIoTools.cpp	0.00%	147 Missing ⚠️
modules/core/include/visp3/core/vpIoTools.h	0.00%	9 Missing ⚠️
modules/io/src/image/vpImageIo.cpp	80.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1263      +/-   ##
==========================================
- Coverage   53.58%   52.47%   -1.12%     
==========================================
  Files         399      401       +2     
  Lines       50509    51826    +1317     
==========================================
+ Hits        27067    27196     +129     
- Misses      23442    24630    +1188     

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[fspindle]

@s-trinh Nice job. Can I merge or do you want to add other functionalities?

[s-trinh]

@fspindle Thanks!

To be done:

• read/write with OpenCV backend for color images
• handling of alpha channel with OpenCV backend
• check if RGBa to RGB conversion is needed with stb lib wrt. to this stride_bytes param
• check endianness handling [POSTPONED]
• should we export only the following functions instead of the whole lib? And potentially export more functions if needed?
  • npz_load
  • npy_save
• add documentation for npz_load and npy_save functions
• show how to check if a data variable is present in a npz file with C++ code?
• add some basic unit tests?
• add some demo code with Python?

I will ping you back when the PR will be ready.

[s-trinh]

@fspindle PR is ready from my side.

To quickly test:

• ./tutorial-mb-generic-tracker-save
• ./tutorial-mb-generic-tracker-read
• ./tutorial-mb-generic-tracker-full --video [...]/visp-images/mbt/cube/image%04d.png --model [...]/visp-images/mbt/cube_and_cylinder.cao --save test/tracking_%06d.png --save-results test/tracking_results.npz
• python3 PlotCameraTrajectory.py -p [...]/test/tracking_results.npz --npz -m [...]/visp-images/mbt/cube_and_cylinder.cao

Note:

• data are not compressed when using visp::cnpy::npz_save() (it is not equivalent to numpy.savez_compressed)
• you can open a npz file on Linux with an archive manager software
• this PR provides basic functions for loading/saving arrays of data into binary file, with easy compatibility with NumPy, as an alternative to simple file text. For more advanced needs, see Comparison of data-serialization formats

---

PR #1311 reintroduced some C++98 compatibility code.
Does it mean ViSP is no more a >= C++11 library (#1267)?

Should we add the minimum C++ version required for the ViSP library here? https://github.com/lagadic/visp/blob/master/README.md

---

TODO one day:

• looks like there is some bug when plotting cylinder object with PlotCameraTrajectory.py script (I think the error is how to draw a cylinder and coordinates)

[fspindle]

@s-trinh Thanks.

As far as c++98 is concerned, we need to reintroduce this functionality for a project we have with a partner. The idea is to make an effort to make it possible to build ViSP with c++98, but certainly not to port all the classes to c++98. In ViSP you can continue to develop and use c++11 and later features, just remember to disable them if c++98.

[s-trinh]

Is there any public news on this project?