8.1. Design Notes Questions & Answers¶
Originally, Patro was a Python implementation of the Valentina pattern making software, which only focus to implement the core engine and not the graphical user interface.
A pattern (in flat pattern design) is build from geometric operations which can be turned to a computer program and is thus a field of applications of CAD (Computer Aided Design). It corresponds more precisely to parametric modelling with dedicated features to fashion modelling and manufacturing.
8.1.1. What is my opinion on Valentina / Seamly2D ?¶
Valentina project was started in 2013 by Roman Telezhinsky (Ukraine) as C++ developer and Susan Spencer (USA) as community manager. Susan previously developed an Inkscape Python plugin to generate pattern, see http://www.taumeta.org.
In late 2017 (see http://libregraphicsworld.org/blog/entry/valentina-seamly2d), Susan and Roman decided to fork the project in two separate projects. Susan renamed the project to Seamly2D. In August 2018, Valentina is still but slowly developed by Roman and Seamly2D looks like a dead project. Roman seems disconnected with the user community and Susan seems unable to do something excepted making noises and complaining about Roman (cf. Seamly2D forum and a lot of nearly empty web site etc.). In summary, it could be Roman is not the perfect developer boy, but it coded a lot of lines of code and released a free software parametric pattern making software. Concerning Susan, many things are unclear on their past relations, what she did / doing, what is her vision, on is ability to communicate, manage and technical expertise.
Valentina is coded in C++ and use the Qt framework. Qt is definitely the best choice for a GUI actually. Notice Valentina uses Qt4 Widgets and Scene Graph, thus it could require a costly upgrade to Qt5 QML (which is a common upgrade issue). The GUI of Valentina represents a huge cost, probably the main part of Valentina.
Actually Valentina has some design flaws, which is usual for a young project developed with a few budget.
For example, since Valentina is designed as a kind of WYSIWYG software, we cannot modify a pattern to a temporally inconsistent state. For example Valentina will not accept to have a line defined by point that don’t exist anymore. For a computer program, your are free to save something which is broken and fix things later. However for a GUI parametric modeller this flexibility is more challenging. Software like Fusion 360 implements a cache for dangling objects.
8.1.2. Can we implement such a tool as a plugin of another software ?¶
Inkscape is a well known free software SVG editor which support Python scripting and plugin. However it is not designed for CAD purpose and not for professional use actually. This solution is thus flawed.
FreeCAD is a free software 3D parametric modeller designed for CAD which also support Python. This solution could be an option, in particular to implement 3D features. However a 3D CAD software is very complex and challenging, moreover it is not competitive with professional software which are available for “free” in some conditions, thus this solution is risky.
8.1.3. What is the requirements of a pattern drafting software ?¶
The core functionality of a CAD system for pattern drafting consists of these two software components :
- an open file format to store and exchange the pattern,
- a geometric engine to compute the pattern, e.g. to generate the layout of each fabric’s piece of a garment.
The XML language is a natural candidate to define an open file format to store and exchange the pattern. Valentina uses XML to sore measurements in .vit files and patterns in .val files.
Another solution to define and store a pattern is to use a programming language, it can be a dedicated language or any programming language with a dedicated API. Many graphical languages was invented for specific usages, e.g. PostScript for printer, Metafont and MetaPost for publishing, G-code for machining etc.
Usually the geometric operations of a pattern are simple in comparison to the requirements of a mechanical or electronic CAD software. In first hand it is only 2D and the number of operations should be handled smoothly by a computer of these days, whereas it is still challenging for other domains.
Finally, a pattern drafting software requires an efficient graphical user interface so as to be used by fashion designers and not only by hackers. This software component is more challenging in therms of software engineering, i.e. in therms of design and cost.
8.1.4. Why Python is a good language for this library ?¶
The Python language has a large audience in engineering, due to its canonical syntax and richness of its ecosystem (scientific libraries).
Python is a high level language and thus more productive.
Python is used as scripting language to extend many softwares, in particular the famous open source 3D creation suite Blender, the parametric 3D modeller FreeCad as well as the SVG editor Inkscape. Moreover the 3D human model generator MakeHuman is written in Python.
Python as other dynamic languages is able to evaluate code on the fly which provide an expression evaluator for free. This feature is even more pertinent in our case because of the canonical nature of the syntax of Python which is natural to somebody initiated to a basic mathematical language level.
8.1.5. Could we implement a full software using Python ?¶
But up to now Python has of course some drawbacks!
Its main drawback is due to the fact the standard interpreter cannot execute more than one Python bytecode thread at once, this limitation so called Global Interpreter Lock is required for implementation simplicity.
Despite a GUI implemented in PyQt is almost of the time more faster than the human perception on a computer of these days. It can be sometime difficult to overcome latency arising from the software stack. Thus yes we can do it, but it could requires some tricks to achieve the performance of a C++ application.
8.1.6. How to generate drawings in standard format like PDF or SVG ?¶
SVG is not difficult to generate from Python since it is based on XML. However the PDF format is more challenging, for efficiency reason PDF is a binary format and is thus much more complicated than PostScript which is a true programming language.
There is several possibilities to generate PDF.
The most disturbing one is to use the LaTeX publishing system in combination with the PGF package which provide an amazing graphical language on top of LaTeX. This solution could terrify many peoples, but it do the job very well for text and graphics. However user must install a LaTeX environment from their Linux distribution or using the TexLive distribution.
A more conventional solution requires a library that can generate PDF from standard graphical operations. Some libraries featuring that are :
8.1.7. What is the requirements of a geometric engine ?¶
The requirements for a geometric engine are :
- Must support 2D, but 3D is not required
- Must handle smoothly a low complexity graphic scene, i.e. a pattern will never contains thousands of graphical entities like in mechanic or electronic PCB
- Required precision is low : a millimetre unit with one digit of precision is enough for pattern accuracy. However we need more digits for computing so as to don’t accumulate errors and for computer display so as to zoom correctly the pattern (e.g. 3 digits to zoom up to 100x).
- Is exact numerical computation useful in our case ? cf. Computational Geometry Algorithms Library (CGAL)
- We could support symbolic computation, excepted when a computation is only numerical.