[Qgis-developer] Make QGIS interact with LibreCAD.

Diego Gnesi Bartolani diego.gnesi at gmail.com
Sun Apr 21 03:52:45 PDT 2013


Yesterday I posted in a wrong thread, so I'm opening a new one. I'll
copy and paste the previous message and add some notes. Sorry for the

Original post:

I am an Italian archaeologist and an enthusiastic QGIS user. I do what
I can to spread the knowledge about QGIS and other open source
softwares to my university students (I am a contract professor at the
Basilicata University and some days ago I've become contract professor
at the Specialization School of Archeology of Udin, Venice and
Trieste). The main difficulty I have with open source GIS is the
advanced editing of vector layers. QGIS doesn't offer too much support
for digitizing, the CAD Tools plugin has some useful functions but is
far from being complete and the kind of interaction that it proposes
is not very productive and time-saving (it's not a critic: CAD Tools
saved my life a lot of time, and I want to thank the developers and
maintainers). Many times, I had to convert my vector layers to dxf and
back in order to perform some complex tasks (with all kind of
conversion problems, like the preservation of the attributes values,
I have some experience in programming (about ten years of
developement, mainly in C# with the .NET Framework, before I switched
to Linux), and I would like to try (only try, without promising
anything!!) to implement complete CAD tools and functions (If there
isn't and analogous project already active; in that case, nevermind!).

I think that developing a CAD plugin from scratch is very hard to me
and maybe not the better way to get good results, so I'm oriented to
try to augment the interaction between QGIS and an existing CAD
software (I'm thinking to LibreCAD). I thought to two possible

1. Use the LibreCAD library and source code to develop a C++ QGIS plugin;
2. Create a shapefile/spatialite/postgis driver for LibreCAD in order
to edit those formats inside LibreCAD.
3. Convert some layers of the QGIS project in a dxf file and back
(each QGIS layer in a different layer of the final dxf file).

I'm asking your opinion about which of those ways could be the faster
or the better to implement, or which one is impossible to undertake
according to your knowledge of the QGIS API.

My doubts are:

Strategy #1: Probably the ideal one, but does the implementation of
complex snaps or constraints against the features of other layers
(i.e. midpoint, parallels, etc.) require changes to the QGIS API, or
all these operations can be contained on a plugin? Is it better to
work directly on QGIS vector layers or to create a plugin layer, edit
it and then commit the changes to the vector layer? The conversion
could help me to manage geometries in the way LibreCAD wants. Is it
possible in your opinion to have an interactive shell (i.e. draw the
first point, and then inserting the coordinates of the second point)?

Strategy #2: Probably the easier, but I should find a way to disable
some LibreCAD functions (i.e. the drawing of splines, or the drawing
of points on a line layer) without disseminating my changes all over
LibreCAD's source files. For this kind of solution I'll ask to the
developers of LibreCAD.

Strategy #3: The problem is on committing the changes of the dxf file
to the original layer: how to specify that a line in the dxf file must
replace the geometry of and existing record of the original spatialite
layer? Maybe it's impossible, it doesn't seem to me that I can put on
a dxf file the necessary metadata to preserve a link to the original

What do you think? And what do you actually do when you have to draw
complex shapes on shapefiles or other vector layers?


- Antonio Locandro is compiling a list of required CAD features. It
would be very useful, so please Antonio, get me a link to this list
when it's finished.
- The more I explore QGIS and LibreCAD source codes, the more I'm
convincing that integrating LibreCAD's tools on QGIS is very
difficult, and it would be easier to develop those tools from scratch.
To get the best results in a short amount of time, I think that I
could work to make shapefiles and other vector layers editable in
LibreCAD (using GDAL). This solution has the issue that it doesn't
allow to snap to other QGIS layers, nevertheless it could be of great
help and available in a reasonable amount of time. Then, when I have
more practice with QGIS and LibreCAD APIs, I'll try to do something
better (i.e. develop a "QGIS driver" for LibreCAD in order to open the
whole QGIS project in LibreCAD, organized by layers).
I'm waiting for the answers to an analogous thread in LibreCAD's mailing list:


I think that I'm going to work on this project in a few weeks.

Let me know what do you think and if you have suggestions.

Diego Gnesi

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