[Qgis-developer] Fwd: [SoC] GSoC GDAL DWG Support [Week 12 report, final]

Tue Aug 16 23:32:34 PDT 2016

Hi all,
does this have any impact on our developmant?
All the best.

-------- Messaggio Inoltrato --------
*Week 12 Report [Final] blogpost link
<http://gsoc2016gdaldwg.blogspot.ru/2016/08/week-12-report-final.html>*

Brief description of the idea.

The aim of my project was to extend GDAL supported formats with DWG.

The state of the project as it was BEFORE your GSOC.

There was a DWG support, but it was not built-in by default, and GDAL
Driver was based on third-party library Teigha (which is not X/MIT
licensed, not even close).

The addition that your project brought to the software.

Libopencad (GDAL CAD Driver engine)

Overview

Libopencad is a library written in C++11, which provides a way to
read/write CAD (DWG/DXF/DXFB) files. It was designed to have a uniformal
API to work with any CAD files. It has a base class - CADFile.
Inheriting this class it’s possible to create a ‘driver’ for any CAD
format, all you need to do - is to overwrite interface functions like
‘GetGeometry(index)’, and others. Now it has an implementation for
DWG2000 (R15), but only for read.

Library comes with ‘cadinfo’ utility, which prints out everything
library can get from file - header variables, CAD custom classes,
presented layers and geometries with their attributes.

Internal structure

Library presents CAD objects in different ways - base class CADObject
and other CAD...Object classes are exact analogues of how DWG format
stores information about its objects. CADGeometry, and other classes
which inherits it, are done for API access to CAD...Object classes.

Library supports 3 modes of reading - READ_ALL (means everything which
can be extracted from CAD file will be read) READ_FAST (skipping some
kind of meanless information - linetypes, CRC check, etc), READ_FASTEST
(only geometries reading, additional info is skipped).

When parsing the file, library does not store any information it does
not need. First, it reads file meta-information, and creates a file map.
It only stores some info about presented layers, and header variables.
Then, when calling application uses CADLayer.GetGeometry(index), it
actually reads a geometry from file, using prepared file map. Sometimes
it will be slower than having everything you have read in cache, but
gives you more flexibility what you want to store in memory, and what
you wont.

Library also has special classes which incapsulates I/O functions, so
it’s possible to reimplement them for your needs - by default it uses
std::fstream, but you can make it possible to work with network by make
a class inherited from CADFileIO.

Supported geometries list:

Point, Circle, Ellipse, Arc, Text, Solid, Spline, Line, Polyline 2D,
Polyline 3D, LWPolyline, Ray, Raster (Images), MText, MLine, XLine,
Polyface Mesh, 3DFace.

GDAL CAD Driver

Overview

GDAL CAD Driver uses libopencad as a datasource. Not everything that
libopencad can read from file is mapped into OGR infrastructure, but it
will be done in near future. Current features are:

 1.

    OpenOptions are presented with 2 options:

- MODE - READ_ALL/READ_FAST/READ_FASTEST (means the same as library
OpenOptions).

- ADD_UNSUPPORTED_GEOMETRIES_DATA (YES/NO) - unstable feature, if some
of the objects cannot be mapped into OGR representation (or its just not
implemented yet), it will be presented as a CADUnknown object -
OGRFeature without geometric representation, but with basic info -
geometry type, some additional params.

 2.

    CAD header variables are mapped into metadata.

 3.

    Unlike DXF driver, CAD driver stores vector data in the appropriate
    layers (not all entities in single layer).

 4.

    Raster subdatasets are also supported.

 5.

    Coordinate system is extracted from DWG file (if it is presented in
    accordance with ESRI Docs for DWG files), or from %FILENAME%.prj
    file in the same directory.

 6.

    CAD Geometry attributes mapping is completely done into
    OGRLayerDefn. So, any ACAD BlockReference attributes will be seen in
    OGRLayerDefn.

Add all the links (hopefully permanent) to access the relevant code and
documentation for the user to get started with testing your application.

During the planning stage, I decided to separate my project into 2
parts: separated CAD formats support library (named libopencad), and
GDAL driver, which uses it. Library can be used by itself for any
reason, and will be maintained after GSoC.

Links:

Libopencad repository
<https://github.com/sandyre/libopencad>(installation is described in
REAME.MD)

Libopencad + GDAL fork <https://github.com/sandyre/gdal_libopencad>

Blog with all reports and notices <http://gsoc2016gdaldwg.blogspot.ru/>

PR with my work into GDAL-trunk. <https://github.com/OSGeo/gdal/pull/145>

GDAL Wiki driver page <https://trac.osgeo.org/gdal/wiki/DWG_driver>

A slide / image should serve to show the main elements of your project.

Raster and vector layers by the same driver (left - ArcGIS, right -
QGIS): image link <https://pbs.twimg.com/media/Cm2N_-nVIAA8jBn.jpg>

A little comparison between original data source (ACAD) and QGIS with
new DWG driver: image link <https://pbs.twimg.com/media/CmwXIBnWcAAz22X.jpg>

Blockreference attributes mapping into OGRLayerDef: image link
<https://pbs.twimg.com/media/CocvxggWYAACiIg.jpg>

Best regards,

Alexandr Borzykh