Table of Contents

Introduction
Prerequisits
Usage
Input
Materials
Examples
Note on the STL file

Introduction

The tool performs conversion of CAD drawings into the GDML format (Geometry Decription Markup Language). GDML is the format used in particle-physics simulation packages, like GEANT4. Input files should be provided in stl format, which is supported by majority of CAD developer software.

How it works: as an input to the tool, a set of stl files is provided where each stl file represents a part of the geometry, corresponding to one single material (input_file_1.stl, input_file_2.stl, ...). The tool converts this set of stl file into the GDML model.

Prerequisits

• python2.7 or higher

Tested Operating Systems

• Mac OS X
• Linux
• Windows 10 (64 bit)

Usage

python  stl_gdml.py  out_name  input_file_1.stl ... input_file_N.stl

This will create:

• out_name.gdml - top level gdml
• input_file_1.gdml
• ...
• input_file_N.gdml

Note, even if there is one input .stl file, two .gdml files will be created:

• out_name.gdml
• input_file_1.gdml

Input

Input files should be in the ASCII STL format. Usually they are assigned an extension .stl. For some CAD software they may also be assigned an extension .ast. A typical ASCII STL file has a following structure:

solid CATIA STL
  facet normal  3.981976e-001 -3.981976e-001 -8.263641e-001
    outer loop
      vertex -4.024131e+002 -4.421275e+002  6.645833e+001
      vertex -4.092353e+002 -4.480332e+002  6.601675e+001
      vertex -4.028713e+002 -4.416693e+002  6.601675e+001
    endloop
  endfacet
....

Materials

Material name should be simply put in the stl file name. For instance, my_geometry_part_Aluminum.stl file will be parsed as made of aluminum. To display a full list of currently available materials see:

python stl_gdml.py --materials 

So far only basic materials are implemented. If not yet in the list, new materials can be added directly in the resulting gdml model following the example of already existing materials.

Examples

1. Basic example - conversion from stl to gdml:

Before running the example, download and unzip this repository. Then do:

cd cad-to-geant4-converter/Example
python ../stl_gdml.py test_model ./*.stl 

As a result, you will get a test_model.gdml file, which you can use as a GEANT4 geometry. Inside the test_model.gdml, one can see different sub-detector parts, each part corresponding to one intial .stl file.

2. Visualization of resulting model in GEANT4.

This example shows how to load and visualize in GEANT4 the gdml model that you obtained after conversion from stl. Running this example requires either GEANT4 installed on your computer, or having access to CERN lxplus. Instructions are given for the lxplus case. This example can be done independently of the first one.

• Login to lxplus machine:

ssh -XY yourcernlogin@lxplus.cern.ch

• Set up python2.7, GCC, GEANT4, xerces (required by GEANT4 gdml module)

source /afs/cern.ch/work/a/andrii/public/GDML/setup_geant4_gdml_xerces_cvmfs.sh

• Get and compile GEANT4 gdml example

cp -r /cvmfs/geant4.cern.ch/geant4/9.6.p03/share/examples/extended/persistency/gdml/G01 ~/
cd ~/G01
make

• Pick up example .stl model, put it into your home directory, and convert into gdml

cp -r /afs/cern.ch/work/a/andrii/public/GDML/Example/IBL_MODULE_EXAMPLE ~/
cd ~/IBL_MODULE_EXAMPLE
python2.7 [path_to_cad_gdml_converter]/stl_gdml.py TEST ./*.stl

• Get a nice GEANT4 visualization config file and put it where your model is, run visualization

cp /afs/cern.ch/work/a/andrii/public/GDML/Example/vis.mac ./
load_gdml TEST.gdml 

Finally you should be able to see your model as in the figure below:

Installing Gean4 with visualization functionality on a local computer

• download geant4.10.X.tar from http://geant4.web.cern.ch/geant4/support/download.shtml
• unpack geant4.10.X.tar in directory <myG4Path>.
• cd <myG4Path>
• tar -zxf geant4.10.X.tar
• mkdir geant4.10.X-build
• cd geant4.10.X-build
• cmake -DCMAKE_INSTALL_PREFIX=<myG4Path>/geant4.10.X-install -DGEANT4_INSTALL_DATA=ON -DGEANT4_USE_GDML=ON -DGEANT4_USE_OPENGL_X11=ON -DGEANT4_INSTALL_EXAMPLES=ON -DGEANT4_USE_QT=ON <myG4Path>/geant4.10.X
• make -j2 (for duo-core machine)
• make install

NOTE: For MAC OSX, you may need to specify the location of QT librarries:

cmake -DCMAKE_INSTALL_PREFIX=../geant4.10.05.p01-install -DGEANT4_INSTALL_DATA=ON -DGEANT4_USE_GDML=ON -DGEANT4_USE_OPENGL_X11=ON -DGEANT4_INSTALL_EXAMPLES=ON -DGEANT4_USE_QT=ON -DCMAKE_PREFIX_PATH=/usr/local/Cellar/qt/5.13.0/lib/cmake/Qt5Core/Qt5CoreConfig.cmake    ../geant4.10.05.p01 

Note on the STL file

The STL model can be created out of CAD drawing (.step files) in most of contemproary CAD packages, for example in CATIA. However, one should keep in mind that the level of details in STL is configurable.

At least in CATIA there is a specific module to create stl file and user can choose the level of details to implement. Also, there you can "decimate" facettes if you original stl file is too detailed. You can find information about this module on the CAD site of CERN:

CERN CAD Support

In the table on page 4, you can see that the stl file of the same model can vary from 45 Kb to 10,419 kb depending on parameters you have chosen.

In a more general way, it is always beneficial to remove details in the geometry before tessellation like holes, chanfers, fillets. The impact on the stl final model size is far from being negligible.