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OpenCascade minimal working example

1. Introduction

This document describes how to set up and compile a small, yet complete, C++ project using OpenCascade to check if the installation was successful.

2. Creating the CMake project

We will assume that we are building our C++ project using CMake. In order to compile a C++ project using CMake from an IDE (such as CLion or Visual Studio) we just need to write a small CMakeLists.txt file. In our case, this file should look something like this:

# Set CMake version
cmake_minimum_required(VERSION 3.14)

# Set project name
set(project_name "open_cascade_minimal_working_example")
project(${project_name})

# Set the C++ standard to C++11
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")

# Set path to header files directories
include_directories("$ENV{OCCT_INCLUDE}")

# Set path to executable directories
link_directories("$ENV{OCCT_LIB}")

# Add source files to compile to the project
set(SOURCE_FILES main.cpp)
add_executable(${project_name}  ${SOURCE_FILES})

# Add OpenCascade libraries
target_link_libraries(${project_name} -Wl,--no-as-needed
	-lTKernel -lTKMath
	-lTKBRep -lTKG2d -lTKG3d -lTKGeomBase
	-lTKGeomAlgo -lTKTopAlgo -lTKPrim -lTKBO -lTKShHealing -lTKFillet -lTKBool -lTKOffset
	-lTKSTEPBase -lTKXSBase -lTKSTEPAttr -lTKSTEP209 -lTKSTEP -lTKIGES)

Where the environmental variables OCCT_INCLUDE and OCCT_LIB point to the directories containing the OpenCascade headers and compiled libraries, respectively.

Defining the environmental variables

You can define the environmental variables used in the CMakeLists.txt file adding (something similar) to these lines to your .bashrc file:

# Path to the OpenCascade installation directory
export OCCT_INCLUDE="/home/robertoa/my_software/open_cascade-7.4.0/open_cascade_installation/include/opencascade"
export OCCT_LIB="/home/robertoa/my_software/open_cascade-7.4.0/open_cascade_installation/lib"

Writing the C++ program

Now that we have prepared the CMakeLists.txt file of the project we can write our C++ code. The code snippet below is a minimal working example showing how to create a box using OpenCascade. You should be able to compile this example straight ahead if the installation was successful. Check out the detailed explanation of the minimal working example if you want to learn more about how OpenCascade works.

// ------------------------------------------------------------------------------------------------------------------- //
//
//  Minimal working example showing the use of OpenCascade
//  Author: Roberto Agromayor
//
// ------------------------------------------------------------------------------------------------------------------ //


// Include standard C++ libraries
#include <iostream>
#include <sys/stat.h>
#include <cmath>


// Include OpenCascade libraries
#include <gp_Pnt.hxx>
#include <TopoDS_Edge.hxx>
#include <TopoDS_Wire.hxx>
#include <TopoDS_Face.hxx>
#include <BRepBuilderAPI_MakeEdge.hxx>
#include <BRepBuilderAPI_MakeWire.hxx>
#include <BRepBuilderAPI_MakeFace.hxx>
#include <BRepPrimAPI_MakePrism.hxx>
#include <STEPControl_Writer.hxx>


// Define namespaces
using namespace std;


// ------------------------------------------------------------------------------------------------------------------ //
// Step file exporter
// ------------------------------------------------------------------------------------------------------------------ //
IFSelect_ReturnStatus
write_step_file(const string &relative_path, const string &model_name, const TopoDS_Shape &model_object) {

    // Create the .step writer object
    STEPControl_Writer step_writer;

    // Set the type of .step representation
    STEPControl_StepModelType step_mode = STEPControl_StepModelType::STEPControl_AsIs;

    // Create the output directory if it does not exist
    mkdir(relative_path.c_str(), 0777);     // 0007 is used to give the user permissions to read+write+execute

    // Get the full path to the step file as a C-string
    string temp = (relative_path + model_name + ".step");
    Standard_CString file_name = temp.c_str();

    // Write the .step file
    step_writer.Transfer(model_object, step_mode);
    IFSelect_ReturnStatus status = step_writer.Write(file_name);
    return status;

}


// ------------------------------------------------------------------------------------------------------------------ //
// Main body
// ------------------------------------------------------------------------------------------------------------------ //
int main() {


    // -------------------------------------------------------------------------------------------------------------- //
    // Define the geometry and topology of a prism
    // -------------------------------------------------------------------------------------------------------------- //

    // Define points that define the geometry of the base-square
    gp_Pnt myPoint1(0., 0., 0.);   // Bottom left
    gp_Pnt myPoint2(1., 0., 0.);   // Bottom right
    gp_Pnt myPoint3(1., 1., 0.);   // Top right
    gp_Pnt myPoint4(0., 1., 0.);   // Bottom left

    // Create the edges using gp_Pnt objects and BRepBuilderAPI_MakeEdge()
    TopoDS_Edge myEdge1 = BRepBuilderAPI_MakeEdge(myPoint1, myPoint2);
    TopoDS_Edge myEdge2 = BRepBuilderAPI_MakeEdge(myPoint2, myPoint3);
    TopoDS_Edge myEdge3 = BRepBuilderAPI_MakeEdge(myPoint3, myPoint4);
    TopoDS_Edge myEdge4 = BRepBuilderAPI_MakeEdge(myPoint4, myPoint1);

    // Create a closed wire giving the edges to the constructor
    TopoDS_Wire myWire = BRepBuilderAPI_MakeWire(myEdge1, myEdge2, myEdge3, myEdge4);

    // Make a face from the plane wire
    TopoDS_Face myFace = BRepBuilderAPI_MakeFace(myWire);

    // Make a prism sweeping the base-square
    gp_Vec sweep_direction(0.00, 0.00, 1.00);
    TopoDS_Shape myPrism = BRepPrimAPI_MakePrism(myFace, sweep_direction);


    // -------------------------------------------------------------------------------------------------------------- //
    // Export the model as a STEP file
    // -------------------------------------------------------------------------------------------------------------- //

    // Set the destination path and the name of the .step file
    string relative_path = "../output/";
    string file_name = "minimal_working_example";

    // Write the .step file
    write_step_file(relative_path, file_name, myPrism);


    // -------------------------------------------------------------------------------------------------------------- //
    // Visualize the geometry in a graphical user interface (for instance the FreeCAD GUI)
    // -------------------------------------------------------------------------------------------------------------- //
    string open_gui = "FreeCAD --single-instance " + relative_path + file_name + ".step";
    system(open_gui.c_str());


    return 0;


}