occt/dox/technical_overview/technical_overview.md

Technical Overview

@tableofcontents

Open CASCADE Technology (OCCT) is an object-oriented C++ class library designed for rapid production of sophisticated domain-specific CAD/CAM/CAE applications.

A typical application developed using OCCT deals with two or three-dimensional (2D or 3D) geometric modeling in general-purpose or specialized Computer Aided Design (CAD) systems, manufacturing or analysis applications, simulation applications, or even illustration tools.

@figure{/technical_overview/images/technical_overview_over.png}

OCCT library is designed to be truly modular and extensible, providing C++ classes for:

• Basic data structures (geometric modeling, visualization, interactive selection and application specific services);
• Modeling algorithms;
• Working with mesh (faceted) data;
• Data interoperability with neutral formats (IGES, STEP);

The C++ classes and other types are grouped into packages. Packages are organized into toolkits (libraries), to which you can link your application. Finally, toolkits are grouped into seven modules.

This modular structure is illustrated in the diagram below.

@figure{/technical_overview/images/technical_overview_schema.png}

• @ref OCCT_TOVW_SECTION_2 "Foundation Classes" module underlies all other OCCT classes;
• @ref OCCT_TOVW_SECTION_3 "Modeling Data" module supplies data structures to represent 2D and 3D geometric primitives and their compositions into CAD models;
• @ref OCCT_TOVW_SECTION_4 "Modeling Algorithms" module contains a vast range of geometrical and topological algorithms;
  • @ref OCCT_TOVW_SECTION_4a "Mesh" toolkit from "Modeling Algorithms" module implements tessellated representations of objects;
• @ref OCCT_TOVW_SECTION_5 "Visualization" module provides complex mechanisms for graphical data representation;
• @ref OCCT_TOVW_SECTION_6 "Data Exchange" module inter-operates with popular data formats and relies on @ref OCCT_TOVW_SECTION_6a "Shape Healing" to improve compatibility between CAD software of different vendors;
• @ref OCCT_TOVW_SECTION_7 "Application Framework" module offers ready-to-use solutions for handling application-specific data (user attributes) and commonly used functionality (save/restore, undo/redo, copy/paste, tracking CAD modifications, etc).

In addition, @ref OCCT_TOVW_SECTION_8 "Open CASCADE Test Harness", also called Draw, provides an entry point to the library and can be used as a testing tool for its modules.

@section OCCT_TOVW_SECTION_2 Foundation Classes

Foundation Classes module contains data structures and services used by higher-level Open CASCADE Technology classes:

• Primitive types, such as Boolean, Character, Integer or Real;
• String classes that handle Unicode strings;
• Collection classes that handle statically or dynamically sized aggregates of data, such as arrays, lists, queues, sets and hash tables (data maps).
• Classes providing commonly used numerical algorithms and basic linear algebra calculations (addition, multiplication, transposition of vectors and matrices, solving linear systems etc).
• Fundamental types like color, date and time information;
• Primitive geometry types providing implementation of basic geometric and algebraic entities that define and manipulate elementary data structures.
• Exception classes that describe situations, when the normal execution of program is abandoned;

This module also provides a variety of general-purpose services, such as:

• Safe handling of dynamically created objects, ensuring automatic deletion of unreferenced objects (smart pointers);
• Standard and specialized memory allocators;
• Extended run-time type information (RTTI) mechanism maintaining a full type hierarchy and providing means to iterate over it;
• Encapsulation of C++ streams;
• Basic interpreter of expressions facilitating the creation of customized scripting tools, generic definition of expressions, etc.;
• Tools for dealing with configuration resource files and customizable message files facilitating multi-language support in applications;
• Progress indication and user break interfaces, giving a possibility even for low-level algorithms to communicate with the user in a universal and convenient way;
• and many others...

See the details in @ref occt_user_guides__foundation_classes "Foundation Classes User's Guide"

See also: our E-learning & Training offerings.

@section OCCT_TOVW_SECTION_3 Modeling Data

Modeling Data supplies data structures to implement boundary representation (BRep) of objects in 3D. In BRep the shape is represented as an aggregation of geometry within topology. The geometry is understood as a mathematical description of a shape, e.g. as curves and surfaces (simple or canonical, Bezier, NURBS, etc). The topology is a data structure binding geometrical objects together.

Geometry types and utilities provide geometric data structures and services for:

• Description of points, vectors, curves and surfaces:
  • their positioning in 3D space using axis or coordinate systems, and
  • their geometric transformation, by applying translations, rotations, symmetries, scaling transformations and combinations thereof.
• Creation of parametric curves and surfaces by interpolation and approximation;
• Algorithms of direct construction;
• Conversion of curves and surfaces to NURBS form;
• Computation of point coordinates on 2D and 3D curves;
• Calculation of extrema between geometric objects.

Topology defines relationships between simple geometric entities. A shape, which is a basic topological entity, can be divided into components (sub-shapes):

• Vertex -- a zero-dimensional shape corresponding to a point;
• Edge -- a shape corresponding to a curve and bounded by a vertex at each extremity;
• Wire -- a sequence of edges connected by their vertices;
• Face -- a part of a plane (in 2D) or a surface (in 3D) bounded by wires;
• Shell -- a collection of faces connected by edges of their wire boundaries;
• Solid -- a finite closed part of 3D space bounded by shells;
• Composite solid -- a collection of solids connected by faces of their shell boundaries;
• Compound -- a collection of shapes of arbitrary type.

Complex shapes can be defined as assemblies (compounds) of simpler entities.

See the details in @ref occt_user_guides__modeling_data "Modeling Data User's Guide"

3D geometric models can be stored in OCCT native BREP format. See @ref occt_user_guides__brep_wp "BREP Format Description White Paper" for details on the format.

See also: our E-learning & Training offerings.

@section OCCT_TOVW_SECTION_4 Modeling Algorithms

Modeling Algorithms module groups a wide range of topological and geometric algorithms used in geometric modeling. Basically, there are two groups of algorithms in Open CASCADE Technology:

• High-level modeling routines used in the real design;
• Low-level mathematical support functions used as a groundwork for the modeling API.

Low-level geometric tools provide the algorithms, which:

• Calculate the intersection of two curves, surfaces, or a curve and a surface;
• Project points onto 2D and 3D curves, points onto surfaces and 3D curves onto surfaces;
• Construct lines and circles from constraints;
• Construct free-form curves and surfaces from constraints (interpolation, approximation, skinning, gap filling, etc).

Low-level topological tools provide the algorithms, which:

• Tessellate shapes;
• Check correct definition of shapes;
• Determine the local and global properties of shapes (derivatives, mass-inertia properties, etc);
• Perform affine transformations;
• Find planes in which edges are located;
• Convert shapes to NURBS geometry;
• Sew connected topologies (shells and wires) from separate topological elements (faces and edges).

Top-level API provides the following functionality:

• Construction of Primitives:
  • Boxes;
  • Prisms;
  • Cylinders;
  • Cones;
  • Spheres;
  • Toruses.
• Kinematic Modeling:
  • Prisms -- linear sweeps;
  • Revolutions -- rotational sweeps;
  • Pipes -- general-form sweeps;
  • Lofting.

@figure{/technical_overview/images/0001.png "Shapes containing pipes with variable radius produced by sweeping"}

• Boolean Operations, which allow creating new shapes from the combinations of source shapes. For two shapes S1 and S2:
  • Common contains all points that are in S1 and S2;
  • Fuse contains all points that are in S1 or S2;
  • Cut contains all points in that are in S1 and not in S2.

See @ref occt_user_guides__boolean_operations "Boolean Operations" User's Guide for detailed documentation.

• Algorithms for local modifications such as:

  • Hollowing;
  • Shelling;
  • Creation of tapered shapes using draft angles;
  • Algorithms to make fillets and chamfers on shape edges, including those with variable radius (chord).

• Algorithms for creation of mechanical features, i.e. depressions, protrusions, ribs and grooves or slots along planar or revolution surfaces.

@figure{/technical_overview/images/0004.png}

See the details in @ref occt_user_guides__modeling_algos "Modeling Algorithms User's Guide".

See also: our E-learning & Training offerings.

@subsection OCCT_TOVW_SECTION_4a Mesh

Mesh toolkit provides the functionality to work with tessellated representations of objects in form of triangular facets. This toolkit contains:

• data structures to store surface mesh data associated to shapes and basic algorithms to handle them;
• data structures and algorithms to build triangular surface mesh from BRep objects (shapes);
• tools for displaying meshes with associated pre- and post-processor data (scalars or vectors).

Open CASCADE SAS also offers Advanced Mesh Products:

• Open CASCADE Mesh Framework (OMF)
• Express Mesh

@figure{/technical_overview/images/0003.png}

@section OCCT_TOVW_SECTION_5 Visualization

Visualization module provides ready-to-use algorithms to create graphic presentations from various objects: shapes, meshes, etc.

In Open CASCADE Technology visualization is based on the separation of CAD data and its graphical presentation. The module also supports a fast and powerful interactive selection mechanism.

Visualization module relies on the following key toolkits:

• TKV3d toolkit defines a high-level API called (Application Interactive Services* (AIS) for working with interactive objects.
• TKService toolkit defines a low-level API for managing and creating presentations from primitive arrays. This toolkit defines an abstraction layer for defining an arbitrary graphic driver responsible for actual rendering.
• TKOpenGl toolkit implements the graphic driver using OpenGL and OpenGL ES libraries.

While low-level API operates with primitive arrays (triangles, lines, points), the higher level includes services for building presentations for B-Rep shapes (shaded and wireframe). A comprehensive list of standard interactive objects includes topological shape, mesh presentation, various dimensions, manipulators and others. It provides a solid basis for rapid application development, while flexible and extensible API allows development of highly customized application-specific presentations.

Here are a few examples of OCCT Visualization features:

• Camera-driven view projection and orientation. Perspective, orthographic and stereographic projections are supported.

• Support of Common (diffuse/ambient/specular) and PBR metallic-roughness material models.

• Possibility to flexibly adjust appearance of dimensions in a 3D view. The 3D text object represents a given text string as a true 3D object in the model space.

• Definition of clipping planes through the plane equation coefficients. Ability to define visual attributes for cross-section at the level or individual clipping planes. In the image below different parts of the rocket are clipped with different planes and hatched. @figure{/technical_overview/images/0008.png, "Display of shape cross-section and dimensions"}

• Support of built-in and application-specific GLSL shaders. @figure{/technical_overview/images/0013.png, "Fragment shader implementing custom clipping surface"}

• Optimization of rendering performance through the algorithms of:

  • View frustum culling, which skips the presentation outside camera at the rendering stage;
  • Back face culling, which reduces the rendered number of triangles and eliminates artifacts at shape boundaries.

• Real-time ray tracing technique using recursive Whitted's algorithm and Bounded Volume Hierarchy effective optimization structure. @figure{/technical_overview/images/0002.png, "Real time visualization by ray tracing method"} @figure{/technical_overview/images/0012.png, "Simulation of a glass cover"}

For more details, see @ref occt_user_guides__visualization "Visualization User's Guide".

The visualization of OCCT topological shapes by means of VTK library provided by VIS component is described in a separate @ref occt_user_guides__vis "VTK Integration Services" User's Guide.

See also: our E-learning & Training offerings.

@section OCCT_TOVW_SECTION_6 Data Exchange

Data Exchange allows developing OCCT-based applications that can interact with other CAD systems by writing and reading CAD models to and from external data.

@figure{/technical_overview/images/0014.png,"Shape imported from STEP"}

Data Exchange is organized in a modular way as a set of interfaces that comply with various CAD formats: IGES, STEP, STL, VRML, etc. The interfaces allow software based on OCCT to exchange data with various CAD/PDM software packages, maintaining a good level of interoperability. This module handles various problems of interoperability between CAD systems, caused by differences in model validity criteria and requirements to internal representation.

• Standardized Data Exchange interfaces allow querying and examining the input file, converting its contents to a CAD model and running validity checks on a fully translated shape. The following formats are currently supported:
  • @ref occt_user_guides__step "STEP" (AP203: Mechanical Design, this covers General 3D CAD; AP214: Automotive Design; AP242).
  • @ref occt_user_guides__iges "IGES" (up to 5.3).
  • glTF 2.0 reader and writer.
  • OBJ mesh file reader.
  • VRML converter translates Open CASCADE shapes to VRML 1.0 files (Virtual Reality Modeling Language).
  • STL converter translates Open CASCADE shapes to STL files. STL (STtereoLithography) format is widely used for rapid prototyping (3D printing).
• @ref occt_user_guides__xde "Extended data exchange" (XDE) allows translating additional attributes attached to geometric data (colors, layers, names, materials etc).
• Advanced Data Exchange Components are available in addition to standard Data Exchange interfaces to support interoperability and data adaptation (also using @ref OCCT_TOVW_SECTION_6a "Shape Healing") with CAD software using the following proprietary formats:
  • ACIS SAT
  • Parasolid
  • DXF
  • IFC
  • JT

These components are based on the same architecture as interfaces with STEP and IGES.

@section OCCT_TOVW_SECTION_6a Shape Healing

Shape Healing library provides algorithms to correct and adapt the geometry and topology of shapes imported to OCCT from other CAD systems.

Shape Healing algorithms include, but are not limited to, the following operations:

• Analyze shape characteristics and, in particular, identify the shapes that do not comply with OCCT geometry and topology validity rules by analyzing geometrical objects and topology:
  • check edge and wire consistency;
  • check edge order in a wire;
  • check the orientation of face boundaries;
  • analyze shape tolerances;
  • identify closed and open wires in a boundary.
• Fix incorrect or incomplete shapes:
  • provide consistency between a 3D curve and its corresponding parametric curve;
  • repair defective wires;
  • fit the shapes to a user-defined tolerance value;
  • fill gaps between patches and edges.
• Upgrade and change shape characteristics:
  • reduce curve and surface degree;
  • split shapes to obtain C1 continuity;
  • convert any types of curves or surfaces to Bezier or B-Spline curves or surfaces and back;
  • split closed surfaces and revolution surfaces.

Each sub-domain of Shape Healing has its own scope of functionality:

Sub-domain	Description	Impact on the shape
Analysis	Explores shape properties, computes shape features, detects violation of OCCT requirements.	The shape itself is not modified.
Fixing	Fixes the shape to meet the OCCT requirements.	The shape may change its original form: modification, removal or creation of sub-shapes, etc.)
Upgrade	Improves the shape to fit some particular algorithms.	The shape is replaced with a new one, but geometrically they are the same.
Customization	Modifies the shape representation to fit specific needs.	The shape is not modified, only the mathematical form of its internal representation is changed.
Processing	Mechanism of shape modification via a user-editable resource file.

For more details, refer to @ref occt_user_guides__shape_healing "Shape Healing User's guide".

See also: our E-learning & Training offerings.

@section OCCT_TOVW_SECTION_7 Application Framework

Open CASCADE Application Framework (OCAF) handles Application Data basing on the Application/Document paradigm. It uses an associativity engine to simplify the development of a CAD application thanks to the following ready-to-use features and services:

• Data attributes managing the application data, which can be organized according to the development needs;
• Data storage and persistence (open/save);
• Possibility to modify and recompute attributes in documents. With OCAF it is easy to represent the history of modification and parametric dependencies within your model;
• Possibility to manage multiple documents;
• Predefined attributes common to CAD/CAM/CAE applications (e.g. to store dimensions);
• Undo-Redo and Copy-Paste functions.

Since OCAF handles the application structure, the only development task is the creation of application-specific data and GUIs.

OCAF differs from any other CAD framework in the organization of application data, as there the data structures are based on reference keys rather than on shapes. In a model, such attributes as shape data, color and material are attached to an invariant structure, which is deeper than the shapes. A shape object becomes the value of Shape attribute, in the same way as an integer number is the value of Integer attribute and a string is the value of Name attribute.

OCAF organizes and embeds these attributes in a document. OCAF documents, in their turn, are managed by an OCAF application.

For more details, see @ref occt_user_guides__ocaf "OCAF User's Guide".

See also: our E-learning & Training offerings.

@section OCCT_TOVW_SECTION_8 Draw Test Harness

Test Harness or Draw is a convenient testing tool for OCCT libraries. It can be used to test and prototype various algorithms before building an entire application. It includes:

• A command interpreter based on the TCL language;
• A number of 2D and 3D viewers;
• A set of predefined commands.

The viewers support operations such as zoom, pan, rotation and full-screen views.

The basic commands provide general-purpose services such as:

• Getting help;
• Evaluating a script from a file;
• Capturing commands in a file;
• Managing views;
• Displaying objects.

In addition, Test Harness provides commands to create and manipulate curves and surfaces (geometry) and shapes, access visualization services, work with OCAF documents, perform data exchange, etc.

You can add custom commands to test or demonstrate any new functionalities, which you develop.

For more details, see @ref occt_user_guides__test_harness "Draw Test Harness Manual".