What are BREP and CSG | Difference between BREP and CSG
Table of Content Constructive Solid Geometry Boundary Representation Differences between BREP and CSG The geometric modeling technique has revolutionized the design and manufacture of products to a great extent. Although there have been various ways of representing an object, the most commonly used modeling technique is Solid Modelling. Boundary Representation modeling and Constructive Solid Geometry modeling are the two main ways to express solid models. Constructive Solid Geometry Constructive solid geometry or C-REP/CREP, previously known as computational binary solid geometry, is a reliable modeling technique that allows the creation of a complex object from simple primitives using Boolean operations. It is based on the fundamental that a physical object can be divided into a set of primitives or basic elements combined in a particular order by following a set of rules (Boolean operations) to create an object. Typically, they are objects of simple shapes such as cuboids, cylinders, prisms, pyramids, spheres, and cones. CSG cannot represent fillets, chamfers, and other context-based features. The primitives are considered valid CSG models, where each primitive is bounded by orientable surfaces (Half-spaces). These simple primitives are in generic form and must be confirmed by the user to be used in the design. The primitive may require scaling, translation, and rotation transformations to be assigned a coveted position. There are two kinds of CSG schemes: Primitive-based CSG: A popular CSG scheme based on bounded solid primitives, R-sets.Half-space-based CSG: This CSG scheme uses unbounded Half-spaces. Bounded solid primitives and their boundaries are considered composite half-spaces and the surfaces of the component half- spaces, respectively. Some attributes of CSG are as follows: Boundary Representation In solid modeling and computer-aided design, boundary representation or B-rep / BREP—is the process of representing shapes using the limits. Here a solid is described as a collection of connected surface elements. BREP was one of the first computer-generated representations to represent three- dimensional objects. BREP defines an object by its spatial boundaries. It details the points, edges, and surfaces of a volume. BREP can also be explained in terms of cell domain combination. A cell is a connected limitation of the underlying geometry. There are four kinds of cells as per the spatial dimension they inhabit: A domain is a set of connected cells grouped to define boundaries. Fields define various components inside a non-manifold object.Boundary representation of models consists of two kinds of information:Topology: The main topological entities are faces, edges, and vertices.Geometry: The main geometrical entities are surfaces, curves, and points. The topological and geometrical entities are intertwined in a way where: BREP comes with its share of advantages and disadvantages, which are: Differences between BREP and CSG Boundary Representation(BREP) Constructive Solid Geometry(CSG) BREP describes only the oriented surface of a solid as a data structure composed of vertices, edges, and faces. A solid is represented as a Boolean expression of primitive solid objects of a simpler structure. A BREP object is easily rendered on a graphic display system. A CSG object is always valid because its surface is closed and orientable and encloses a volume, provided the primitives are authentic in it. Basic operations include reviewing the possible surface types, the winged-edge representation schema, and the Euler operators for BREP. Basic operations include classifying points, curves, and surfaces concerning a solid, detecting redundancies in the representation, and approximating CSG objects systematically.
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Faceted Modeling and NURBS
Modern CAD systems and CAD packages enable designers to model objects and retrieve them in their formats. Some formats are interchangeable while some enforce restrictions, upon which, it becomes difficult to transfer an object model from one form to another. This article describes some of the most used CAD formats in the industry. But before we look into various CAD formats, it is essential to understand the concept of Faceted geometry and Analytic geometry (NURBS). FACETED GEOMETRY Faceted geometry, also known as discrete geometry, are models which consist of groups of polygons which is often triangles. Most Computer-Aided Design (CAD) systems typically use continuous surface and edge definitions based on NURBS. CAE simulations break down this NURBS representation into facets by a process known as meshing. The faceted models are quite appealing to engineering marketing, as such simulations are less bothered with exact physical reality and tend to emphasize on creating eye-catching visuals, such as airflow over a car, which can be incorporated into a marketing brochure. File formats typically used for faceted models are: .3ds, .dxf, .obj, .stl (Stereolithography). Almost all the faceted formats, except for STL, reflect material properties such as glass and metal by providing groupings of facets. However, such groupings are inadequate for a CAE simulation. ANALYTIC GEOMETRY (NURBS) NURBS or Non-uniform rational basis spline describes curves and surfaces with mathematical functions, and form the most common analytic geometry representations. The NURBS geometry has unlimited resolution. The NURBS definition defines the location of the boundary points and uses control points with slope definition to determine the internal shape of curves and surfaces, thereby enabling a great deal of flexibility. NURBS geometry is typically produced in CAD systems such as CATIA, Pro/Engineer, Solidworks, NX, etc. A significant drawback of NURBS geometry is that they are generally specific to the CAD packages that created them, and interchanging formats can be error-prone and inaccurate. DIFFERENCE BETWEEN FACETED GEOMETRY AND ANALYTIC GEOMETRY (NURBS) Faceted geometry NURBS geometry Facets are always guaranteed to comply with the original definition In NURBS geometry, different levels of model detail are created without losing fidelity Faceted geometry describes a shape as a mesh, points usually connected by triangles Analytic geometry defines curves and surfaces with mathematical functions Faceted geometry has limited resolution NURBS geometry has unlimited resolution Evaluating a faceted surface, one can get a shape defined by linear interpolation between known discrete points One can assess a NURBS surface anywhere and get coordinates lying on the surface Simple definition Includes topology Cons of Faceted geometry Cons of NURBS geometry Fixed resolution More computing intense No topology High data exchange Although faceted geometry has its use, NURBS geometry is superior for design and manufacturing processes. Due to the high demands on geometric precision, NURBS geometry finds its place in CAE applications. But if the modeling requirements ask for stunning visuals, faceted models are worth giving a try.
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Types of Geometric Modeling
Table of Content Solid Modeling Surface Modeling Wireframe Modeling The previous edition gave a brief introduction to Geometric Modeling and its features. Geometric modeling is the mathematical representation of an object’s geometry. It incorporates the use of curves to create models. It can be viewed either in 2D or 3D perspective. The general design is applied to different geometric structures, including sets and graphs. This article introduces geometric models which represent geometric objects and their properties mathematically. They can be viewed as collections of squares of different colors. On the other hand, geometric shapes can be considered mathematical equations. Geometric models can represent data in context, such as a digital image. Regardless of the modeling approach, all students should understand how CAD software works. A basic understanding of geometric modeling is essential for any aspiring designer. In contrast, button- push robots often neglect this aspect of CAD training. However, preparing students with the fundamental knowledge of geometric modeling is also critical for their competitive edge. This edition details the primary types of geometric modeling. Geometric modeling can be classified into the following: Solid Modeling Also known as volume modeling, this is the most widely used method, providing a complete description of solid modeling. Solid modeling defines an object by its nodes, edges, and surfaces; therefore, it gives a perfect and explicit mathematical representation of a precisely enclosed and filled volume. Solid modeling requires topology rules to guarantee that all covers are stitched together correctly. This geometry modeling procedure is based upon the “Half-Space” concept. A solid model begins with a solid, which is then stitched together using topology rules. Solid modeling has many benefits, including improved visualization and functional automation. CAD software can quickly calculate the actual geometry of complex shapes. A cube, for example, has six faces, a radius of 8.4 mm, and many radii. It has many angles and a shallow pyramid on each face. By applying solid modeling concepts, CAD programs can quickly calculate these attributes for a given cube. Similarly, a cube with rounded edges has many radii and faces. There are two prevalent ways of representing solid models:Constructive solid geometry: Constructive solid geometry combines primary solid objects (prism, cylinder, cone, sphere, etc.). These shapes are either added or deleted to form the final solid shape.Boundary representation: In boundary representation, an object’s definition is determined by its spatial boundaries. It describes the points, edges, and surfaces of a volume and issues the command to rotate and sweep bind facets into a third-dimensional solid. The union of these surfaces enables the formation of a surface that explicitly encloses a volume. Solid Modeling is the most widely used geometric modeling in three dimensions, and it serves the following purpose: Different solid modeling techniques are as follows: Surface Modeling Surface modeling represents the solid appearing object. Although it is a more complicated representation method than wireframe modeling, it is not as refined as solid modeling. Although surface and solid models look identical, the former cannot be sliced open the way solid models can be. This model makes use of B-splines and Bezier for controlling curves. When polygons or NURBS represent surfaces, the computer can convert the commands into mathematical models. These models are saved in files and can be opened for editing and analysis at any time. The process of importing models from other programs is often complex and problematic, resulting in ambiguous results. In contrast, surface modeling allows for precise changes to difficult surfaces. A typical surface modeling process involves the following steps: Surface modeling is used to: Geometric surface modeling has proven to be extremely useful in computer graphics. Multiresolution modeling involves the generation of various surfaces at different levels of detail and accuracy. The resulting surfaces are then applied to various problems, including general surface estimation from structured and unstructured data. One such application is a subdivision surface, which begins from a simple primitive and gradually adds tools and details. Wireframe Modeling A wireframe model is composed of points, lines, curves, and surfaces connected by point coordinates. Because the model is not solid, it is difficult to visualize, but it helps generate simple geometric shapes. The resulting model contains information on every object’s point, edge, face, and vertices. This model is beneficial for creating an orthographic isometric or perspective view. The lines within a wireframe connect to create polygons, such as triangles and rectangles, representing three-dimensional shapes when bound together. The outcome may range from a cube to a complex three-dimensional scene with people and objects. The number of polygons within a model indicates how detailed the wireframe 3D model is. Wireframe modeling helps in matching a 3D drawing model to its reference. Planar objects can be moved to a 3D location once they have been created. It allows the creator to match the vertex points to align with the desired reference and see the reference through the model. Although Wireframe modeling is a quick and easy way to demonstrate concepts, creating a fully detailed, precisely constructed model for an idea can be highly time-consuming. If it does not match what was visualized for the project, all that time and effort is wasted. In wireframe modeling, one can skip the detailed work and present a very skeletal framework that is simple to create and apprehensible to others. Using a wireframe model as a reference geometry will help you create a solid or surface model that follows a definite shape. In this way, you can easily visualize your model and select objects. The wireframe model is decomposed into a series of simple wireframes and the desired face topology.
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Point Clouds | Point cloud formats and issues
Table of content Different 3D point cloud file formats Challenges with point cloud data Whether working on a renovation project or making information data about an as-built situation, it is understandable that the amount of time and energy spent analyzing the object/project can be pretty debilitating. Technical literature over the years has come up with several methods to make a precise approach. But inarguably, the most prominent method is the application of Point Clouds. 3D scanners gather point measurements from real-world objects or photos for a point cloud that can be translated to a 3D mesh or CAD model. But what is a Point Cloud? A standard definition of point clouds would be – A point cloud collection of data points defined by a given coordinates system. In a 3D coordinates system, for example, a point cloud may determine the shape of some real or created physical system. Point clouds are used to create 3D meshes and other models used in 3D modeling for various fields, including medical imaging, architecture, 3D printing, manufacturing, 3D gaming, and various virtual reality (VR) applications. When taken together, a point is identified by three coordinates that correlate to a precise point in space relative to the end of origin. Different 3D point cloud file formats Scanning a space or an object and bringing it into designated software lets us manipulate the scans further, and stitch them together, which can be exported to be converted into a 3D model. Now there are numerous file formats for 3D modeling. Different scanners yield raw data in different formats. One needs other processing software for such files, and each & every software has its exporting capabilities. Most software systems are designed to receive a large number of file formats and have flexible export options. This section will walk you through some known and commonly used file formats. Securing the data in these common formats enables using different software for processing without approaching a third-party converter. Standard point cloud file formatsOBJ: It is a simple data format that only represents 3D geometry, color, and texture. And this format has been adopted by a wide range of 3D graphics applications. It is commonly ASCII (American Standard Code for Information Interchange). ASCII is a rooted language based on a binary that conveys information using text. Standard ASCII represents each character as a 7-bit binary number. In reverse engineering, characters are the focus of data. E57: E57 is a compact and widely used open, vendor-neutral file format for point clouds, and it can also be used to store images and data produced by laser scanners and other 3D imaging systems. Its compact, binary-based format combines the speed and accessibility of ASCII with the precision and accuracy of binary. E57 files can also represent normals, colors, and scalar field intensity. However, E57 is not universally compatible across all software platforms. PLY: The full form of PLY is the polygon file format. PLY was built to store 3D data. It uses lists of nominally flat polygons to represent objects. The aim is to store a more significant number of physical elements. It makes the file format capable of representing transparency, color, texture, coordinates, and data confidence values. It is found in ASCII and binary versions. PTS, PTX & XYZ: These three formats are familiar and compatible with most BIM software. It conveys data in lines of text. They can be easily converted and manipulated. PCG, RCS & RCP: These three formats were developed by Autodesk to meet their software suite’s demands. RCS and RCP are relatively newer. Binary point cloud file formats are better than ASCII or repurposed file types. It is because the latter is more universal and has better long-term storage capabilities. However, this type of format can be used to create a backup of the original data. If you need to convert binary point cloud files to ASCII, back up your binary files before reformatting them. This way, you can restore your data if something goes wrong. Challenges with point cloud data In reverse engineering, you may encounter several Point Cloud issues. The laser scanning procedure has catapulted product design technology to new heights. 3D data capturing system has come a long way, and we can see where it’s headed. As more and more professionals and end users are using new devices, the scanner market is rising at a quick pace. But along with a positive market change, handling and controlling the data available becomes a vital issue. These problems can result in poor quality point cloud data. Read on to learn more about five key challenges professionals working with point cloud face are: Data Quality: You must identify the quality issues in reverse engineering point cloud data. Reconstruction algorithms differ in their behavior based on the properties of point clouds. Many studies have classified the properties of point clouds by their effect on algorithms. The quality of point clouds affects the precision of the reconstructions. Point clouds produced by body scanners typically contain many duplicated and overlapping patches. These features cause a large amount of noise and redundancy in the final data. Reconstruction of free-form surfaces requires the use of clean-up meshes. This data must be transformed into a model that is consistent and accurate. Fortunately, this task is possible with the help of cloud-to-cloud alignment tools. Data Format: New devices out there in the market yield back data in a new form. Often, one needs to bring together data in different formats from different devices against a compatible software tool. It presents a not-so-easy situation. Identifying the wrong point cloud file format in reverse engineering is of great importance. Often when a company attempts to perform reverse engineering, it will be presented with a point cloud file in the wrong format. It can cause problems because the data is not in the correct format. EMPA has made it their business to work with point clouds as soon as possible. However, this doesn’t mean that you should give up
Read MoreWhat is CAD | Types of CAD Models and CAD Formats
Table of content What is CAD? Types of CAD models Types of CAD formats Use of CAD What is CAD? Computer-Aided Design, aka CAD, is undoubtedly a crucial stage in product development. By definition, CAD is the acronym for Computer-Aided Design. It covers various design tools used by multiple professionals like artists, game designers, manufacturers, and design engineers. After a meshed part is aligned, it goes through surface modeling in tools such as Polyworks. It generates a non-parametric model (IGES or STEP format) or parametric modeling where a sketch of the meshed part is created instead of putting it through surfacing (.PRT format). The resultant is generally called a 3D computer-aided model or CAD model. The technology of CAD systems has tremendously helped users by performing thousands of complex geometrical calculations in the background without anyone dropping a sweat for it. CAD has its origin in early 2D drawings where one could draw objects using basic views: top, bottom, left, right, front, back, and the angled isometric view. 3D CAD programs allow users to take 2D pictures and convert them into a 3D object on the screen. In a simple definition, CAD design is converting primary design data into a more perceptible and more understandable design. Each CAD system has its algorithm for describing geometry mathematically and structurally. Types of CAD models. Everything comes with its variety, and CAD modeling is no stranger. As the technology evolved, CAD modeling came up in different styles. There are many methods of classifying them, but a broad general classification can be as follows: Two-dimensional or 2D CAD: The early version of CAD that most of us are aware of. These are 2-dimensional drawings on a flat sheet with dimensions, layouts, and other information needed to manufacture the object. The 2D CAD objects consist of lines, ovals, circles, ovals, curves, and slots. 2D CAD platforms generally come with a library of geometric images and the ability to create Bezier curves, polylines, and splines. They are also capable of generating a bill of materials (BOM). 2.5D CAD:The 2.5 D CAD are prismatic, which means they represent the depth of an object. They fall between 2D and 3D CAD, and the objects consist of geometric patterns like in 2D CAD. Three-dimensional or 3D CAD:The purpose of both 2D and 3D models is the same. But what sets 3D models apart is their ability to present more excellent details about the individual component and assembly by projecting it as a full-scale 3-dimensional object. 3D CAD offers a realistic portrayal of the CAD model. 3D models can be viewed and rotated in X, Y, or Z axes. It also shows how two objects can fit and operate, which is impossible with 2D CAD. 3D models can be further classified into three categories: 3D Wire-frame Models:These models resemble an entire object made of just wires, with the background visible through the skeletal structure. Surface Models:Surface models are the next stage of wireframe models, and they are created by joining the 3D surfaces together and look like real-life objects. Solid Models:They best represent real physical objects in a virtual environment. Unlike other models, solid models have weight, volume, and density properties. They are the most used models and serve as prototypes for engineering projects. The Boundary Representation (BREP) solid modeling links Constructive Solid Geometry (CSG) images while a hybrid systems mix CSG and BREP to attain the intended design. Types of CAD formats Different professionals use different software platforms for various reasons like cost, project requirements, features, etc. Although the software comes with its file formats, there are instances where one needs to share their project with someone else, either partners or clients, who are using different software. In such cases, it is necessary that both parties’ software understand each other’s file formats or, in other words, interoperable. As a result of this situation, it is essential to have file formats that can be accommodated in various software. CAD file formats can be broadly classified into two types: STEP: This is the most popular CAD file format of all. It is widely used and highly recommended as most software support STEP files. STEP is the acronym for Standard for the Exchange of Product Data. IGES: IGES is the acronym for Initial Graphics Exchange Specification. It is an old CAD file format that is vendor-neutral. IGES has fallen out lately since it lacks many features that newer file formats have. Parasolid: Parasolid was initially developed by Shape Data and is currently owned by Siemens PLM Software. STL: STL stands for Stereolithography which is the format for 3D information created by 3D systems. STL finds its usage mostly in 3D printers. STL describes only the outer structure or surface geometry of a physical object but doesn’t give out the color, texture, and other attributes of an object. VRML: VRML stands for Virtual Reality Modeling Language. Although it gives back more attributes than STL, a handful of software can read it. Prototyping & pilot runs (preliminary design stage) In this stage, prototypes are built and tested after several iterations, and a pilot run of the manufacturing process is conducted. This stage involves creating rapid prototypes for a concept deemed to have business relevance and value. Prototype means a ‘quick and dirty’ model rather than a refined one that will be tested and marketed later. Adjustments are carried out as required before finalizing the design. X3D: X3D is an XML-based file format for representing 3D computer graphics. COLLADA: COLLADA stands for Collaborative Design Activity and is mostly used in gaming and 3D modeling. DXF: DXF stands for Drawing Exchange Format, a pure 2D file format native to AutoCAD. Use of CAD CAD technology has placed the entire engineering process in an adrenaline mode. It is possible to mold or fold, modify, or make a new part from scratch, all with the help of CAD modeling software. The many uses of CAD are as follows:CAD generates design and layouts, details and calculations, and 3-D
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