1. The Diversity of 3D Model Formats

1.1 Two Major Categories of Digital Models

The 3D modeling industry encompasses numerous file formats, including OBJ, STL, WRL, STP, IGES, MAX, C4D, and many others. These formats generally fall into two distinct categories: Digital Content Creation (DCC) and Computer Aided Design/Manufacturing (CAD/CAM).

1.2 Distinctive Characteristics of Each Category

DCC software specializes in creating models using mesh/polygon structures, where models are manipulated through vertices, line segments, and patch meshes. This approach offers exceptional flexibility for creating complex, organic shapes.

CAD/CAM software, on the other hand, primarily utilizes NURBS (Non-Uniform Rational B-Spline) surfaces, generating sophisticated surface models through spline manipulation. This method provides precise control over geometric forms.

To draw an analogy, the distinction between these approaches resembles that between raster and vector graphics. DCC software excels at crafting organic, intricate designs with complex details - perfect for creating creatures, natural environments, and fantasy elements. Meanwhile, CAD/CAM software shines in producing precise mechanical components and smooth, mathematically-defined surfaces, ideal for engineering applications like automotive design.

Regarding precision, CAD software typically generates models with higher accuracy compared to DCC software. In terms of industry applications, we can observe a hierarchy of detail complexity: gaming models ≈ VR/AR models < film/television models < industrial models. The corresponding polygon counts typically range from hundreds of thousands for games, to millions for film, and tens of millions for industrial applications. Mobile platforms and real-time rendering applications generally favor low-polygon models for optimal performance.

1.3 Industry-Standard Modeling Software

• DCC Software Suite: Blender, Cinema 4D, 3ds Max, Maya, Modo, ZBrush, etc.
• CAD/CAM Software Suite: FreeCAD, UG, ProE, SOLIDWORKS, CATIA, AutoCAD, Rhino, Alias, etc.

1.4 Model Format Categories

Each 3D modeling software typically employs its proprietary file format - for instance, 3ds Max uses .max files, while SOLIDWORKS utilizes .slprt files. However, to facilitate interoperability between different software platforms, several standardized exchange formats have emerged. In the Digital Content Creation (DCC) sphere, obj and fbx formats dominate, while the Computer-Aided Design/Manufacturing (CAD/CAM) industry primarily relies on stp, iges, and stl formats. It’s worth noting that these universal formats often cannot preserve all scene information from the source software, instead capturing only the attributes specifically supported by the format specification.

2. Comprehensive Guide to 3D Model Formats

2.1 Point Data Storage Formats

• PCD (Point Cloud Data): The official format of Point Cloud Library (PCL), specifically designed for comprehensive point cloud data storage and manipulation.
• TXT: A straightforward format for point cloud data storage, offering simplicity and compatibility with standard text processing workflows.

2.2 Combined Point and Topology Storage Formats

• VTK: Advanced format that captures both point data and their topological relationships.
• PLY: Specialized 3D mesh format dedicated to polygonal model representation.
• OFF: Efficient format for storing geometric polygon information.
• OBJ: Versatile geometry-based format with primary support for polygon/mesh structures.
• STL: Comprehensive format storing both point data and topology, ideal for representing closed surfaces and volumetric models.

2.3 Binary Format

• BIN: A generic binary file format that, while capable of storing various data types, doesn’t inherently specify content structure. May or may not contain point cloud data.

2.4 STL Format in Detail

STL (STereoLithography) format, introduced by 3D SYSTEMS in 1988, was developed as a standard for rapid prototyping manufacturing. While commonly associated with 3D printing, its original purpose extended beyond this specific application.

Unlike feature-rich CAD formats, STL employs a simplified triangular mesh representation of 3D models. It focuses solely on geometric information, excluding properties such as color and material data. This streamlined approach results in a more basic but widely compatible 3D model format.

STL files are available in two variants:

• ASCII format: Text-based, offering broader compatibility
• Binary format: More compact but potentially less universally supported

2.5 PCD Format Evolution

The Point Cloud Data (PCD) format has undergone significant evolution since its inception. Prior to PCL 1.0, multiple versions were developed, designated as PCD_Vx (including PCD_V5, PCD_V6, and PCD_V7), corresponding to versions 0.5, 0.6, and 0.7 respectively. The current standard in PCL implementations is version 0.7 (PCD_V7).

PCD files employ a structured format comprising:

1. Header section: Defines fundamental characteristics of the point cloud dataset
2. Point cloud data section: Contains the actual point data

Below is a representative example of a PCD_V7 header structure and associated point cloud data: structure and associated point cloud data:

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# .PCD v.5 - Point Cloud Data file format
    VERSION .5
    FIELDS x y z
    SIZE 4 4 4
    TYPE F F F
    COUNT 1 1 1
    WIDTH 397
    HEIGHT 1
    POINTS 397
    DATA ascii
    0.0054216 0.11349 0.040749
    -0.0017447 0.11425 0.041273
    -0.010661 0.11338 0.040916
    0.026422 0.11499 0.032623
    0.024545 0.12284 0.024255
    0.034137 0.11316 0.025079

• FIELDS: Defines the dimensional attributes and supplementary information for each point in the dataset. For instance, FIELDS x y z indicates three-dimensional spatial coordinates, while FIELDS x y z rgb includes both spatial coordinates and color information.

• SIZE: Specifies the byte allocation for each dimensional attribute:

  • 1 byte: char type data
  • 2 bytes: short type data
  • 4 bytes: int/float type data
  • 8 bytes: double type data

• TYPE: Designates data types using character indicators:

  • ‘I’: Signed integers (int8/char, int16/short, int32/int)
  • ‘U’: Unsigned integers (uint8, uint16, uint32)
  • ‘F’: Floating-point numbers (float, double)

• COUNT: Indicates the number of elements per dimension.

• POINTS: Represents the total point count in the cloud (Note: This parameter may be deprecated in future versions post-0.7).

• DATA: Determines the storage format - either ASCII or binary.

2.6 OBJ Format

The OBJ (Object) format, pioneered by Alias|Wavefront, stands as an industry-standard 3D model format. Originally designed for their “Advanced Visualizer” software, it has evolved into a universal medium for 3D model exchange across platforms. Its widespread adoption makes it particularly valuable for workflows involving multiple software tools, such as transferring models from 3ds Max to Maya.

Key characteristics:

• Focuses on polygon-based model representation
• Excludes advanced features like animation, materials, textures, and dynamics
• Enjoys broad compatibility across CAD software and 3D printing platforms

While chronologically newer than STL and offering some enhancements, the fundamental capabilities remain similar between the two formats.

2.7 PLY Format

The PLY (Polygon File Format) employs a structured hierarchy:

1. Header Section
2. Vertex Data
3. Face Information
4. Supplementary Elements

The header comprises descriptive text lines that outline the file’s contents, including element specifications (e.g., edges), quantity information, and associated properties. It also indicates whether the data is stored in ASCII or binary format. The subsequent sections contain the actual geometric data in the sequence specified by the header.

2.8 AMF Format

The Additive Manufacturing File (AMF) format represents an evolution of the STL standard, specifically engineered for modern 3D printing requirements. Built on XML architecture, it offers several advantages:

• Enhanced data representation including colors, materials, and internal structures
• Human-readable while maintaining machine processability
• Extensible framework through XML tags
• Support for:
  • Multi-material specifications
  • Gradient material transitions
  • Mathematical descriptions of internal structures
  • Surface texturing
  • Optimal print orientation
  • Metadata (creator, model information)

2.9 3MF Format

Introduced by the Microsoft-led 3MF Consortium in 2015, the 3D Manufacturing Format (3MF) represents a modern approach to 3D printing file standards. Its key features include:

• Comprehensive model description capabilities
• Support for:
  • Geometric data
  • Internal structures
  • Color information
  • Material properties
  • Texture mapping
  • Additional metadata

Built on XML foundations, 3MF offers excellent extensibility and enjoys widespread industry support, making it particularly valuable for both consumers and professional users in the 3D printing ecosystem.

3. The 3D Formats Used in Computer-Aided Engineering (CAE)

The evolution of computer graphics, hardware capabilities, and 3D printing technology has led to the widespread adoption of mesh-based graphic rendering and associated file formats. In the realm of Finite Element Analysis (FEA), engineers frequently utilize mesh-based geometric files, particularly STL formats, for analytical purposes. However, a fundamental distinction exists between surface meshes and finite element solid meshes, necessitating a conversion process before surface meshes can be employed in finite element calculations. This section explores the characteristics of both STL surface meshes and finite element meshes, while providing practical guidance on the conversion process between these formats.

3.1 Understanding Triangular Mesh

Triangular mesh represents a sophisticated surface modeling approach where triangular grids, composed of points and edges, form intricate spatial patches that approximate three-dimensional surfaces. This methodology serves as the foundation for most three-dimensional mesh file representations, effectively approximating complex solid models through triangular elements. The popularity of triangular mesh stems from its computational efficiency, superior visualization capabilities, and robust performance characteristics. As computer graphics technology has advanced, triangular mesh models have become integral to diverse applications, including 3D animation, e-commerce platforms, computer-aided design systems, industrial manufacturing processes, and digital entertainment solutions. The emergence of 3D printing technology has further cemented triangular mesh as a predominant method for three-dimensional model representation.

3.2 Three-dimensional Solid Mesh in Finite Element Analysis

While finite element analysis encompasses various specialized elements - including plane elements for plates and shells, and line elements for cables and beams - three-dimensional solid elements remain the most prevalent. Tetrahedral and hexahedral elements, in particular, play crucial roles in structural, thermal, fluid, acoustic, and electromagnetic simulations. These solid elements incorporate both surface information and internal mesh divisions, resulting in significantly more complex structures and larger data volumes compared to STL surface meshes.

The quality requirements for finite element mesh are particularly stringent. Key considerations include:

• Mesh Integrity: Distorted meshes must be avoided
• Optimal Density: Balance between accuracy and computational efficiency
• Adaptive Refinement: Higher mesh density in geometrically complex regions

Critical quality parameters include:

• Jacobian Ratio: A measure of element distortion based on coordinate transformation matrices (optimal value: 1, must exceed 0)
• Aspect Ratio: The relationship between an element’s longest and shortest edges (ideal: 1)
• Skewness: A metric for element face regularity (range: 0-1, with lower values preferred)

Element order significantly impacts analysis accuracy. While second-order elements typically provide superior results compared to linear first-order elements, they introduce additional computational complexity due to increased nodal points.

3.3 Converting STL Mesh to FEA Mesh

3.3.1 Professional Software Solutions

For STL to FEA mesh conversion, several professional tools are available:

• HyperMesh and GMSH: Preferred options for converting STL files into FEA-compatible formats
• Abaqus: Offers specialized plugins for STL file processing
• Format Considerations:
  • STEP format is recommended over IGES (which is limited to line representations)
  • For STL or OBJ files, utilize PDElib or GMSH for conversion to 3D formats before Abaqus import

Note: Major FEA platforms like Abaqus and ANSYS typically require intermediate conversion tools for STL file processing.