An AMF can represent one object, or multiple objects arranged in a constellation. Each object is described as a set of non-overlapping volumes. Each volume is described by a triangular mesh that references a set of points (vertices). These vertices can be shared among volumes belonging to the same object. An AMF file can also specify the material and the color of each volume, as well as the color of each triangle in the mesh. The AMF file is compressed using the zip compression format, but the ".amf" file extension is retained. A minimal AMF reader implementation must be able to decompress an AMF file and import at least geometry information (ignoring curvature).
Basic file structure
The AMF file begins with the XML declaration line specifying the XML version and encoding. The remainder of the file is enclosed between an opening <amf> element and a closing </amf> element. The unit system can also be specified (millimeter, inch, feet, meter or micrometer). In absence of a units specification, millimeters are assumed.
Within the AMF brackets, there are five top level elements. Only a single object element is required for a fully functional AMF file.
<object> The object element defines a volume or volumes of material, each of which are associated with a material ID for printing. At least one object element must be present in the file. Additional objects are optional.
<material> The optional material element defines one or more materials for printing with an associated material ID. If no material element is included, a single default material is assumed.
<texture> The optional texture element defines one or more images or textures for color or texture mapping, each with an associated texture ID.
<constellation> The optional constellation element hierarchically combines objects and other constellations into a relative pattern for printing.
<metadata> The optional metadata element specifies additional information about the object(s) and elements contained in the file.
Geometry specification
The format uses a Face-vertex polygon mesh layout. Each top-level <object> element specifies a unique id. The <object> element can also optionally specify a material. The entire mesh geometry is contained in a single <mesh> element. The mesh is defined using one <vertices> element and one or more <volume> elements. The required <vertices> element lists all vertices that are used in this object. Each vertex is implicitly assigned a number in the order in which it was declared, starting at zero. The required child element <coordinates> gives the position of the point in 3D space using the <x>, <y> and <z> elements. After the vertex information, at least one <volume> element must be included. Each volume encapsulates a closed volume of the object, Multiple volumes can be specified in a single object. Volumes may share vertices at interfaces but may not have any overlapping volume. Within each volume, the child element <triangle> is used to define triangles that tessellate the surface of the volume. Each <triangle> element will list three vertices from the set of indices of the previously defined vertices given in the <vertices> element. The indices of the three vertices of the triangles are specified using the <v1>, <v2> and <v3> elements. The order of the vertices must be according to the right-hand rule, such that vertices are listed in counter-clockwise order as viewed from the outside. Each triangle is implicitly assigned a number in the order in which it was declared, starting at zero.
Color specification
Colors are introduced using the <color> element by specifying the red, green, blue and alpha (transparency) channels in the sRGBcolor space as numbers in the range of 0 to 1. The <color> element can be inserted at the material, object, volume, vertex, or triangle levels, and takes priority in reverse order (triangle color is highest priority). The transparency channel specifies to what degree the color from the lower level is blended in. By default, all values are set to zero.
A color can also be specified by referring to a formula that can use a variety of coordinate-dependent functions.
Texture maps
Texture maps allow assigning color or material to a surface or a volume, borrowing from the idea of Texture mapping in graphics. The <texture> element is first used to associate a texture-id with particular texture data. The data can be represented as either a 2D or a 3D array, depending on whether the color or material need to be mapped to a surface or a volume. The data is represented as a string of bytes in Base64 encoding, one byte per pixel specifying the grayscale level in the 0-255 range.
Once the texture-id is assigned, the texture data can be referenced in a color formula, such as in the example below.
Usually, however, the coordinates will not be used directly as shown above, but transformed first to bring them from object coordinates to texture coordinates. For example, tex(1,f1(x,y,z),f2(x,y,z),f3(x,y,z)) where f1(), f2(), f3() are some functions, typically linear.
Material specification
Materials are introduced using the <material> element. Each material is assigned a unique id. Geometric volumes are associated with materials by specifying a material-id within the <volume> element.
Mixed, graded, lattice, and random materials
New materials can be defined as compositions of other materials. The element <composite> is used to specify the proportions of the composition, as a constant or as a formula dependent of the x, y, and z coordinates. A constant mixing proportion will lead to a homogenous material. A coordinate-dependent composition can lead to a graded material. More complex coordinate-dependent proportions can lead to nonlinear material gradients as well as periodic and non-periodic substructure. The proportion formula can also refer to a texture map using the tex(textureid,x,y,z) function. Reference to material-id "0" (void) is reserved and may be used to specify porous structures. Reference to the rand(x,y,z) function can be used to specify pseudo-random materials. The rand(x,y,z) function returns a random number between 0 and 1 that is persistent for that coordinate.
Print constellations
Multiple objects can be arranged together using the <constellation> element. A constellation can specify the position and orientation of objects to increase packing efficiency and to describe large arrays of identical objects. The <instance> element specifies the displacement and rotation an existing object needs to undergo to arrive into its position in the constellation. The displacement and rotation are always defined relatively to the original position and orientation in which the object was defined. A constellation can refer to another constellation as long as cyclic references are avoided.
If multiple top-level constellations are specified, or if multiple objects without constellations are specified, each of them will be imported with no relative position data. The importing program can then freely determine the relative positioning.
Meta-data
The <metadata> element can optionally be used to specify additional information about the objects, geometries and materials being defined. For example, this information can specify a name, textual description, authorship, copyright information and special instructions. The <metadata> element can be included at the top level to specify attributes of the entire file, or within objects, volumes and materials to specify attributes local to that entity.
Optional curved triangles
A curved triangle patch. Normals at vertices are used to recursively subdivide the triangle into four sub-triangles
In order to improve geometric fidelity, the format allows curving the triangle patches. By default, all triangles are assumed to be flat and all triangle edges are assumed to be straight lines connecting their two vertices. However, curved triangles and curved edges can optionally be specified in order to reduce the number of mesh elements required to describe a curved surface. The curvature information has been shown to reduce the error of a spherical surface by a factor of 1000 as compared to a surface described by the same number of planar triangles.[1] Curvature should not create a deviation from the plane of the flat triangle that exceeds 50% of the largest dimension of the triangle.
To specify curvature, a vertex can optionally contain a child element <normal> to specify desired surface normal at the location of the vertex. The normal should be unit length and pointing outwards. If this normal is specified, all triangle edges meeting at that vertex are curved so that they are perpendicular to that normal and in the plane defined by the normal and the original straight edge. When the curvature of a surface at a vertex is undefined (for example at a cusp, corner or edge), an <edge> element can be used to specify the curvature of a single non-linear edge joining two vertices. The curvature is specified using the tangent direction vectors at the beginning and end of that edge. The <edge> element will take precedence in case of a conflict with the curvature implied by a <normal> element.
When curvature is specified, the triangle is decomposed recursively into four sub-triangles. The recursion must be executed five levels deep, so that the original curved triangle is ultimately replaced by 1024 flat triangles. These 1024 triangles are generated "on the fly" and stored temporarily only while layers intersecting that triangle are being processed for manufacturing.
Formulas
In both the <color> and <composite> elements, coordinate-dependent formulas can be used instead of constants. These formulas can use various standard algebraic and mathematical operators and expressions.
Compression
An AMF can be stored either as plain text or as compressed text. If compressed, the compression is in ZIP archive format. A compressed AMF file is typically about half the size of an equivalent compressed binary STL file.[3]:275 The compression can be done manually using compression software, or automatically by the exporting software during write. Both the compressed and uncompressed files have the .amf extension and it is the responsibility of the parsing program to determine whether or not the file is compressed, and if so to perform decompression during import.
Design considerations
When the ASTM Design subcommittee began developing the AMF specifications[when?], a survey of stakeholders[4] revealed that the key priority for the new standard was the requirement for a non-proprietary format. Units and buildability issues were a concern lingering from problems with the STL format. Other key requirements were the ability to specify geometry with high fidelity and small file sizes, multiple materials, color, and microstructures. In order to be successful across the field of additive manufacturing, this file format was designed to address the following concerns
Technology independence: The file format must describe an object in a general way such that any machine can build it to the best of its ability. It is resolution and layer-thickness independent, and does not contain information specific to any one manufacturing process or technique. This does not negate the inclusion of properties that only certain advanced machines support (for example, color, multiple materials, etc.), but these are defined in such a way as to avoid exclusivity.
Simplicity: The file format must be easy to implement and understand. The format should be readable and editable in a simple text viewer, in order to encourage understanding and adoption. No identical information should be stored in multiple places.
Scalability: The file format should scale well with increase in part complexity and size, and with the improving resolution and accuracy of manufacturing equipment. This includes being able to handle large arrays of identical objects, complex repeated internal features (e.g. meshes), smooth curved surfaces with fine printing resolution, and multiple components arranged in an optimal packing for printing.
Performance: The file format must enable reasonable duration (interactive time) for read and write operations and reasonable file sizes for a typical large object.
Backwards compatibility: Any existing STL file should be convertible directly into a valid AMF file without any loss of information and without requiring any additional information. AMF files are also easily convertible back to STL for use on legacy systems, although advanced features will be lost.
Future compatibility: In order to remain useful in a rapidly changing industry, this file format must be easily extensible while remaining compatible with earlier versions and technologies. This allows new features to be added as advances in technology warrant, while still working flawlessly for simple homogenous geometries on the oldest hardware.
History
Since the mid-1980s, the STL file format has been the de facto industry standard for transferring information between design programs and additive manufacturing equipment. The STL format only contained information about a surface mesh, and had no provisions for representing color, texture, material, substructure, and other properties of the fabricated target object. As additive manufacturing technology evolved from producing primarily single-material, homogenous shapes to producing multi-material geometries in full color with functionally graded materials and microstructures, there was a growing need for a standard interchange file format that could support these features. A second factor that ushered the development of the standard was the improving resolution of additive manufacturing technologies. As the fidelity of printing processes approached micron scale resolution, the number of triangles required to describe smooth curved surfaces resulted in unacceptably large file sizes.[4]
During the 1990s and 2000s, a number of proprietary file formats have been in use by various companies to support specific features of their manufacturing equipment, but the lack of an industry-wide agreement prevented widespread adoption of any single format. In 2006, Jonathan D. Hiller and Hod Lipson presented an initial version of AMF dubbed "STL 2.0".[3] In January 2009, a new ASTM Committee F42 on Additive Manufacturing Technologies was established, and a design subcommittee was formed to develop a new standard. A survey was conducted in late 2009[4] leading to over a year of deliberations on the new standard. The resulting first revision of the AMF standard became official on May 2, 2011.[5]
During the July 2013 meetings of ASTM's F42 and ISO's TC261 in Nottingham (UK), the Joint Plan for Additive Manufacturing Standards Development was approved. Since then, the AMF standard is managed jointly by ISO and ASTM.
Sample file
Object produced by the sample AMF code
Below is a simple AMF file describing a pyramid made of two materials, adapted from the AMF tutorial[6] (548 bytes compressed). To create this AMF file, copy and paste the text below text into a text editor or an xml editor, and save the file as "pyramid.amf". Then compress the file with ZIP, and rename the file extension from ".zip" to ".zip.amf".
Texture mapping is a method for mapping a texture on a computer-generated graphic. Texture here can be high frequency detail, surface texture, or color.
In 3D computer graphics and solid modeling, a polygon mesh is a collection of vertices, edges and faces that defines the shape of a polyhedral object. The faces usually consist of triangles, quadrilaterals (quads), or other simple convex polygons (n-gons), since this simplifies rendering, but may also be more generally composed of concave polygons, or even polygons with holes.
In computer graphics, a shader is a computer program that calculates the appropriate levels of light, darkness, and color during the rendering of a 3D scene - a process known as shading. Shaders have evolved to perform a variety of specialized functions in computer graphics special effects and video post-processing, as well as general-purpose computing on graphics processing units.
In computer graphics, a computer graphics pipeline, rendering pipeline or simply graphics pipeline, is a conceptual model that describes what steps a graphics system needs to perform to render a 3D scene to a 2D screen. Once a 3D model has been created, for instance in a video game or any other 3D computer animation, the graphics pipeline is the process of turning that 3D model into what the computer displays. Because the steps required for this operation depend on the software and hardware used and the desired display characteristics, there is no universal graphics pipeline suitable for all cases. However, graphics application programming interfaces (APIs) such as Direct3D and OpenGL were created to unify similar steps and to control the graphics pipeline of a given hardware accelerator. These APIs abstract the underlying hardware and keep the programmer away from writing code to manipulate the graphics hardware accelerators.
OBJ is a geometry definition file format first developed by Wavefront Technologies for its Advanced Visualizer animation package. The file format is open and has been adopted by other 3D graphics application vendors.
In 3D computer graphics, polygonal modeling is an approach for modeling objects by representing or approximating their surfaces using polygon meshes. Polygonal modeling is well suited to scanline rendering and is therefore the method of choice for real-time computer graphics. Alternate methods of representing 3D objects include NURBS surfaces, subdivision surfaces, and equation-based representations used in ray tracers.
STL is a file format native to the stereolithography CAD software created by 3D Systems. STL has several backronyms such as "Standard Triangle Language" and "Standard TessellationLanguage". This file format is supported by many other software packages; it is widely used for rapid prototyping, 3D printing and computer-aided manufacturing. STL files describe only the surface geometry of a three-dimensional object without any representation of color, texture or other common CAD model attributes. The STL format specifies both ASCII and binary representations. Binary files are more common, since they are more compact.
UV mapping is the 3D modeling process of projecting a 3D model's surface to a 2D image for texture mapping. The letters "U" and "V" denote the axes of the 2D texture because "X", "Y", and "Z" are already used to denote the axes of the 3D object in model space, while "W" is used in calculating quaternion rotations, a common operation in computer graphics.
PLY is a computer file format known as the Polygon File Format or the Stanford Triangle Format. It was principally designed to store three-dimensional data from 3D scanners. The data storage format supports a relatively simple description of a single object as a list of nominally flat polygons. A variety of properties can be stored, including color and transparency, surface normals, texture coordinates and data confidence values. The format permits one to have different properties for the front and back of a polygon. There are two versions of the file format, one in ASCII, the other in binary.
3D computer graphics, sometimes called CGI, 3D-CGI or three-dimensional computer graphics are graphics that use a three-dimensional representation of geometric data that is stored in the computer for the purposes of performing calculations and rendering digital images, usually 2D images but sometimes 3D images. The resulting images may be stored for viewing later or displayed in real time.
A vertex buffer object (VBO) is an OpenGL feature that provides methods for uploading vertex data to the video device for non-immediate-mode rendering. VBOs offer substantial performance gains over immediate mode rendering primarily because the data reside in video device memory rather than system memory and so it can be rendered directly by the video device. These are equivalent to vertex buffers in Direct3D.
3DS is one of the file formats used by the Autodesk 3ds Max 3D modeling, animation and rendering software.
Synopsys Simpleware ScanIP is a 3D image processing and model generation software program developed by Synopsys Inc. to visualise, analyse, quantify, segment and export 3D image data from magnetic resonance imaging (MRI), computed tomography (CT), microtomography and other modalities for computer-aided design (CAD), finite element analysis (FEA), computational fluid dynamics (CFD), and 3D printing. The software is used in the life sciences, materials science, nondestructive testing, reverse engineering and petrophysics.
OpenSCAD is a free software application for creating solid 3D computer-aided design (CAD) objects. It is a script-only based modeller that uses its own description language; parts can be previewed, but cannot be interactively modified by mouse in the 3D view. An OpenSCAD script specifies geometric primitives and defines how they are modified and combined to render a 3D model. As such, the program does constructive solid geometry (CSG). OpenSCAD is available for Windows, Linux and macOS.
A vertex in computer graphics is a data structure that describes certain attributes, like the position of a point in 2D or 3D space, or multiple points on a surface.
In 3D computer graphics, 3D modeling is the process of developing a mathematical coordinate-based representation of any surface of an object in three dimensions via specialized software by manipulating edges, vertices, and polygons in a simulated 3D space.
The Open Game Engine Exchange (OpenGEX) format is a text-based file format designed to facilitate the transfer of complex 3D scene data between applications such as modeling tools and game engines. The OpenGEX format is built upon the data structure concepts defined by the Open Data Description Language (OpenDDL), a generic language for the storage of arbitrary data in human-readable format. The OpenGEX file format is registered with the Internet Assigned Numbers Authority (IANA) as the model/vnd.opengex media type.
3D Manufacturing Format or 3MF is an open source file format standard developed and published by the 3MF Consortium.
This is a glossary of terms relating to computer graphics.
JMesh is a JSON-based portable and extensible file format for the storage and interchange of unstructured geometric data, including discretized geometries such as triangular and tetrahedral meshes, parametric geometries such as NURBS curves and surfaces, and constructive geometries such as constructive solid geometry (CGS) of shape primitives and meshes. Built upon the JData specification, a JMesh file utilizes the JSON and Universal Binary JSON (UBJSON) constructs to serialize and encode geometric data structures, therefore, it can be directly processed by most existing JSON and UBJSON parsers. The JMesh specification defines a list of JSON-compatible constructs to encode geometric data, including N-dimensional (ND) vertices, curves, surfaces, solid elements, shape primitives, their interactions and spatial relations, together with their associated properties, such as numerical values, colors, normals, materials, textures and other properties related to graphics data manipulation, 3-D fabrication, computer graphics rendering and animations.
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