Polygon mesh

Example of a triangle mesh representing a dolphin.

A polygon mesh is a collection of vertices, edges and faces that defines the shape of a polyhedral object in 3D computer graphics and solid modeling. The faces usually consist of triangles (triangle mesh), quadrilaterals, or other simple convex polygons, since this simplifies rendering, but may also be composed of more general concave polygons, or polygons with holes.

The study of polygon meshes is a large sub-field of computer graphics and geometric modeling. Different representations of polygon meshes are used for different applications and goals. The variety of operations performed on meshes may include Boolean logic, smoothing, simplification, and many others. Network representations, "streaming" and "progressive" meshes, are used to transmit polygon meshes over a network. Volumetric meshes are distinct from polygon meshes in that they explicitly represent both the surface and volume of a structure, while polygon meshes only explicitly represent the surface (the volume is implicit). As polygonal meshes are extensively used in computer graphics, algorithms also exist for ray tracing, collision detection, and rigid-body dynamics of polygon meshes.

Elements of mesh modeling

Objects created with polygon meshes must store different types of elements. These include vertices, edges, faces, polygons and surfaces. In many applications, only vertices, edges and either faces or polygons are stored. A renderer may support only 3-sided faces, so polygons must be constructed of many of these, as shown above. However, many renderers either support quads and higher-sided polygons, or are able to convert polygons to triangles on the fly, making it unnecessary to store a mesh in a triangulated form. Also, in certain applications like head modeling, it is desirable to be able to create both 3- and 4-sided polygons.

A vertex is a position along with other information such as color, normal vector and texture coordinates. An edge is a connection between two vertices. A face is a closed set of edges, in which a triangle face has three edges, and a quad face has four edges. A polygon is a coplanar set of faces. In systems that support multi-sided faces, polygons and faces are equivalent. However, most rendering hardware supports only 3- or 4-sided faces, so polygons are represented as multiple faces. Mathematically a polygonal mesh may be considered an unstructured grid, or undirected graph, with additional properties of geometry, shape and topology.

Surfaces, more often called smoothing groups, are useful, but not required to group smooth regions. Consider a cylinder with caps, such as a soda can. For smooth shading of the sides, all surface normals must point horizontally away from the center, while the normals of the caps must point straight up and down. Rendered as a single, Phong-shaded surface, the crease vertices would have incorrect normals. Thus, some way of determining where to cease smoothing is needed to group smooth parts of a mesh, just as polygons group 3-sided faces. As an alternative to providing surfaces/smoothing groups, a mesh may contain other data for calculating the same data, such as a splitting angle (polygons with normals above this threshold are either automatically treated as separate smoothing groups or some technique such as splitting or chamfering is automatically applied to the edge between them). Additionally, very high resolution meshes are less subject to issues that would require smoothing groups, as their polygons are so small as to make the need irrelevant. Further, another alternative exists in the possibility of simply detaching the surfaces themselves from the rest of the mesh. Renderers do not attempt to smooth edges across noncontiguous polygons.

Mesh format may or may not define other useful data. Groups may be defined which define separate elements of the mesh and are useful for determining separate sub-objects for skeletal animation or separate actors for non-skeletal animation. Generally materials will be defined, allowing different portions of the mesh to use different shaders when rendered. Most mesh formats also support some form of UV coordinates which are a separate 2d representation of the mesh "unfolded" to show what portion of a 2-dimensional texture map to apply to different polygons of the mesh.

Representations

Polygon meshes may be represented in a variety of ways, using different methods to store the vertex, edge and face data. These include:

Each of the representations above have particular advantages and drawbacks, further discussed in Smith (2006).[1]

The choice of the data structure is governed by the application, the performance required, size of the data, and the operations to be performed. For example, it is easier to deal with triangles than general polygons, especially in computational geometry. For certain operations it is necessary to have a fast access to topological information such as edges or neighboring faces; this requires more complex structures such as the winged-edge representation. For hardware rendering, compact, simple structures are needed; thus the corner-table (triangle fan) is commonly incorporated into low-level rendering APIs such as DirectX and OpenGL.

Vertex-vertex meshes

Vertex-vertex meshes represent an object as a set of vertices connected to other vertices. This is the simplest representation, but not widely used since the face and edge information is implicit. Thus, it is necessary to traverse the data in order to generate a list of faces for rendering. In addition, operations on edges and faces are not easily accomplished.

However, VV meshes benefit from small storage space and efficient morphing of shape. The above figure shows a four-sided box as represented by a VV mesh. Each vertex indexes its neighboring vertices. Notice that the last two vertices, 8 and 9 at the top and bottom center of the "box-cylinder", have four connected vertices rather than five. A general system must be able to handle an arbitrary number of vertices connected to any given vertex.

For a complete description of VV meshes see Smith (2006).[1]

Face-vertex meshes

Face-vertex meshes represent an object as a set of faces and a set of vertices. This is the most widely used mesh representation, being the input typically accepted by modern graphics hardware.

Face-vertex meshes improve on VV-mesh for modeling in that they allow explicit lookup of the vertices of a face, and the faces surrounding a vertex. The above figure shows the "box-cylinder" example as an FV mesh. Vertex v5 is highlighted to show the faces that surround it. Notice that, in this example, every face is required to have exactly 3 vertices. However, this does not mean every vertex has the same number of surrounding faces.

For rendering, the face list is usually transmitted to the GPU as a set of indices to vertices, and the vertices are sent as position/color/normal structures (in the figure, only position is given). This has the benefit that changes in shape, but not geometry, can be dynamically updated by simply resending the vertex data without updating the face connectivity.

Modeling requires easy traversal of all structures. With face-vertex meshes it is easy to find the vertices of a face. Also, the vertex list contains a list of faces connected to each vertex. Unlike VV meshes, both faces and vertices are explicit, so locating neighboring faces and vertices is constant time. However, the edges are implicit, so a search is still needed to find all the faces surrounding a given face. Other dynamic operations, such as splitting or merging a face, are also difficult with face-vertex meshes.

Winged-edge meshes

Introduced by Baumgart 1975, winged-edge meshes explicitly represent the vertices, faces, and edges of a mesh. This representation is widely used in modeling programs to provide the greatest flexibility in dynamically changing the mesh geometry, because split and merge operations can be done quickly. Their primary drawback is large storage requirements and increased complexity due to maintaining many indices. A good discussion of implementation issues of Winged-edge meshes may be found in the book Graphics Gems II.

Winged-edge meshes address the issue of traversing from edge to edge, and providing an ordered set of faces around an edge. For any given edge, the number of outgoing edges may be arbitrary. To simplify this, winged-edge meshes provide only four, the nearest clockwise and counter-clockwise edges at each end. The other edges may be traversed incrementally. The information for each edge therefore resembles a butterfly, hence "winged-edge" meshes. The above figure shows the "box-cylinder" as a winged-edge mesh. The total data for an edge consists of 2 vertices (endpoints), 2 faces (on each side), and 4 edges (winged-edge).

Rendering of winged-edge meshes for graphics hardware requires generating a Face index list. This is usually done only when the geometry changes. Winged-edge meshes are ideally suited for dynamic geometry, such as subdivision surfaces and interactive modeling, since changes to the mesh can occur locally. Traversal across the mesh, as might be needed for collision detection, can be accomplished efficiently.

See Baumgart (1975) for more details.[2]

Render dynamic meshes

Winged-edge meshes are not the only representation which allows for dynamic changes to geometry. A new representation which combines winged-edge meshes and face-vertex meshes is the render dynamic mesh, which explicitly stores both, the vertices of a face and faces of a vertex (like FV meshes), and the faces and vertices of an edge (like winged-edge).

Render dynamic meshes require slightly less storage space than standard winged-edge meshes, and can be directly rendered by graphics hardware since the face list contains an index of vertices. In addition, traversal from vertex to face is explicit (constant time), as is from face to vertex. RD meshes do not require the four outgoing edges since these can be found by traversing from edge to face, then face to neighboring edge.

RD meshes benefit from the features of winged-edge meshes by allowing for geometry to be dynamically updated.

See Tobler & Maierhofer (WSCG 2006) for more details.[3]

Summary of mesh representation

Operation Vertex-vertex Face-vertex Winged-edge Render dynamic
V-V All vertices around vertex Explicit V → f1, f2, f3, ... → v1, v2, v3, ... V → e1, e2, e3, ... → v1, v2, v3, ... V → e1, e2, e3, ... → v1, v2, v3, ...
E-F All edges of a face F(a,b,c) → {a,b}, {b,c}, {a,c} F → {a,b}, {b,c}, {a,c} Explicit Explicit
V-F All vertices of a face F(a,b,c) → {a,b,c} Explicit F → e1, e2, e3 → a, b, c Explicit
F-V All faces around a vertex Pair search Explicit V → e1, e2, e3 → f1, f2, f3, ... Explicit
E-V All edges around a vertex V → {v,v1}, {v,v2}, {v,v3}, ... V → f1, f2, f3, ... → v1, v2, v3, ... Explicit Explicit
F-E Both faces of an edge List compare List compare Explicit Explicit
V-E Both vertices of an edge E(a,b) → {a,b} E(a,b) → {a,b} Explicit Explicit
Flook Find face with given vertices F(a,b,c) → {a,b,c} Set intersection of v1,v2,v3 Set intersection of v1,v2,v3 Set intersection of v1,v2,v3
Storage size V*avg(V,V) 3F + V*avg(F,V) 3F + 8E + V*avg(E,V) 6F + 4E + V*avg(E,V)
Example with 10 vertices, 16 faces, 24 edges:
10 * 5 = 50 3*16 + 10*5 = 98 3*16 + 8*24 + 10*5 = 290 6*16 + 4*24 + 10*5 = 242
Figure 6: summary of mesh representation operations

In the above table, explicit indicates that the operation can be performed in constant time, as the data is directly stored; list compare indicates that a list comparison between two lists must be performed to accomplish the operation; and pair search indicates a search must be done on two indices. The notation avg(V,V) means the average number of vertices connected to a given vertex; avg(E,V) means the average number of edges connected to a given vertex, and avg(F,V) is the average number of faces connected to a given vertex.

The notation "V → f1, f2, f3, ... → v1, v2, v3, ..." describes that a traversal across multiple elements is required to perform the operation. For example, to get "all vertices around a given vertex V" using the face-vertex mesh, it is necessary to first find the faces around the given vertex V using the vertex list. Then, from those faces, use the face list to find the vertices around them. Notice that winged-edge meshes explicitly store nearly all information, and other operations always traverse to the edge first to get additional info. Vertex-vertex meshes are the only representation that explicitly stores the neighboring vertices of a given vertex.

As the mesh representations become more complex (from left to right in the summary), the amount of information explicitly stored increases. This gives more direct, constant time, access to traversal and topology of various elements but at the cost of increased overhead and space in maintaining indices properly.

Figure 7 shows the connectivity information for each of the four technique described in this article. Other representations also exist, such as half-edge and corner tables. These are all variants of how vertices, faces and edges index one another.

As a general rule, face-vertex meshes are used whenever an object must be rendered on graphics hardware that does not change geometry (connectivity), but may deform or morph shape (vertex positions) such as real-time rendering of static or morphing objects. Winged-edge or render dynamic meshes are used when the geometry changes, such as in interactive modeling packages or for computing subdivision surfaces. Vertex-vertex meshes are ideal for efficient, complex changes in geometry or topology so long as hardware rendering is not of concern.

Other representations

Streaming meshes store faces in an ordered, yet independent, way so that the mesh can be transmitted in pieces. The order of faces may be spatial, spectral, or based on other properties of the mesh. Streaming meshes allow a very large mesh to be rendered even while it is still being loaded.

Progressive meshes transmit the vertex and face data with increasing levels of detail. Unlike streaming meshes, progressive meshes give the overall shape of the entire object, but at a low level of detail. Additional data, new edges and faces, progressively increase the detail of the mesh.

Normal meshes transmit progressive changes to a mesh as a set of normal displacements from a base mesh. With this technique, a series of textures represent the desired incremental modifications. Normal meshes are compact, since only a single scalar value is needed to express displacement. However, the technique requires a complex series of transformations to create the displacement textures.

File formats

There exist many different file formats for storing polygon mesh data. Each format is most effective when used for the purpose intended by its creator. Some of these formats are presented below:

File suffix Format name Organization(s) Program(s) Description
.raw Raw mesh Unknown Various Open, ASCII-only format. Each line contains 3 vertices, separated by spaces, to form a triangle, like so: X1 Y1 Z1 X2 Y2 Z2 X3 Y3 Z3
.blend Blender File Format Blender Foundation Blender 3D Open source, binary-only format
.fbx Autodesk FBX Format Autodesk Various Proprietary. Binary and ASCII specifications exist.
.3ds 3ds Max File Autodesk 3ds Max
.dae Digital Asset Exchange (COLLADA) Sony Computer Entertainment, Khronos Group N/A Stands for "COLLAborative Design Activity". A universal format designed to prevent incompatibility.
.dgn MicroStation File Bentley Systems MicroStation
.3dm Rhino File Robert McNeel & Associates Rhinoceros 3D
.dxf Drawing Exchange Format Autodesk AutoCAD
.obj Wavefront OBJ Wavefront Technologies Various ASCII format describing 3D geometry alone. All faces' vertices are ordered counter-clockwise, thus removing the need to specify normals.
.ply Polygon File Format Stanford University Unknown Binary and ASCII
.pmd Polygon Movie Maker data Yu Higuchi MikuMikuDance Proprietary binary file format for storing humanoid model geometry with rigging, material, and physics information.
.stl Stereolithography Format 3D Systems N/A Binary and ASCII format originally designed to aid in "3D printing".
.amf Additive Manufacturing File Format ASTM International N/A Like the STL format, but with added native color, material, and constellation support.
.wrl Virtual Reality Modeling Language Web3D Consortium Web Browsers ISO Standard 14772-1:1997
.wrz VRML Compressed Web3D Consortium Web Browsers
.x3d, .x3db, .x3dv Extensible 3D Web3D Consortium Web Browsers ISO Standard 19775/19776/19777
.x3dz, .x3dbz, .x3dvz X3D Compressed Binary Web3D Consortium Web Browsers
.c4d Cinema 4D File MAXON CINEMA 4D
.lwo LightWave 3D object File NewTek LightWave 3D
.msh Gmsh Mesh GMsh Developers GMsh Project Open source, providing an ASCII mesh description for linear and polynomially interpolated elements in 1 to 3 dimensions.
.mesh OGRE XML OGRE Development Team OGRE, purebasic Open Source. Binary (.mesh) and ASCII (.mesh.xml) format available. Includes data for vertex animation and Morph target animation (blendshape). Skeletal animation data in separate file (.skeleton).
.z3d Z3d Oleg Melashenko Zanoza Modeler -
.vtk VTK mesh VTK, Kitware VTK, Paraview Open, ASCII or binary format that contains many different data fields, including point data, cell data, and field data.

See also

References

  1. ↑ 1.0 1.1 Colin Smith, On Vertex-Vertex Meshes and Their Use in Geometric and Biological Modeling, http://algorithmicbotany.org/papers/smithco.dis2006.pdf
  2. ↑ Bruce Baumgart, Winged-Edge Polyhedron Representation for Computer Vision. National Computer Conference, May 1975. http://www.baumgart.org/winged-edge/winged-edge.html
  3. ↑ Tobler & Maierhofer, A Mesh Data Structure for Rendering and Subdivision. 2006.

External links