DREAM3D Review (Geometry)
This Filter creates a Geometry object within a Data Container. The Data Container must not already contain a Geometry; empty Data Containers may be created using the Create Data Container Filter. This Filter will also create the necessary Element Attribute Matrices on which to store Attribute Arrays. The type of Attribute Matrices created depends on the kind of Geometry being created:
|Edge||Vertex + Edge|
|Triangle||Vertex + Face|
|Quadrilateral||Vertex + Face|
|Tetrahedral||Vertex + Cell|
|Hexahedral||Vertex + Cell|
This Filter requires the user to enter information that defines the topological information for the chosen Geometry. Choosing valid information for a given Geometry necessitates an understanding of how DREAM.3D stores and interprets this information. A general overview of the data model used in DREAM.3D may be found here. More specific information for Geometry objects is provided below.
An Image Geometry is a grid-like Geometry, and is the simplest and most widely used of the basic Geometry types. An Image Geometry is a regular, rectilinear grid; if the dimenionality of the image is d, then only 3d_ numbers are needed to completely define the Geometry*: three _d-vectors for the dimensions, origin, and resolution. Dimensions define the extents of the grid, the resolution defines the spacing between grid planes for each orthogonal direction (constant along a given direction), and the origin defines the physical location of the bottom left grid point in d-dimensional space. The dimensions are stored as unsigned 64-bit integers, while the origin and resolution are stored as 32-bit floats.
All Image Geometries in DREAM.3D are defined using 3-vectors (3D images). A 2D image is assumed when one of the dimension values is exactly 1; the 2D image is then considered a plane. Most DREAM.3D Filters will properly take account for the Image dimension if it matters (for example, the Find Feature Shapes Filter accounts for whether the Image is 2D or 3D when computing values such as aspect ratios or axis Euler angles). No dimension may be negative or equal to 0. The resolution and origin have no value restrictions. This Filter requires the user to enter the nine values for the dimenions, origin, and resolution.
Since all Image Geometries are implicitly 3D (even when plane-like), the fundamental building-block of an image is a voxel, which is a 3D object; therefore, the basic Element type for an Image Geometry is Cell. Attribute Arrays associated with Image Cells are assumed to raster x-y-z, fastest to slowest.
A Rectilinear Grid Geometry is a grid-like Geometry. Similar to an Image Geometry, a Rectilinear Grid Geometry has grid extents (dimensions), but is allowed to have variable resolution along each orthogonal direction. The Geometry then requires a total of (xdim + 1) + (ydim + 1) + (zdim + 1) numbers to define the topology. The values are stored in three separate arrays termed the x bounds, y bounds, and z bounds. These bounds arrays store the spatial location of all the planes along a given orthogonal direction. The spacing for a given plane (equivalent to the resolution for an Image Geometry) is then the difference between two of these contiguous array values. An origin does not need to be defined for a Rectilinear Grid Geometry, since the grid's location in space is explicitly encoded in its bounds arrays. This Filter requires the user to select Attribute Arrays that define the three bounds arrays. These arrays must be single component, 32-bit float arrays. Additionally, the values for each of the bounds arrays must be strictly increasing, which guarantees that computing the spacing for a given plane yields a postive value.
A Rectilinear Grid Geometry may be defined as 2D; the associated bounds array for the plane dimension is then exactly two. No bounds arrays may have less than two values. Since all Rectilinear Grid Geometries are implicitly 3D (even when plane-like), the fundamental building-block of an image is a voxel, which is a 3D object; therefore, the basic Element type for an Image Geometry is Cell. Attribute Arrays associated with Rectilinear Grid Cells are assumed to raster x-y-z, fastest to slowest.
Unstructured and Mesh-Like Geometries¶
A Vertex Geometry is an unstructured Geometry. An unstructured Geometry requires explicit definition of point coordinates. Sometimes referred to as a point cloud, a Vertex Geometry is simply a collection of points. Defining this topology requires a total number of values equal to d times the total number of points, where d is the dimensionality of the point cloud; within DREAM.3D, d is always taken to be three. The point coordinates are stored as 32-bit floats; no other range restrictions are enforced. This Filter requires the user to select an Attribute Array that defines these point coordinates. The array must have three components and consist of 32-bit floats. The number of tuples in the array defines the number of vertices in the resulting Vertex Geometry.
The fundamental Element type of a Vertex Geometry is vertices. Data stored in a Vertex Attribute Matrix is ordered according to Vertex Ids. Therefore, the nth tuple in the supplied Vertex list corresponds to the data stored in the nth column of the Vertex Attribute Matrix. By convetion, Vertex Ids are zero indexed.
The following Geometries are considered mesh-like, and all share similar features concerning their storage and interpretation. A mesh-like Geometry is an unstructured Geometry that additionally requires explicit definition of the connectivity of its Elements and its Vertices. The Element type defines the kind of Geometry and the number of Vertices needed to define that Element:
|Name||Element Type||Number of Vertices Per Element|
The storage scheme adopted by DREAM.3D requires at least two arrays to define mesh-like Geometries: a list of Vertices (i.e., the vertex coordinates) and the Element connectivities (i.e., which vertices belong to a given Element). To maintain simplicity, flexibility, and small memory overhead, DREAM.3D uses the concept of shared vertex lists. In this paradigm, the vertex coordinates are stored only once per unique vertex. Consider a Quadrilateral Geometry that consists of just two squares that share one side. In this example, there are exactly six unique vertices. The Attribute Array that defines the coordinates of these Vertices would then have six tuples, with three values at each tuple (the x, y, and z positions of that Vertex). Writing each tuple on one line, the array could look like this:
0.0 0.0 0.0 // Vertex Id 0 1.0 0.0 0.0 // Vertex Id 1 0.0 1.0 0.0 // Vertex Id 2 1.0 1.0 0.0 // Vertex Id 3 2.0 0.0 0.0 // Vertex Id 4 2.0 1.0 0.0 // Vertex Id 5
Element connectivities are stored in Attribute Arrays that have a number of tuples equal to the total number of Elements, with a number of components at each tuple equal to the number of vertices per element. In this example, a quadrilateral list would have two tuples, with four values stored at each tuple (the four vertex Ids that define that quadrilateral). When defining Elements, the order in which the Vertex Ids are listed, called the winding, is important, since this ordering defines the direction of the normal. By convention, the right hand rule used. Thus, given the above vertex positions, the following list of Vertex Ids defines two quadrilaterals whose normals point along the positive z direction:
0 1 3 2 // Quad Id 0 1 4 5 3 // Quad Id 1
Creating any mesh-like Geometry requires the user to supply two arrays: one that defines the vertex coordinates (the shared vertex list), which is a three component array of floats; and one that defines the Element connectivities, which is a n-component array (where n is the number of vertices per element) of signed 64-bit integers. Note that any Element Id values (Vertex or otherwise) are zero indexed.
The shared list schema for mesh storage has the benefit of being space efficient, time efficient when iterating in sequence over vertices or elements, and capable of storing nonmanifold meshes. An example of a nonmanifold mesh is a Triangle Geometry that has more than two triangles sharing the same edge. This specific example of nonmanifold meshes occurs frequently in DREAM.3D surface meshes of polycrystals, where many nonmanifold entities may exist (i.e., triple lines and quad points). A significant downside of shared lists is that computing adjacency information, such as the neighbors of a given element or the elements that share a vertex, requires iterating over the entire Geometry; other mesh data structures avoid this limitation. Additionally, since the lists are stored as Attribute Arrays, which hold information contiguously in memory, adding or removing vertices or elements is tedious and potentially slow.
Note that although the default interpretation of lists that define mesh-like Geometries is shared, no undefined behavior should be observed if the information is not stored shared (i.e., if the same Vertex is stored more than once with a different Id). Additionally, not all Vertices are required to be associated with an Element. The primary requirement is that the largest Vertex Ids listed in the Element list must not be larger than the total number of Vertices.
An Edge Geometry is the simplest mesh-like Geometry, consisting of a collection of edges connecting two vertices. Creating an Edge Geometry requires supplying a shared Vertex list and an Edge list.
A Triangle Geometry is a mesh-like Geometry, consisting of a collection of triangles connecting three vertices; it is a type of surface mesh. Creating a Triangle Geometry requires supplying a shared Vertex list and a Triangle list.
A Quadrilateral Geometry is a mesh-like Geometry, consisting of a collection of quadrilaterals connecting four vertices; it is a type of surface mesh. Creating a Quadrilateral Geometry requires supplying a shared Vertex list and a Quadrilateral list.
A Tetrahedral Geometry is a mesh-like Geometry, consisting of a collection of tetrahedra connecting four vertices; it is a type of volume mesh. Creating a Tetrahedral Geometry requires supplying a shared Vertex list and a Tetrahedral list. The winding that define tetrahedra require one additional convention to complement the right hand rule. By convention, the first three vertices define the tetrahedra base; the winding of these vertices by the right hand rule defines a normal that points towards the fourth vertex. This convention is useful since applying it consistently allows for the volume of the tetrahedra to be signed, which is important for determining if a tetrahedron is "inverted".
A Hexahedral Geometry is a mesh-like Geometry, consisting of a collection of hexahedra connecting eight vertices; it is a type of volume mesh. Creating a Hexahedral Geometry requires supplying a shared Vertex list and a Hexahedral list.
Defining Geometries with Attribute Arrays¶
For Geometries that require the selection of Attribute Arrays (all Geometries except Image), the arrays will be copied to create the new Geometry. Therefore, any operations on the original array will not affect the topology of the Geometry, and any geometric operations will not affect the original array. This behavior can be adjusted in the filter by using the Array Handling boolean.¶
This Filter will validate that the arrays selected to define a Geometry "make sense", given the above information for how Geometries are stored in DREAM.3D (for example, no dimension for an Image may be less than or equal to zero, no bounds arrays for a Rectilinear Grid may have less than two values, and no Vertex Ids stored in a shared Element list may be larger than the total number of Vertices in the shared Vertex list). The checks that require accessing the actual array values (as opposed to just descriptive information) will be performed at run time. By default, these checks will only produce warnings, allowing the Pipeline to continue; the user may opt to change these warnings to errors by selecting the Treat Geometry Warnings as Errors option.
Generally, arrays used by this Filter to create Geometries must be supplied by the user. One method to import geometric information into DREAM.3D is to read the information in from a text file using the Import ASCII Data Filter. For example, imagine having an external simulation code that creates a tetrahedral volume mesh with two associated field values, one stored on the mesh vertices and one stored on the mesh tetrahedra. It is possible to import this mesh and corresponding information into DREAM.3D for further analysis. The user must supply at least two files: one that contains the vertex information and one that contains the tetrahedra information. The vertex file would contain, on each line, the three coordinates of the vertex and the value of the field array on that vertex. It may, for example, look like this:
# Some header information # Some more header information x_pos y_pos z_pos value 1.235 2.323 1.562 465.2 -12.3 3.456 2.323 567.4 3.450 9.782 6.567 120.2 .....
In this above example, the vertex information begins on line 4; thus, line 4 defines Vertex Id 0, line 5 defines Vertex Id 1, etc. Similarly, a file containing information about tetrahedra is needed:
# Some header information # Some more header information vert_0 vert_1 vert_2 vert_3 value 1 2 0 3 12.42 2 7 5 4 14.71 6 9 7 8 16.78 .....
Again, the real information begins on line 4, which defines the connectivity for Tetrahedra Id 0. These Id values refer to the Vertex Ids from the first file (for example, a Vertex Id of 0 corresponds to the information on line 4 of the first file). Remember to consider the right hand rule when dealing with mesh-like Geometries, as this will affect the Vertex ordering!
Assuming it is possible to get the mesh into files similar to the above ones, it is straightforward to import the information into DREAM.3D. First, create an emtpy Data Container. Then, run the Import ASCII Data Filter for the vertex file. In the above example, line 3 could be used as headers to define the array names (remember that Vertex positions must be of type float!). Allow the reader wizard to create an Attribute Matrix in which to store the arrays. Repeat the process with another Import ASCII Data Filter to read the tetrahedra information (remember in this case that Id values must be of type int64_t!). At this point, there will be two Attribute Matrices in the Data Container, one with 4 arrays (the Vertex information) and one with 5 arrays (the Tetrahedra information). The Create Geometry Filter wants the Vertex list as a three component array and the Tetrahedra list as a four component array. To combine the individual arrays into ones of the proper component dimension, run the Combine Attribute Arrays Filter. At this point, the Create Geometry Filter may be used to create a Tetrahedral Geometry using the combined arrays. After this Filter, the Move Data Filter may be used to move the arrays that represent the values stored on the Vertices and Tetrahedra into the created Vertex and Cell Attribute Matrices.
When creating Geometries, remember to consider all the various rules for how a Geometry is stored and interpreted. In particular, remeber that Element Ids are always zero indexed, mesh-like Geometries obey the right hand rule for windings and normal directions, and Element lists are by default considered shared. Note that although the storage scheme used by DREAM.3D (shared lists) is highly generic, some Filters may assume that the Geometry is reasonably well formed.
|Geometry Type||Enumeration||The type of Geometry to create|
|Treat Geometry Warnings as Errors||bool||Whether run time warnings for Geometries should be treated as errors|
|Array Handling||bool||Determines if the arrays that make up the geometry primitives should be Moved or Copied to the created Geometry object.|
|Dimensions||size_t (3x)||The number of cells in each of the X, Y, Z directions, if Image is chosen|
|Origin||float (3x)||The origin of each of the axes in X, Y, Z order, if Image is chosen|
|Resolution||float (3x)||The length scale of each voxel/pixel, if Image is chosen|
|Kind||Default Name||Type||Component Dimensions||Description|
|Data Container||None||N/A||N/A||Data Container in which to place the created Geometry|
|Various Attribute Arrays||None||float/int64_t||1/2/3/4||Various Attribute Arrays used to define the Geometry topology; see the above documentation for a detailed discussion of what kinds of information are needed for each Geometry|
|Kind||Default Name||Type||Component Dimensions||Description|
|Various Attribute Matrices||None||Vertex/Edge/Face/Cell||N/A||Various Attribute Matrices used to store information for the new Geometry; see the above documentation for a detailed discussion of what kinds of Attribute Matrices are created for each Geometry|
License & Copyright¶
Please see the description file distributed with this plugin.
DREAM3D Mailing Lists¶
If you need more help with a filter, please consider asking your question on the DREAM3D Users mailing list: https://groups.google.com/forum/?hl=en#!forum/dream3d-users