Reporter Selector Controller Projection Re-present (abstract or transform) information from a model. Models can be complex. Finding and using the relevant parts of a model can be tedious and error-prone. Further, "parts" of a model may only be implied---computation may be needed to construct them from primary model data. Use this pattern when you need to use some aspect of a model in another process or another part of the model. Models can be arbitrarily complex and therefore hard to understand. Further, they can imply far more information than they directly express. Such information is typically uncovered through functions\index{function} applied to the model. Using a Reporter allows you to present only the information you need to another part of the model. This makes your model structure more clear and helps you communicate with other people who may use your model. Reporters may abstract (simplify) or transform (re-present). They report a design or its parts from a different point of view. In analogy to a relational database, the Reporter pattern is akin to a view table extracted from a database. Data from a Reporter must be conceived, extracted and envisioned. Deciding what to report can be non-trivial, for example, when reporting planarity in facade elements, the most appropriate report may, or may not, be the minimal vertex movement that restores planarity. Extracting the data may involve a substantial algorithm\index{algorithm}, for example, a convex hull\index{convex hull} of a set of points. Envisioning that data so that it makes sense to the person receiving it has been the subject of entire books \cite{Tufte1986}, \cite{Tufte1990}, \cite{Tufte1997} -- it is likely that a simple, textual list will not best. Of course, if the purpose of the Reporter is to provide data to another program, a textual or numeric list might be exactly right. You can use the Reporter pattern in many different ways.

1. Displaying properties of an object. For example, the coordinates of a collection of points might be displayed numerically in a table, or their minima and maxima might be instanced as two points.
2. Defining an element in a different way. For example, a point defined in one coordinate system can be reported in another.
3. Conditionally selecting parts from your model. In this use, a Reporter is very similar to a Selector. For example, a roof surface comprising polygons might be reported by the degree of non-planarity in each.
4. Creating new objects from the reported objects. In this case of indirection, a property of an object is used to define another object. For example, a Reporter on line segments might comprise new segments coincident with the orginals' central third. Another Reporter example produces a dual polygon mesh (the mesh generated by replacing centroids with vertices, and edges between vertices with edges between centroids).
5. Sampling a model and then reporting this simplified version somewhere else and with different conditions to create a more complex model.
6. Copying. Generally, copying is reporting the model as many times as needed in different places, therefore features such as copy, mirror, clone and rotate are all kinds of reporting.

The Reporter pattern typically combines with other patterns to make a complete task. This pattern feeds information to downstream objects or directly to the designer. In many ways, it is a normal part of parametric modeling in that models are defined in terms of other models. The difference is that a Reporter is often not a part of the design, but rather a view on the design or an intermediary in the model construction process. In some sense, the Reporter pattern is an anti-Controller pattern. In a Controller, information flows from the control to the target -- typically from a simple model to a complex one. In a Reporter information flows the other way -- it typically is an abstraction of a larger model. Subtended Angle Envision information from the model as text. In a circle\index{circle}, any inscribed angle subtended by a chord of a given length is constant. A good demonstration is to justapose as text multiple angles\index{angle} subtended by a common chord. This model comprises a circle and two pairs of lines that represent two angles and share a common subtending chord. Showing both of those angles together demonstrates this simple geometric theorem. Text is perhaps the most simple Reporter. Sometimes it is actually effective. Click to download RepoSubtendedAngle.gct Array Depth Extract properties of a collection of points organized as an array. Collections of objects such as points\index{point} imply many properties, some pertaining the points themselves and some to their organization in a collection. Examples include the extrema coordinates of the points, the longest path in the collection (the array depth) and the number of elements in the collection. In this example, the Reporter pattern extracts such data from the model. In this sample, a function iterates over each element in a collection, accumulating the desired measures as it proceeds. Click to download RepoArrayDepth.gct Fabrication Transform design data for fabrication. Slicing a solid with planes gives a set of closed curves. A common fabrication technique is to use such curves to cut stations out of a sheet material and use the stations as internal formwork that is covered with other sheet or strip material. The curves\index{curve} forming the stations can be reported in variety of ways, in both 2D and 3D and through mediating transformations\index{transformation}. In this sample, one report is a scaled 3D version and the other is a 2D layout suitable (to a first approximation) as a cutting plan from sheet material. \begin{bodyNote} \pbOneCol {\input{\GCFigsPath/Patterns/ReporterFabricationExplain0.tex}} \end{bodyNote} Click to download RepoFabrication.gct Mirror Mirror a curve\index{curve} through an axis. Define a curve based on a point collection, and a line to use as a mirror. In 3D the line would be a plane. Now project poles to the axis (perpendicular to the line) and then create points on the other side of the axis at the same distance as between the original points and their projections. Finally create a curve from the new poles on the other side of the axis, this curve is a mirror of the first curve. In general, it will be an enantiomorph, that is, identical except for handedness. Click to download RepoMirror.gct Out of Plane Report the out-of-plane polygons of a surface both by color and text. Create a surface\index{surface} with curvature\index{surface!curvature} and then subdivide the surface with polygons or create an array of polygons on the surface. When the surface has curvature, some of the polygons may be non-planar. The degree of non-planarity depends on the local curvature of the surface. Using a function that iterates over the collection, extract the polygons that exceed a threshold out-of-plane measure. Visualize the resulting polygons\index{polygon} both in-situ with colour and in a tabular report. \begin{bodyNote} \pbOneCol {\input{\GCFigsPath/Patterns/ReporterOutofPlaneExplain0.tex}} \end{bodyNote} Click to download RepoOutOfPlane.gct Snapper Report a triangle snapped onto a grid. Parametric "sketches" (yes, a parametric model can be sketch-like!) are continuous. They may be abstracted into a discrete system, such as a grid. This Reporter maintains the original model and ability to smoothly interact with it and reports the model as it would appear were it snapped onto a specified grid. The reporting occurs in two steps. First, report the position of the original triangle in a new coordinate system. Second, select and report the nearest point on the grid to each triangle\index{triangle} vertex. Construct the reported triangle on these abstracted points\index{point}. Click to download RepoSnapper.gct Triangle Rotate and scale a triangle\index{triangle}. This sample separates the shape of an object from the space in which it is embedded, giving independent control of each. It reports a triangle developed in one coordinate system (the baseCS) in another system (the reporterCS) and provides rotation\index{frame!rotation} and scaling\index{frame!scaling} controls for that new coordinate system. Ironically, this Reporter uses a Reporter internally. In order to compute the rotation of the reporterCS system, a point is defined in the baseCS system at the origin of the reporterCS system, but one unit along the $x$-axis of the baseCS system. Reporting this point in the reporterCS system provides the arguments needed for the function Atan2 to correctly compute the rotation angle. Click to download RepoTriangle.gct