0024336: Content of OCCT documentation should be updated. Iter 2
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bf62b306 1OCAF White-Paper {#ocaf_wp}
3 @tableofcontents
5@section ocaf_wp_1 What is OCAF ?
7@subsection ocaf_wp_1_1 Purpose of OCAF
9 The Open CASCADE Application Framework (OCAF) is an easy-to-use platform for rapidly developing
10 sophisticated domain-specific design applications.
11 A typical application developed using OCAF deals with two or three-dimensional (2D or 3D) geometric modeling
12 in trade-specific Computer Aided Design (CAD) systems, manufacturing or analysis applications,
13 simulation applications or illustration tools.
15 Developing a design application requires addressing many technical aspects.
16 In particular, given the functional specification of your application, you must at least:
18 * Design the architecture of the application — definition of the software components and the way they cooperate
19 * Define the data model able to support the functionality required — a design application operates on data maintained during the whole end-user working session
20 * Structure the software in order to
21 * synchronize the display with the data — commands modifying objects must update the views
22 * support generalized undo-redo commands — this feature has to be taken into account very early in the design process
23 * Implement the function for saving the data — if the application has a long life cycle, the compatibility of data between versions of the application has to be addressed
24 * Build the application user interface
26 By providing architectural guidance and ready-to-use solutions to these issues,
27 OCAF helps you to develop your application significantly faster: you concentrate on the application's functionality.
29@subsection ocaf_wp_1_2 Overview of the Architecture
31 OCAF provides you with an object-oriented Application-Document-Attribute model.
32 This consists of C++ class libraries. The main class, Application, is an abstract class
33 in charge of handling documents during the working session. Services provided by this class include:
34 * Creating new documents
35 * Saving documents and opening them
36 * Initializing document views
38 The document, implemented by the concrete class Document, is the container for the application data.
39 Its main purpose is to centralize notifications of data editing in order to provide Undo-Redo.
40 Each document is saved in a single flat ASCII file defined by its format and extension (a ready-to-use format is provided with OCAF).
42 Application data are attributes, that is, instances of classes derived from the *Attribute* abstract class,
43 organized according to the OCAF Data Framework.
44 The OCAF Data Framework references aggregations of attributes using persistent identifiers in a single hierarchy (the Data Framework is described <a href="#ocaf_wp_2">in the next chapter</a>). A wide range of attributes come with OCAF, including:
46 * Primitive attributes such as *Integer, Real, Name* and *Comment*;
47 * Shape attribute containing the geometry of the whole model or elements of it;
48 * Other geometric attributes such as Datums (points, axis and plane) and Constraints (*tangent-to, at-a-given-distance, from-a-given-angle, concentric,* etc.)
49 * Modeling step and Function attributes — the purpose of these attributes is to rebuild objects after they have been modified (parameterization of models)
50 * Visualization attributes — these attributes allow data to be visualized in a 2D or 3D viewer
51 * User attributes, that is, attributes typed by the application
52 * In addition, application-specific data can be added by defining new attribute classes;
53 naturally, this changes the standard file format. The only functions that have to be implemented are:
54 * Copying the attribute
55 * Converting it from and to its persistent homolog (persistence is briefly presented in the paragraph <a href="#ocaf_wp_2_3">Persistent Data Storage</a>)
57@image html ocaf_wp_image003.png "The Application-Document-Attribute model "
58@image latex ocaf_wp_image003.png "The Application-Document-Attribute model "
60OCAF uses other modules of Open CASCADE Technology — the Shape attribute is implemented with the geometry supported by the <a href="#user_guides__modeling_data">Modeling Data</a> module and the viewer is the one provided with the <a href="#user_guides__visualization">Visualization</a> module. Modeling functions can be implemented using the <a href="#user_guides__modeling_algos">Modeling Algorithms</a> module.
62@subsection ocaf_wp_1_3 Getting Started
64 At the beginning of your development, you first define an application class by inheriting from the Application abstract class.
65 You only have to create and determine the resources of the application
66 for specifying the format of your documents (you generally use the standard one) and their file extension.
68 Then, you design the application data model by organizing attributes you choose among those provided with OCAF.
69 You can specialize these attributes using the User attribute. For example, if you need a reflection coefficient,
70 you aggregate a User attribute identified as a reflection coefficient
71 with a Real attribute containing the value of the coefficient (as such, you don't define a new class).
73 If you need application specific data not provided with OCAF, for example,
74 to incorporate a finite element model in the data structure,
75 you define a new attribute class containing the mesh,
76 and you include its persistent homolog in a new file format.
78 Once you have implemented the commands which create and modify the data structure
79 according to your specification, OCAF provides you, without any additional programming:
81 * Persistent reference to any data, including geometric elements — several documents can be linked with such reference;
82 * Document-View association;
83 * Ready-to-use functions such as :
84 * Undo-redo;
85 * Save and open application data.
87 Finally, you develop the application's graphical user interface using the toolkit of your choice, for example:
88 * KDE Qt or GNOME GTK+ on Linux;
89 * Microsoft Foundation Classes (MFC) on Windows Motif on Sun;
90 * Other commercial products such as Ilog Views.
92 You can also implement the user interface in the Java language using
93 the Swing-based Java Application Desktop component (JAD) provided with OCAF.
95@subsection ocaf_wp_1_4 Benefits of OCAF
97 As you use the architecture provided by OCAF, the design of your application is made easy:
98 the application developer concentrates on the functionality
99 instead of the underlying mechanisms required to support this functionality.
101 Also, thanks to the coupling with the other Open CASCADE Technology modules,
102 your application can rapidly be prototyped. In addition, the final application
103 can be developed by industrializing the prototype — you don't need to restart the development from scratch.
105 Last but not least, you base your application on an Open Source component:
106 this guarantees the long-term usefulness of your development.
108@section ocaf_wp_2 A Look Inside OCAF
110@subsection ocaf_wp_2_1 The Design of OCAF
112@subsubsection ocaf_wp_2_1_1 Reference-key model
114 In most existing geometric modeling systems, the data are topology driven.
115 They usually use a boundary representation (BRep), where geometric models
116 are defined by a collection of faces, edges and vertices,
117 to which application data are attached. Examples of data include:
119 * a color;
120 * a material;
121 * information that a particular edge is blended.
123 When the geometric model is parameterized, that is, when you can change
124 the value of parameters used to build the model (the radius of a blend, the thickness of a rib, etc.),
125 the geometry is highly subject to change.
126 In order to maintain the attachment of application data, the geometry must be distinguished from other data.
128 In OCAF, the data are reference-key driven. It is a uniform model in which reference-keys
129 are the persistent identification of data. All *accessible </b>*data, including the geometry,
130 are implemented as attributes attached to reference-keys.
131 The geometry becomes the value of the Shape attribute, just as a number is the value
132 of the Integer and Real attributes and a string that of the Name attribute.
134 On a single reference-key, many attributes can be aggregated;
135 the application can ask at runtime which attributes are available.
136 For example, to associate a texture to a face in a geometric model,
137 both the face and the texture are attached to the same reference-key.
139@image html ocaf_wp_image004.png "Figure 2. Topology driven versus reference-key driven approaches"
140@image latex ocaf_wp_image004.png "Figure 2. Topology driven versus reference-key driven approaches"
142@subsubsection ocaf_wp_2_1_2 Topological naming
144 Reference-keys can be created in two ways:
146 * At programming time, by the application
147 * At runtime, by the end-user of the application (providing that you include this capability in the application)
149 As an application developer, you generate reference-keys in order to give semantics to the data.
150 For example, a function building a prism may create three reference-keys:
151 one for the base of the prism, a second for the lateral faces and a third for the top face.
152 This makes up a semantic built-in the application's prism feature.
153 On the other hand, in a command allowing the end-user to set a texture to a face he/she selects,
154 you must create a reference-key to the selected face
155 if it has not previously been referenced in any feature
156 (as in the case of one of the lateral faces of the prism).
158 When you create a reference-key to selected topological elements (faces, edges or vertices),
159 OCAF attaches to the reference-key information defining the selected topology — the Naming attribute.
160 For example, it may be the faces to which a selected edge is common to.
161 This information, as well as information about the evolution of the topology at each modeling step
162 (the modified, updated and deleted faces), is used by the naming algorithm to maintain the topology
163 attached to the reference-key. As such, on a parametrized model,
164 after modifying the value of a parameter, the reference-keys still address the appropriate faces,
165 even if their geometry has changed.
166 Consequently, you change the size of the cube shown in the figure 2 above,
167 the user texture stay attached to the right face.
169 <b>Note</b> As Topological naming is based on the reference-key and attributes such as Naming
170 (selection information) and Shape (topology evolution information),
171 OCAF is not coupled to the underlying modeling libraries.
172 The only modeling services required by OCAF are the following:
174 * Each algorithm must provide information about the evolution of the topology
175 (the list of faces modified, updated and deleted by the algorithm)
176 * Exploration of the geometric model must be available
177 (a 3D model is made of faces bounded by close wires,
178 themselves composed by a sequence of edges connected by their vertices)
180 Currently, OCAF uses the Open CASCADE Technology modeling libraries.
182@subsubsection ocaf_wp_2_1_3 Aggregation of attributes
184 To design an OCAF-based data model, the application developer is encouraged to aggregate
185 ready-to-use attributes instead of defining new attributes by inheriting from an abstract root class.
186 There are two major advantages in using aggregation rather than inheritance:
188 * As you don't implement data by defining new classes, the format of saved data
189 provided with OCAF doesn't change; so you don't have to write the Save and Open functions
190 * The application can query the data at runtime if a particular attribute is available
192@subsubsection ocaf_wp_2_1_4 Summary
194 * OCAF is based on a uniform reference-key model in which:
195 * Reference-keys provide persistent identification of data;
196 * Data, including geometry, are implemented as attributes attached to reference-keys;
197 * Topological naming maintains the selected geometry attached to reference-keys in parametrized models ;
198 * In many applications, the data format provided with OCAF doesn't need to be extended;
199 * OCAF is not coupled to the underlying modeling libraries.
201@subsection ocaf_wp_2_2 The Data Framework
203@subsubsection ocaf_wp_2_2_1 Data structure
205 The OCAF Data Framework is the Open CASCADE Technology realization
206 of the reference-key model presented in the previous paragraph.
207 It implements the reference-key as label objects,
208 organized in a tree structure characterized by the following features:
210 * A document contains only one tree of labels
211 * Each label has a tag expressed as an integer value unique at its level in the tree
212 * A label is identified by a string — the entry — built by concatenation of tags from the root of the tree, for example [0:1:2:1]
213 * Attributes are of a type identified by a universal unique identifier (GUID)
214 * Attributes are attached to labels; a label may refer to many attributes as long as each has a different GUID
216 As such, each piece of data has a unique persistent address made up of the document path,
217 its entry and the GUID of its class.
219 In the image the application for designing coffee machines first allocates
220 a label for the machine unit. It then adds sub-labels for the main features
221 (glass coffee pot, water receptacle and filter) which it refines as needed
222 (handle and reservoir of the coffee pot and spout of the reservoir).
224 You now attach technical data describing the handle — its geometry and color —
225 and the reservoir — its geometry and material.
226 Later on, you can modify the handle's geometry without changing its color —
227 both remain attached to the same label.
229@image html ocaf_wp_image005.png "Figure 3. The data structure of the coffee machine"
230@image latex ocaf_wp_image005.png "Figure 3. The data structure of the coffee machine"
232 The nesting of labels is key to OCAF. This allows a label to have its own structure
233 with its local addressing scheme which can be reused in a more complex structure.
234 Take, for example, the coffee machine. Given that the coffee pot's handle has a label of tag [1],
235 the entry for the handle in the context of the coffee pot only (without the machine unit) is [0:1:1].
236 If you now model a coffee machine with two coffee pots, one at the label [1],
237 the second at the label [4] in the machine unit,
238 the handle of the first pot would have the entry [0:1:1:1]
239 whereas the handle of the second pot would be [0:1:4:1].
240 This way, we avoid any confusion between coffee pot handles.
242@subsubsection ocaf_wp_2_2_2 Compound documents
244 As the identification of data is persistent, one document can reference data contained in another document,
245 the referencing and referenced documents being saved in two separate files.
247 Lets look at the coffee machine application again. The coffee pot can be placed in one document.
248 The coffee machine document then includes an *occurrence </b>*— a positioned copy — of the coffee pot.
249 This occurrence is defined by an XLink attribute (the external Link)
250 which references the coffee pot of the first document
251 (the XLink contains the relative path of the coffee pot document and the entry of the coffee pot data [0:1] ).
253@image html ocaf_wp_image006.png "The coffee machine compound document"
254@image latex ocaf_wp_image006.png "The coffee machine compound document"
256 In this context, the end-user of the coffee machine application can open the coffee pot document,
257 modify the geometry of, for example, the reservoir, and overwrite the document without worrying
258 about the impact of the modification in the coffee machine document.
259 To deal with this situation, OCAF provides a service which allows the application to check
260 whether a document is up-to-date. This service is based on a modification counter included in each document:
261 when an external link is created, a copy of the referenced document counter is associated to the XLink
262 in the referencing document. Providing that each modification of the referenced document increments its own counter,
263 we can detect that the referencing document has to be updated by comparing the two counters
264 (an update function importing the data referenced by an XLink into the referencing document is also provided).
267@subsubsection ocaf_wp_2_2_3 Transaction mechanism
269 The Data Framework also provides a transaction mechanism inspired from database management systems:
270 the data are modified within a transaction which is terminated either by a Commit
271 if the modifications are validated or by an Abort if the modifications are abandoned —
272 the data are then restored to the state it was in prior to the transaction.
273 This mechanism is extremely useful for:
275 * Securing editing operations (if an error occurs, the transaction is abandoned and the structure retains its integrity)
276 * Simplifying the implementation of the Cancel function (when the end-user begins a command,
277 the application may launch a transaction and operate directly in the data structure;
278 abandoning the action causes the transaction to Abort)
279 * Executing Undo (at commit time, the modifications are recorded in order to be able to restore the data to their previous state)
281 The transaction mechanism consists simply of managing a backup copy of attributes.
282 During a transaction, attributes are copied before their first modification.
283 If the transaction is validated, the copy is destroyed.
284 If the transaction is abandoned, the attribute is restored to its initial value
285 (when attributes are added or deleted, the operation is simply reversed).
287 Transactions are document-centered, that is, the application starts a transaction on a document.
288 So, modifying a referenced document and updating one of its referencing documents requires
289 two transactions, even if both operations are done in the same working session.
291@subsection 2.3 Persistent Data Storage
293 In OCAF, persistence, that is, the mechanism used to save a document in a file,
294 is based on an explicit formal description of the data saved.
296 When you open a document, the application reads the corresponding file
297 and first creates a memory representation of it.
298 This representation is then converted to the application data model —
299 the OCAF-based data structure the application operates on.
300 The file's memory representation consists of objects defined by classes known as persistent.
301 The persistent classes needed by an application to save its documents make the application's data schema.
302 This schema defines the way the data are organized in the file — the format of the data.
303 In other words, the file is simply an ASCII dump of the persistent data defined by the schema,
304 the persistent data being created from the application data model during the save process.
306 Only canonical information is saved. As a matter of fact,
307 the application data model usually contains additional data to optimize processing.
308 For example, the persistent Bézier curve is defined by its poles, whereas
309 its data model equivalent also contains coefficients used to compute a point at a given parameter.
310 The additional data is calculated when the document is opened.
312 The major advantages of this approach are the following:
314 * Providing that the data format is published, files created by OCAF-based applications
315 can be read without needing a runtime of the application (openness)
316 * Although the persistence approach makes the data format more stable,
317 OCAF provides a framework for managing compatibility of data between versions of the application —
318 modification of the data format is supported through the versioning of schema.
320 OCAF includes a ready-to-use schema suitable for most applications.
321 However, it can be extended if needed. For that, the only things you have to do are:
323 * To define the additional persistent attributes
324 * To implement the functions converting these persistent attribute to and from the application data model.
326Applications using compound documents extensively (saving data in many files linked together) should implement data management services. As a matter of fact, it's out the scope of OCAF to provide functions such as:
327* Version and configuration management of compound documents;
328* Querying a referenced document for its referencing documents.
330In order to ease the delegation of document management to a data management application, OCAF encapsulates the file management functions in a driver (the meta-data driver). You have to implement this driver for your application to communicate with the data management system of your choice.