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Editing of three-dimensional geometry at performance of design works
While the systems based on stories of construction, are trained in possibilities of direct interaction with geometry and become more “obvious” systems without history are trained in ways of indirect management by geometry, becoming more “parametrical”.

So, what editing of 3D-geometry at performance of design works includes? When we use 3D-CAD-system for the purpose of designing, we basically describe and we transform geometry in a virtual environment. Today this process is carried out or “directly” or “indirectly”. The indirect way is characterised by usual methods when for management of geometry are defined and modified structure and means of intellectualization of model. The direct way is characterised by obvious methods of interaction with 3D-geometry. It is interesting enough that today both these of a way of interaction with model exist as within the limits of 3D-CAD on a basis of history of construction, and in systems without history.

We will make the detailed review of how the 3D-geometry is edited and modified in 3D-CAD-systems. It is probable, when we are users CAD – we learn more that occurs in our CAD-systems today, We begin to use is better our tools.

We will look, as indirect editing works in an environment with history of construction and in an environment without history. Then we will discuss, as direct editing works in each of these two environments.

Let's consider indirect editing of 3D-geometry at performance of design works.

At indirect editing in model parametres, the sizes and other means of intellectualization with which help possible transformations and relations between model elements are defined are added. In systems with construction history these means of intellectualization remain in a tree of construction at the moment of model creation. In systems without history of construction of means of intellectualization become attached to boundary representation of geometry (BRep) or at the moment of model creation, or later. Means of intellectualization of model finally should define, what elements and will change how at editing.
Let's consider direct editing of 3D-geometry at performance of design works.

Direct editing does not demand management by means of intellectualization means. This technology provides manipulation possibility directly with boundary geometry BRep. Direct interaction with BRep-model not trivial, but we receive more intellectual means. When editing is carried out directly, it is necessary to choose geometry and obviously to set transformations.

In the end of article results and analysis of how these technologies influence your process of working out of a product are resulted.

Let's consider indirect editing at performance of design works.

Indirect editing is usually used in systems of parametrical modelling with construction history. At indirect editing parametres, the sizes and other means of intellectualization which then are used for definition of possible transformations and mutual relations associate with 3D-model. In systems on a history basis these means of intellectualization are brought in a construction tree (history-tree) during model creation. In systems without history, on the contrary, intellectualization means associate with BRep-model during time or after its creation. In any case (with history or without it) intellectualization means are used for the task of geometrical transformations. Added in model “intelligence” describes that will change, i.e. geometry and as it will change, i.e. transformation.

After updating and processing put in model “intelligence” the resultant 3D-geometry can change. What exactly varies in geometry, completely depends on what means of intellectualization have been set, and as they have been structured. It also is “indirect transformation” 3D-geometry. Advantage of this technology (indirect editing) consists that you can set and associate with model demanded behaviour. When you can really expect the future changes of your model, the given technology can be very useful. It also is applied in need of a positive control of mutual relations between sides, constructive elements and details during editing.

Let's consider indirect editing with history at performance of design works.

The majority of users of 3D-CAD-systems is familiar with the given environment. It is a standard method of change of 3D-geometry in CAD-systems on a basis of history of construction. Management in parametres occurs in a context of a tree of construction where all elements are structured by means of the relation the parent/child. Structure, relations and parametres are always created and ordered in a tree simultaneously with model working out. Process of the correct organisation and management of this structure is not optional. It what each user of systems with history of construction for an effective utilisation of tools should seize. In systems on a basis of history you always or add new structure, or edit existing (i.e. carry out indirect editing). When there are changes in the history (in sketches, parametres, constructive elements or in their order), the geometrical model is recycled and it is authorised linearly on the basis of tree structure.

In this case it is important to know structure of a tree and to expect, as concrete change will affect affiliated elements (so-called, “wave effect”). In certain cases this effect is wished, and in many other things is on the contrary.

In systems on a basis of history of construction users actually edit process and structure, i.e. “the recipe” which has been defined during model creation. You do not edit 3D-geometry, and change the recipe and ask system to reconstruct geometrical model on the basis of the changed recipe. In this context the 3D-geometry always is created anew, instead of modified.

At indirect editing it is possible to make inadvertently changes which will make model incorrect. Ability to work with such situations is critical for good modelling on construction stories.

The technology of indirect editing with construction history has worried recently “facelift” connected with occurrence of means of dynamic or instant updating of model. By means of these means the user can choose a side or a constructive element of model and dynamically modify the means of intellectualization connected with it (parametres and the sizes). The subsequent permission of parametres and is carried out the sizes by linear evolution of model – as well as in case of usual editing, – but for the user it looks as instant updating. This method is similar to direct editing of geometry, but is not that (see more low about the present methods of direct editing in a context of systems with construction history) as its result completely depends on structure of means of intellectualization of model.

Let's consider indirect editing without history at performance of design works.

The given technology is not new, but it is unknown to the majority of users CAD. It gives possibility of indirect manipulation with boundary geometry (BRep) by means of means of intellectualization added to it (parametres and the sizes) – as well as in case of indirect editing with construction history. However in this case parametres are not a tree part (which in these systems is absent), and associate directly with elements BRep, i.e. with tops, edges and faces. All mutual relations imposed on model are resolved simultaneously that usually nonlinear depends on the size of model. Thus has no value an order in which parametres are added in model. The given technology does not possess “wave effect” based on mutual relations the parent/child.

Indirect editing without construction history really modifies geometry, instead of creates it anew – as in case of modelling on a history basis. There are some different technologies which can be used for updating solid-state BRep models. For a guarantee of a correctness of model, and also for maintenance of expected result of operation of indirect editing the system should possess certain intelligence.

As well as in case of indirect editing on a basis of history of construction the user should know, how parametres are structured. However, as the order of parametres is not important, to operate intellectualization means rather simply.

Today exists not so many CAD-systems without history of construction which provide possibility of indirect editing. Work of one of them – PTC CoCreate Modeling – is shown by the following roller:

There are pros and cons of each approach – indirect editing without history and that on a history basis. The understanding of what of them in the best way can support your requirements, can be a challenge which we will analyse in the end of article.

Let's consider direct editing at performance of design works.

Possibilities of direct editing of 3D-geometry are present for today at the majority of systems with construction history though historically they developed within the limits of systems without history (with direct modelling). Direct editing means that the model is not burdened by any means of intellectualization, and consists in possibility of direct manipulation with geometry BRep. In most cases solid-state model BRep can be modified methods of direct editing – irrespective of, when and as it has been created. Direct manipulations with BRep-model not trivial, but this technology has considerably developed for last twenty years, and now is quite mature.

Boundary representation (English Boundary REPresentation, BREP) – a method of representation of the volume form by the description of its borders. The 3D-body is represented a set of the surfaces connected with each other setting border between the represented body and other space. Simply speaking, points (top) communicate edges, edges communicate with each other, forming sides, and the sheaf of sides forms “water-proof” a body. All these communications are called as model topology. If direct editing means topology change, i.e. addition or removal of sides, edges and tops, to model of Boolean operation should be applied. In case editing does not cause topology change, it is possible to manage local operations on model. When we try to carry out change of geometry by means of direct editing, the system should be enough intellectual to know, what function will provide the best and most expected result.

Unlike indirect editing, direct demands from the user of the obvious specification of that (geometry) will vary and as it will vary (transformation).

Instructions “that”. The elementary way of the task of geometry at direct editing is a choice of a side the index. If it is necessary to change some sides, the CAD-system can provide function for a plural choice (multi-selection). Some systems also use recognition of constructive elements in real time (real-time feature recognition) to help the user at a choice of several sides which represent one ledge, a pocket, an edge or an aperture. The constructive elements defined by the user can be accessible to a choice. There are also other technologies for the help to the user in a geometry choice at direct editing. For example, at a choice the system can allocate with the user of a planar side all coplanar sides. Or to choose all adjacent sides, tangents given, for example. In base BRep-models enough intelligence contains that systems have distinguished conditions of this kind. In some more advanced systems the choice of geometry for editing is possible in the several isolated details simultaneously that allows to operate relations between details during editing.

Instructions “as”. At the task of transformation the user usually specifies a 3D-axis or a 3D-vector with distance or corner instructions. Many systems offer 3D-icons or special tools to help the user to set desirable transformation. By means of such means the user can choose at first directing vector or a rotation axis, and then to drag geometry on some distance or to turn on some corner.

At the task of size of carrying over or an angle of rotation many systems allow to print desirable value directly also. In some systems it is possible to refer to other geometry at the task of size of carrying over/turn, including on other details in assemblage.

After the user has defined as how to move, updating can be finished. The geometry changes immediately as updating is applied directly to its internal representation – solid-state BRep models.

Advantage of the given technology consists that users should not worry about how in their model intellectualization means are structured. They simply manipulate geometry irrespective of, when and as it has been created.

Let's consider direct editing on a history basis at performance of design works.

The given technology is a novelty for the majority of users CAD. It provides possibility of direct interaction with “a resultant” BRep the model received on a tree of construction. Each operation of direct editing should be written down in a tree, and each time when indirect editing is used and is created new BRep the model, all operations of direct editing should be applied to it anew.

The way instant or dynamic or direct shape editing of parametres with direct editing. Similar operations of indirect editing modifies in real time operating parametres of constructive elements. This technology is definitely useful, but, as well as in case of use of other means of indirect editing, the end result depends on an order of creation of constructive elements. At performance of operations of direct editing the system with construction history cannot use any information on the geometrical mutual relations, found in topology BRep as these mutual relations can change at the subsequent updating of the constructive elements standing above in a tree of construction. Therefore the majority of operations of direct editing in systems with construction history will be always limited in the possibilities. For example, there can not be powerful methods of a choice of geometry, such as a choice of several sides simultaneously or recognition of constructive elements, no less than a choice of elements in several details simultaneously. Also, in systems with construction history there can not be powerful tools for the task of transformations, and these systems cannot process difficult topological changes.

Direct editing in systems with construction history also becomes complicated that fact that all operations of editing should be structured and ordered in a construction tree together with other structures containing there. As a result the construction tree becomes more difficult. Operations of direct editing depend on parental constructive elements and influence affiliated constructive elements. In practice direct editing in systems with construction history causes more questions, than adds values. Therefore it is better to avoid direct editing in such systems – at least, at current level of their development. However, this possibility is definitely useful at work with the imported models.

Let's consider direct editing without history at performance of design works.

If you use means of direct modelling similar CoCreate Modeling, KeyCreator or SpaceClaim the given technology should you be familiar. Direct editing is the basic method of change of 3D-geometry in systems without construction history. Editing is carried out directly at level BRep of model, does not register anywhere and is not structured.

As direct editing is the basic method of manipulation with geometry in systems without construction history, it is important to provide users with a set of powerful tools and methods for a geometry choice. Such tools and methods include plural a choice, a choice by means of a rectangular parallelepiped (boxing), recognition of constructive elements, the conditional choice, constructive elements defined by the user, templates, lists/filters for a choice the choice in several details simultaneously etc. Should be given. Besides, such systems should give a tooling and methods for the task of transformations. For example, the task of several transformations within the limits of one operations can be accessible. Intellectual extraction of the data from BRep and use of this information is critical for the organisation of reliable process of direct editing.

Certainly, after the choice of geometry is carried out and transformations are set, it is important to provide result expected by the user as frequently there are some decisions.

Direct editing definitely is motive power of our industry. It provides huge jump in flexibility in comparison with indirect editing, especially that is based on construction history. By means of the advanced methods of a choice of geometry which are accessible now in some systems without construction history direct editing it can to be used for management of mutual relations between details and assemblages – that was possible earlier only within the limits of indirect editing.

Thus, as modelling on base of history of construction was many years prepotent technology, it continues to be used widely in designing. But after modelling without history became more reliable and useful, we can choose between two very viable and very various technologies. As such choice influences the data or “intellectual property” your company, and also will affect your process of working out of the products, the given choice should be based on something more, than simply user preferences.

Let's consider 3D-geometry editing on a history basis at performance of design works.

Within the limits of technology on a basis of history geometry editing usually is indirect, and intellectual level for management of editing is set simultaneously with creation of the model. Here it is very important to create correctly structured tree of construction. The given environment demands use of joint methodology of modelling and careful attention to that as as will change in model in the future. The basic advantage of such environment is that thanks to record of each step of modelling, editing of any of the previous steps, expressed in reception of the model which are distinct from the initial is possible. The basic call here is the compatibility requirement in modelling methods, inflexibility, complexity for studying, the requirement of good knowledge of structure prior to the beginning of use of the data. That you see, is not that you receive, in systems with construction history. From the point of view of the correct organisation of process, it is necessary for you to be assured that value which you receive from well structured tree of construction, will justify your investments.

Direct editing will continue to extend in systems with construction history, bringing the advantages, but remaining limited in the possibilities.

If your process and products depend from the highly structured engineering data and if this data has sufficient life cycle and use which will justify investments then modelling on a history basis can be a good choice.

Let's consider editing of 3D-geometry without history at performance of design works.

Within the limits of technology without history of construction editing usually is to straight lines. By default in model there is no data which describe how it has been created or as one constructive element influences another (the relation the parent/child). That you see in models without construction history, is the same that you receive at editing. This technology provides very fast and flexible environment for the modelling, not demanding understanding of conceptual integrity (design intent) prior to the beginning of modelling process. The basic call of this technology is editing of difficult forms. Though the technology constantly improves, editing of the forms consisting of B-splines or NURBS of surfaces with a considerable quantity of roundings off, can sometimes be difficult realizable. Here all again rests against topology. Maintenance “connectivity” during time operation extracting and movings is not trivial. Such difficult forms definitely can be modified by means of technology without history, but it not so is simple, as change of parametre and history playing.

We are waited by new possibilities of indirect editing within the limits of modelling without history. PTC CoCreate many years uses technology DCM of company D-Cubed (departament of Siemens PLM Software) for management of possibilities of indirect editing. Company LEDAS also develops very similar technology which you can see working in system Rhino.

From the point of view of the process organisation if you need to make only the project as soon as possible and as it is possible more flexibly the technology without construction history can be the best choice. It is very cheap technology. She allows to use smaller quantity of the data and it is natural to process the big assemblages.

The above-stated comments concern mainly model of use of the tool in which there are many distinctions influencing process of designing. However the data is more important, than use model. Though both technologies (on a basis of history and without that) create BRep model of a 3D-body, there are important distinctions in how with their help the intellectual property which then is used by decades throughout product life cycle is created.

At use of technology without construction history 3D-body BRep in itself is the main document. Without it you do not have anything. At technology use on a basis of history BRep the model is only result of processing of a tree of construction. This tree also is your main document. Without it (in a context of the given technology) you do not have anything – only “mute” a body. The most important question concerning management of a construction tree as the basic document consists that the history tree is private (internal): there are no industrial standards for an exchange of such type of the data. Therefore the majority of the engineering workings out executed by means of technology on a basis stories of construction, have rather short life cycle. Pay special attention on the intellectual property concerning working out of a product, the size of your investments into its creation and influence on your future workings out.

It is interesting that some companies-manufacturers of CAD-systems speak about a technology combination on a basis of history of construction with technology without history. It will be interesting to see, to what they as a result will come. Today CAD-given or contain construction history, or have no it. And if the history was gone, it really was gone for ever. The reconstruction of a tree of construction for model without history will be similar to a reconstruction of the picture album which has burnt down on fire. CAD-systems on a basis of history of construction and CAD-system without history definitely can co-exist and supplement each other, but it is necessary to understand accurately how it is possible to operate intellectual property in such environment.

Now, when exist a choice between two efficient technologies, consumers need to analyse attentively process of working out of the product and to consider, what of technologies in the best way satisfies their current and future requirements. There are many parametres for such analysis.


Sources:
1. Hamilton P. Editing in 3D-geometry//isicad. – 2009 [http://isicad.ru/ru/articles.php?article_num=13263]
2. Boundary representation [http://plmpedia.ru/ wiki/BREP]

The author: Челябэнергопроект
Date: 03/23/2010

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