Modifications

Applying controlled modifications to a CAD model preserving their integrity and final quality

The global modelling of shapes developed in ThinkDesign, more commonly known as Global Shape Modelling, allows 3D entities like points, curves, surfaces, solids and meshes to be modified using controlled modification rules. All the Features applied to the objects like joints, trimming and ribbing remain unaltered and preserve their shape remaining perfectly welded to the model even when the object radically changes its shape.

More simply, the GSM algorithm developed in ThinkDesign which manages the modifications allows an object's structure to be modified setting, where necessary, one or more constraints in order to "block" precisely one or more specific areas of the CAD model. Naturally, in view of all this, it is possible to ensure that the modification satisfies the tangency and curvature requirements between the surfaces. This detail should not be underestimated; rather it is of fundamental importance when the object to be modified must present continuity between surfaces. In the case of objects of a mechanical nature this aspect might be of secondary importance but in the case of "aesthetic" surfaces typically applicable to the industrial design sector, it is very important to be able to rely on the quality of the resulting modified surfaces. Indeed, the quality of the aesthetic surfaces typically used in Industrial Design is perceived by the human eye through the characteristic curves and ambient reflection reproduced on them.

GSM

Global Shape Modelling (GSM) can be used in various applications:

• Deforming a model
• Plate mould or other compensation to control the distortion which occurs during the mould manufacturing process (springback)
• Local modifications (GSM Zone Modelling) like closing a gap between two surfaces or faces of a solid 
• Aesthetic modifications or revisiting a 3D model in order to produce n types of variants
• Making aesthetic features, twisting, folding and stretching the existing model

The figure above shows two curves. The source curve is shown in blue whereas the target curve is in red

The curvature graph shows the quality of the source curve

The curve shown in green is the result of the modification. The curvature graph activated on the modified green curve confirms the validity of the operation

Features

There's modification and modification...
In general terms, nearly all CAD systems on the market currently offer tools to modify an object's structure; however, few are capable of guaranteeing surface accuracy and quality. To take a 3D model and apply a "lattice deformation" type frame is quite simple and indeed, if the purpose is merely to have a 3D display/animation of the object, these simple tools are more than sufficient. However, if the final objective is to modify and correctly preserve continuity between the modified surfaces, various issues come into play. For example, the object must remain complete, it must not disintegrate and, very importantly, the quality of the modified surfaces must remain where possible of the same initial order. For example, various CAD systems modify an object converting it to a lower order. For example, if I have a 4th order surface, following the modification the resulting surface must be of the same order

How GSM works
The GSM controlled modification environment provided with ThinkDesign produces native surfaces which differ from the traditional topology based on NURBS (Non Uniform Rational B-splines). Using special controls GSM entities can be transformed into NURBS surfaces or curves adopting various transformation strategies. One of the most significant is to maintain the initial order of the modified surfaces/curve. As GSM bypasses the traditional NURBS topology, it offers tools which can help the designer to solve various problems; one of the main ones is the possibility of easily editing surfaces with n sides (trimmed surfaces). Finally, another advantage of GSM is the possibility of moving the edge of a trimmed surface in space, tangency or continuity, fixing it in a different place which can be the edge of a surface, a point, etc


The blue surface must be modified; on the outer edge a fixed modification rule is applied whereas point 1 must collide with point 2


Following the modification the isophote analysis is shown from which it can be seen that the curvature of the surface is constant and has no inflections.

Constrains

Setting constraints
As indicated above, the distinctive feature of the GSM modification environment are the constraints that can be set on the objects. The constraints are used to selectively preserve the object's shape. The object shown below, although very simple, clarifies this important aspect of ThinkDesign's GSM.

The red edges represent the areas to be preserved whereas the movement is shown by the numbers 1 and 2

As can be seen, the model was modified correctly and the areas fixed beforehand remained the same. The mesh topology seen at the base of the cylinder, although it may seem to be an artifact, confirms that the tangency rule set on the lower edge was respected; indeed, both cylinders (the source object and the modified object) merge uniformly in this area

Quality

Why modify
Modifying a CAD object using a controlled modification is successful in a variety of settings. Simply think of a designer who has to reassess the aesthetic shape of a model while working on a CAD model from, for example, a CAD environment different from the one normally used. Traditionally, moving data from one CAD modelling environment to another is done using the IGES or STEP transfer formats. The final result is a CAD model, with no history, on which the modifications are made. These modifications may be of a global nature (on the whole model) or, more simply, localised in specific areas. It almost goes without saying that if the CAD model is complicated, with lots of features, the modification could be tiresome and probably also very long.  A controlled modification environment such as ThinkDesign's GSM makes it easy to reassess the shape of a model quickly without abandoning the most important aspect of the modification; quality.

Zone Modelling by GSM

Zone modelling is a particular variant of GSM for working locally and selectively on solids. Zone modelling includes the normal GSM tools so it operates in a very similar way. The zone to be modified is established, the source curves are chosen, then the target curves, and finally the constraints are set: position, tangency or curvature



The IGES CAD model shown in the picture above is static and has no history. To make a modification preserving the model's structure inevitably means deleting the "old" surfaces, redesigning the curves and consequently redefining the new surfaces



The purpose of the modification is to raise the car's bonnet according to a new curve (red). Following the modification, corresponding to the dotted yellow edge, the second order curvature between the surfaces must be respected



Result. The bonnet is raised according to the target curve and maintains, where set, the curvature between the surfaces

Surface control

.GSM to control surface quality
ThinkDesign's GSM is also useful for checking the quality of the surfaces. In some circumstances, it may be necessary to correct inflections on the surface highlighted following isophote analysis. As an initial step, you might think of solving the problem by moving the normal control points of the surface. This modification, which takes a long time, is not the best solution as moving the point could produce a local modification. In addition, the point moved does not lie on the surface so it would be necessary to proceed by trial and error without a precise reference. Thanks to ThinkDesign's GSM it is possible to exploit the light lines to produce an effective and precise modification. In brief, the wrong light lines (source curves) are extracted and the correct curves (target) redesigned. More simply, GSM allows constraints to be put on the surfaces in such a way as to obtain a new curve of the light line coinciding with the new curve that has been created.



The isophote analysis of the surface highlights an artifact. To resolve this problem GSM modification is used combined with the light lines



The light line is extracted in COS (Curve on Surfaces) mode. This will be the line from which the modification will start



In view, a new curve is designed which is accurate and has no inflections. This will be the target curve


GSM corrects the defect automatically using the two curves as reference.

Zone Modeling

Zone Modelling by GSM
Zone modeling is a particular variant of GSM for working locally and selectively on solids. Zone modeling includes the normal GSM tools so it operates in a very similar way. The zone to be modified is established, the source curves are chosen, then the target curves, and finally the constraints are set: position, tangency or curvature.

 

The figure above shows the object to be modified. It is a solid so we will use the Zone Modeling command to apply to the protruding pipe.



A tangency constraint is set corresponding to the part shown by the yellow dotted line, and then the surfaces to be modified are selected. The source curves are shown in green and the target curves in red



The final result following Zone Modelling

Projections

Another advantage of GSM over competitors' systems is the possibility not only of setting constraint rules but also of respecting the proportions of the modified model. The following example shows a typical modification. Without the possibility of maintaining the modification projection, the central zone of the model, which clearly refers to a circle, bends in the direction of the Y axis



The hole shown in the centre of the object in the figure must be modified according to the yellow dotted projection line





Following the modification we can clearly see the excessive stretching of the central part. If it becomes necessary to control this aspect, a projection constraint can be set using different criteria. In this case the yellow dotted line will act as guide



The correct result after the modification