AR1AE015 WORKSHOP 1: Editing NURBS geometry
Editing NURBS geometry
We can alter the NURBS geometry in several ways.
Maya, as the most other 3d modeling software make use of History. History is an option which keeps track of the commands used to generate a surface. In the case of curve based geometry there is still a direct link between the surface and the curves from which they where generated. Altering the curve will alter also the surface. If the geometry was not generated by curves but by isoparms on other surfaces the alteration of these surfaces will affect the surface which was created from the isoparms. The use of history can be an effective tool to relate translations and deformations of parts the geometry to other parts of the geometry. If surfaces have a direct relation the surface generation of parts of the surface which it has a relation to can be very effective and save time in altering the 3d model. The strategy in building the objects is important to use this functionality to its maximum.
A geometry based on curves. Parts of the geometry is based on other geometry by using the isoparms to generate new surfaces. The CURVE RANGE - PARTIAL option of some of the tools will have their full effect made visible in this set up. The length of the isoparm which is used can be shortened but will still be attached to the geometry to which the isoparm belongs.
When surfaces are attached they will form a new surface. The NURBS surface is basically a rectangular surface based on two sets of Isoparms describing the surface in two directions, the U and V direction. The Isoparms behave like bezier curves and can be adjusted by moving the vertices of the surface which are linked to the Isoparms. The Isoparm is a continuous line on a surface.
- EDIT NURBS – ATTACH SURFACE: When we attach two surfaces, the Isoparms will have to be combined in one to generate the new attached surface. If the Isoparms don’t line up they could be moved, possibly altering the curvature of the surface or extra isoparms are intergarted. If the amount of isoparms don’t match new ones will be created to support both sets of the surfaces. The surface will move to close the gap between the two surfaces. If the isoparms are connected the curvature of the surfaces may alter due to the way the isoparms connect with each other. They can blend together or keep their curvature and make a hard connect without influencing the other isoparm.
- EDIT NURBS – ATTACH WITHOUT MOVEMENT.: With this option the surfaces are blended together. They don’t move towards each other. The surfaces themselves will retain their shape, their will be a surface created to fill the gap and the surface will try to align its tangency with the existing surfaces.
- EDIT NURBS – ALIGN SURFACES.: With this option the surfaces are aligned . Just as the curves there are three methods of alignment. The C0 –positional alignment, the C1 – tangency alignment ( the surface will make a smooth connection with the least impact on its original shape) C2 – Curvature. The curvatures will be matched to generate a smooth transition. The in and out curvature will be the same, the effect on the existing surface will be larger. In the option box the settings of the alignment can be altered.
- EDIT NURBS – OPEN-CLOSE SURFACE: Just as a curve a surface can be closed. The function works like the attach option with the difference that the attached edges belong to the same object. A surface can be closed in both directions or one of the two directions.
Deleting parts of the surface
There are basically two kinds of methods to delete parts of a surface.
- EDIT NURBS – DETACH SURFACEOne is to detach a part of the surface using an Isoparm as a cutting edge. This option will break the surface in multiple parts at the Isoparm or Isoparms selected on the surface. The advantage of this method is that the resulting geometry still will be defined by a set of crosswise isoparms and will have the full functionality of a NURBS surface.
The other option is to trim the surface. Although this option is very effective in creating holes and cutting parts of the surface it will have an effect on the topology of the surface. With trimming parts of the surface which are trimmed will become invisible. However the topology of the original surface remains. The geometry will basically behave as the original shape when trimming or aligning. This will have an effect on the possibilities to align the trimmed surface to another surface. The alignment will take place on the original untrimmed surface, ignoring the trim. Another limitation is that it is impossible to attach a trimmed surface. Because the topology of the trimmed surface is still the untrimmed variant of the surface the geometry can’t align with the trimmed surface. If a trim gives to much trouble the surface can be UNTRIMMED and be restored to its original state.
- EDIT NURBS – TRIM TOOL: The tool will make it possible to cut out or cut off a part of a surface. The cutting edges are closed curves on the surface or curves moving van one edge of the surface to another or the same edge of the surface. The surface within the curves can be deleted or kept and the surface outside the curves deleted.
- EDIT NURBS – PROJECT CURVE ON SURFACE:To be able to trim a surface a closed curve has to be placed on the surface itself. Maya has the option of projecting a curve on a surface. The curve will be projected along the axis of the view port or the normal of the surface. The result will be a curve on both sides of the surface.
Another less accurate method is making the surface LIVE draw the curve directly onto the surface. The way the curve reacts on the surface is difficult to predict an can be problematic.
- EDIT NURBS – INTERSECT SURFACES: The curves on a surface can be projected, drawn on and defined by the intersection of two surfaces. Curves of the intersected surfaces can be added to both objects and later used for trimming. This method is especially usefull for different objects which have to connect precisely.
Adjusting the surface
If we look at the difference of the edit commands of NURBS and polygons , one thing is obvious. The amount of edit tools for polygons are much larger and are mostly focused on editing on a component level of the geometry. The options of editing on component level with NURBS is limited by adding Isoparms into the surface. This difference of emphasis illustrates the functional difference of both geometry types.
- EDIT NURBS – INSERT ISOPRAM.: Just like the curves it is possible to refine the geometry by adding vertices. In the case of the surface we add Isoparms which in their turn are defined by the vertices. Adding an Isoparm will increase the vertices of the surface. If after generation of a surface, the surface locally have to be adjusted and more vertices are needed to support the transformation inserting a new Isoparm can be enough to help the transformation.
- EDIT NURBS – EXTEND SURFACE. : Will extend the surface with a certain value defined in the option box of the command. The direction and starting point of the extension can be altered. However there is no option to alter the settings afterwards.
- EDIT NURBS – OFFSET SURFACE: Offsetting the surface is a quick way of generating an object with a certain thickness. Due to the same topology of the surface, the offset can later be easily closed by lofting the edges of the surfaces . Attaching the surfaces will generate a closed tube.
The smooting of corners
- EDIT NURBS – ROUND TOOL: The round tool is used to generate beveled edges on surfaces. The tool has the advantage in contrast to the bevel option that a rectangular edge can be beveled without first adjusting the surfaces themselves.
A quick way of smoothing edges between two objects is the use of the Fillet tool. With this option two intersecting surfaces can be blended together with the option of creating curves on surfaces. The curves on surfaces will make it possible to trim the surfaces.
- EDIT NURBS - SURFACE FILLET - CIRCULAR FILLET:The use of circular fillet makes it possible to make smooth transitions between the intersections of two objects. The curvature of the blend is fixed and can’t be altered. Curvature of the fillet tries to accommodate a tangent continuity. Radius of the fillet can be adjusted. If the radius is to big the fillet can possibly fail.
- EDIT NURBS - SURFACE FILLET - FREE FORM FILLET : Free form fillet is basically the same function as the circular fillet. The difference is the way the fillet is defined. With a circular fillet, the fillet is created between two objects. With the free form fillet the edges of the fillet is defined by curves on surfaces. This will create the option to make very complex fillets, with the fillets still tangency aligned with the surfaces.
- EDIT NURBS - SURFACE FILLET - SURFACE BLEND FILLET The surface blend fillet is a good example how the topology of NURBS surfaces influence the possibilities to adjust a surface. With the Surface Blend Fillet tool it is possible to blend multiple surfaces together. By creating the surface the tangency will be aligned with all the surfaces where the blend is applied to. In the selection option of the tool, the first thing which is asked is to select the left edge and after that the right edge. This will mean that the direction of the blend surface is defined in one direction by the selection. Twisting of the surface will mean that the surface direction has to be altered. This can be done after the blend is made.