Modeling Notes

The following information is new for Version 9.0:

The following information exists from previous releases:

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Note: For installation and release notes for your EvalViewer product, see the Quick Install & Release Notes for EvalViewer.

New Limitations

Limitation

The Move CV tool in the Control Panel treats the Step Size in U and V differently based on the mode of the tool.

In the XYZ mode, the step size works as expected. In the Normal UV and Slide modes, the step size will be used as a sensitivity control and will not produce movements equal to the step size.

Limitation

The bounding box used by the Control Panel > Cross sections tool may not update correctly when switching from construction plane mode to world space or vice versa.

Workaround

Click the Control Panel > Cross section On button to update the bounding box.

Limitation

The Evaluation > Highlight tool will leave a curve-on-surface attached to the surface being evaluated if the Visual option is selected. The curve-on-surface that is left can only be picked in the SBD and not the Modeling windows.

Workaround

You must select and delete the curve-on-surface through the SBD.

Limitation

Certain operations will not work in a consistent manner on geometry in symmetric layers. For example, the Edit > Duplicate object menu item with the Group option turned off does not work with symmetry turned on. Warps also do not work properly on geometry in symmetric layers.

Workaround

Turn off layer symmetry with the Layers > Symmetry > Off menu item.

Limitation

The Evaluate > Min/Max curvature tool does not work with shell geometry.

Limitation

If an object has been scaled, the curvature on its cross section, created by the Evalute > Cross section tool, is scaled twice.

Workaround

Zero transform the geometry before evaluating the cross sections on it.

Limitation

Degenerate geometry (sides are collapsed and CVs are piled on top of each other) should be avoided with the Boolean tools.

Existing Limitations

Limitation

IGES point clouds do not tessellate.

Workaround

If the EvalViewer is available to you, use the tessellation capabilities in that package for your tessellation needs. You can also use the tessellation facility in the Point Cloud tools on Cyberware format cloud data.

Limitation

Scaling stitched objects can create problems in subsequent stitch/ unstitch operations.

After scaling a stitched model, on unstitching and restitching you might find that the surfaces do not stitch anymore. This is because the scaling operation can increase the gaps between surfaces, thereby causing any subsequent stitch operations to fail (within the current tolerance settings).

Workaround

Perform the stitch operation after all modifications and transformations have been applied to the surface.

Limitation

When making large size plots of curves or surfaces, they may appear faceted.

Workaround

To smooth the problem curves and surfaces, pick them in Alias and increase their curve precision using ObjectEdit > Curve precision.

The curve precision can be increased to 129. However, note that the higher the curve precision, the larger will be the plot file. Also, when you have finished plotting, you should set the curve precision back to the default value of 16. High curve precision will degrade the interactive performance in Alias, causing it to draw more slowly.

General Modeling Notes

Limitation

Geometry seems to disappear in some modeling windows at large scale.

In large models, some or all of the geometry can seem to disappear. This can happen if a file was not saved with windows and cameras, and when creating new windows.

Workaround

Use the Cameras > Lookat tool to view the geometry.

Limitation

Surfaces > Fillet surfaces > Bevel and Object Edit > Offset have problems with curves or faces that have multiple CVs or piled up CVs.

Workaround

When building geometry to be offset or bevelled, use multiple knots instead of multiple CVs to create sharp corners.

Limitation

If a surface (or curve) has been built with construction history, it cannot be moved, scaled or rotated even if its constructor objects are transformed along with it.

Workaround

Only select the constructor objects and apply the desired transformation(s). This will cause the object with history to be automatically reconstructed in the new position and orientation.

If an object must be moved independently with respect to its constructor objects, then its construction history must be deleted.

Working with Intersections and Projections

The following comments apply to Surface Edit > Create CurvesOnSurface > Project, Surface Edit > Create CurvesOnSurface > Intersect, and Evaluate > Cross section.

Known problems:

  • Generally, intersection does not work if the intersecting surface passes exactly through `degenerate' points, such as the pole of a sphere, where many CVs are placed on top of each other, or where a surface wraps back on itself. If this situation occurs, move one of the surfaces a bit (a minuscule amount will do), perform the intersection, and move the surface back.
  • Complex saddle point regions may cause problems for the intersection code.

Known restrictions:

  • Occasionally, the order in which intersecting objects are picked can make a difference. To guarantee the simplest curve of intersection (the one with the fewest knots), pick the simplest object first. Usually, the order of picking does not make a difference.
  • The curve on surface resulting from an intersection has multiple knots. This means that they are not as easily editable as other curves on surface might be.

Working with Surfaces

Limitation

Retrieving Alias files from releases prior to version 5.0 that contain trimmed geometry may produce dense trim edges.

The representation of trimmed surface data was changed in version 5.0 and later releases. Prior to version 5.0, trim curves were represented as linear curves. In release 5.0 and later, trim curves are represented by splines. In the conversion process, the old trim data is converted into a large amount of data in the new trim format.

Workaround

To solve this problem, untrim the surfaces and trim them again. For the most common case, the curves on surface are cubic and the trim operation in the new release of Alias will produce spline trim curves.

If the trimmed surfaces originate from a release of Alias prior to version 3.2, the curves on surface will be described by linear geometry. In this case, the best results are obtained by recreating the trim curves, re-projecting, re-intersecting, and re-trimming.

If the data file originated recently from an IGES file, the best results are obtained by reading the IGES file into the newest version of Alias.

Limitation

You may not be able to revolve an unpicked curve if Num Lock is on.

When using Surfaces > Revolve, if the curve is not selected before the revolve function is selected and the Num Lock key is pressed, the revolve may not work.

Workaround

Turn Num Lock off and try again.

Limitation

Under specific circumstances, Surfaces > Boundary surfaces > Square may produce surfaces of poor quality.

Workaround

There are several techniques to try when Square does not produce the intended results:
  • Turn Rebuild on for all boundaries with trimmed surfaces or curves on surface, and have the system recalculate the surface.
  • Adjust the influence sliders to adjust the shape of the resulting surface.
  • Set the Blend Type to LINEAR instead of CUBIC.
  • If trying to maintain continuity with trim edges or curves on surface, it may be necessary to decrease the curve fit tolerance in Preferences > Construction settings.
  • Verify that the square surface doesn't have a corner where the U and V edges are colinear.

Limitation

Circular filleting and Surfaces > Fillet surfaces > Round will give error messages when surfaces with discontinuities are used.

Surfaces that contain tangent discontinuities along multiple knots are considered poor input to the circular filleting and round functions. These types of surfaces can be produced by creating surfaces from curves that were attached, but did not meet with tangent continuity.

Workaround

To solve this problem, detach the surfaces at the discontinuities and execute the fillet or round function again.

Limitation

Surfaces > Fillet surfaces > Round will produce bad results when creating fillets over surface poles.

Surfaces that have all of their CVs along an edge collapsed to a single point are considered poor input to the Round function. Such singularities are found at the apex of cones, the poles of spheres, and the center of capping faces on primitives. They can also be created when two curves which meet end to end are patched (or skinned) together.

Workaround

The planar surfaces that are used to cap solids are best represented as trimmed surfaces. Use Surfaces > Set planar to easily create a trimmed surface that has boundaries of given planar curves. These trimmed surfaces will be better accepted by the Round function. Try to avoid building geometry with singularities (poles).

Limitation

Surfaces > Swept surfaces > Extrude - (the TUBE option) has problems with multiple CVs on the path curve.

Workaround

When using TUBE mode in Extrude, avoid path curves with multiple CVs on them, as the resulting extruded surface may have undesired twists near the locations of the multiple CVs in the path.

Limitation

Sometimes Object Edit > Extend will produce undesirable results.

This may happen when the default option Extrapolate is used. The resulting surface sometimes bends sharply from its initial direction.

Workaround

Use the Linear option in this case. Note that using Linear will not produce curvature continuity - only tangent continuity with the extended object.

Limitation

Scaling the constructor curve of a revolve surface that uses local axes causes the axis of revolution to be translated.

Moving or rotating the constructor curve modifies the position or orientation of its local axes, thereby modifying the axis of revolution of the surface. However, scaling the curve should have no effect on the axis of revolution.

Workaround

Select the surface, select Surfaces > Revolve again and use the interactive handles to reposition the axis of revolution. Alternatively, set the Axes option to Global to ensure that the axis of revolution is not affected by transformations to the constructor curve.

Limitation

Adjusting settings in the control window for a Swept, Birail or Square surface with construction history will not cause the surface to be updated. The After Modification option is turned off in Preferences > Interface options.

Workaround

Turn the After Modification option on, which will cause the surface to be reconstructed. Alternatively, turn the Create History option off in the control window before modifying other parameters in that window.

Limitation

Birail can produce poor surfaces with long generation curves.

Workaround

If possible, try reversing the choice of generation and path curves, and choose the shorter, simpler curves for the generation curves, and use the long, winding curves as the paths.

Working with Curves

Limitation

The successful use of Curve Edit > Create > Combine curves sometimes depends on the order of picking the input curves.

Some input curves to Combine curves may result in unsatisfactory results (particularly some curves created with Object Edit > Fit b-spline), accompanied by the following warning message: 



Curves are too complex. Approximation may be 
inaccurate.

Workaround

Picking the curves in a different order often corrects the problem. Try picking the second curve first.

Limitation

Using Curve Edit > Create > Duplicate curve with the Rebuild Type set to UNIFORM KNOTS on a curve on surface creates an inexact copy.

If the Duplicate curve Rebuild Type option is set to UNIFORM KNOTS and Duplicate curve is used on a curve on surface, the results may not be within the specified tolerance of the original curve.

Workaround

To avoid this problem, use the CHORD parameterization Rebuild Type when duplicating curves on surface.

Limitation

Object Edit > Smooth may produce unpredictable results on curves with multiple knots.

The Object Edit function can cause the CVs in the vicinity of a multiple knot to be placed unpredictably. This is caused by the extremely non-uniform nature of the parameterization of the curve in that vicinity.

Workaround

Before smoothing the curve, rebuild it to remove multi-knots.

Limitation

End tangents do not necessarily stay locked when using Curve Edit > Modify > Curve editor.

It is possible to change the end tangent direction for a curve with locked end tangents, since end tangents are not guaranteed to stay locked when using the Curve editor.

Limitation

Using Curves > Autotrace may cause sluggish system response.

After using Curves > Autotrace on a picture with a large number of edges, you may experience sluggish system response.

Workaround

If this problem occurs, save the data on a disk and exit and restart Alias.

Limitation

When using the same curve to build several fillets with construction history and trim curves both set to ON, editing the radius to be a smaller value is usually not possible.

This happens mostly when using some of the Curves tools such as lines or arcs to build fillets as these tools automatically trim the constructor curves. If the same curve is used as a constructor for two (or more) different fillets, all but the last will not accept a smaller radius value (as set in Windows > Information window). Also, entering a larger value followed by an Edit > Undo will not update the curves correctly and leaves a gap.

Workaround

To avoid this problem, use Surfaces > Fillet surfaces > Fillet (rather than the Curves tools with the Trim Curves option set to off (the default), and then trim the curves manually once the radius has been adjusted.

Limitation

A curve being modified with Curve Edit > Modify > Stretch doesn't update when its locator position is changed using Windows > Information > Information window.

If you open the Information window before entering Stretch, the active handle being moved by Stretch appears as a Space locator in the Locator information window. This allows you to see the numeric position of the handle. If you try typing in a value in the Locator information window, the locator will move, but the curve doesn't update to the new space locator position.

Workaround

Simply click a mouse button in any modeling window. The curve will then update.

Limitation

Implied tangency across edges of triangles in Curve Networks might take a long time to compute.

Workaround

Keep the curves forming the triangular region as simple as possible. When faced with a situation where the triangles contain long curves with a lot of variation in them, try to split up the triangle into a combination of four sided regions and much simpler triangles.

Limitation

Manipulators in Blend curves and Project Tangent will line up to either the U or V value, depending on which direction of a surface you point to initially.

Unless the U and V directions are at 90 degrees in world space, then you cannot use the manipulator to get the other alignment.

Workaround

Align the curve by eye.

Limitation

Snapping the alignment of a curve tangent to the manipulator axes does not keep the alignment fixed in the same way as setting the alignment in the Project tangent editor window.

Workaround

To fix a specific U, V, or Normal alignment, you must select that alignment from the editor window menu.



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