Learning Technical Surfacing Advanced Surface Modeling > Blend Curve Constraint to a Curve
Advanced Surface Modeling
Learning objectives
In this overview, you will gain an understanding of the critical tools required for class A surfacing, and will gain control over the quality of surfaces built by using the surface evaluation tools.
An overview of the core competencies required for advanced surface modeling.In the Foundations of Surface Modeling tutorial you were exposed to the degree of craftsmanship required of a surface modeler. And you were introduced to SurfaceStudio, your new tool that allows you to perform surface modeling in the same manner as traditional methods. You may have traded in the workshop for a virtual space, but the core competencies of surface modeling are still the same.
Required competencies for mastering the craft of surface modeling
To provide you with the tools and knowledge necessary to become a master craftsman in the arena of surface modeling, the following topics will be addressed in this tutorial:
Master the Critical Tools
Master the Geometry Elements
Master Surface Evaluation
Handling Blend Curves
Blend curves are special free curves whose characteristics are constrained by the placement of blend points upon their shape.
Available placement types for blend points
Blend points are the sole avenue for modifying blend curves. Before a blend curve can be modified, the blend points must first be selected. To select blend points, a new tool has been added to the Pick tab in SurfaceStudio 10.0 - the Pick > Point Types > Blend point
tool.
Selected blend points can be modified or deleted in the same manner as other geometry items.
As well, in the same fashion that the visibility of CVs can be manipulated via the Control Panel, the visibility characteristics of blend points are also available for manipulation.
SurfaceStudio allows for the simultaneous selection of multiple blend points. In the image below, three blend points have been selected for modification with the BlendCrv Tools > Blend curve create
tool.
Crucial Blend Curve Tools
Blend curve create
To create a blend curve, use the BlendCrv Tools > Blend curve create
tool. Double-clicking the icon in the blend curve toolbox will produce an option box associated with the tool.
The Knot Spacing option is set to Chord by default and should not be changed unless warranted by extraordinary circumstances.
Enabling the Auto Align at Surface Corners button creates a PARAMETRIC Curvature type when a blend point is attached to a surface corner. The resulting curvature will align the blend curve along the edge at the corner. If the Auto Align at Surface Corners button is not enabled, a GEOMETRIC curvature type will be employed. The resulting blend curve will not be specifically aligned to an edge.
Blend curve edit tangent
By clicking the BlendCrv Tools > Blend curve create
tool, a manipulator is generated from the selected blend point. This tool does not change the type of curvature.
Blend curve planarize
The BlendCrv Tools > Blend curve planarize
tool is useful for pressuring blend curves into a plane. Although employing a GEOMETRIC Curvature type will result in a blend curve constrained to a plane, there are situations that require a construction plane. And, there are situations that require blend curves of a PARAMETRIC Curvature type associated with one blend point while the other blend point(s) remain free.
The image below depicts a surfacing example in which the transition curves between the base surface and the top surface are to be modeled. The primary restriction of the situation is that the curves must be oriented at the U/V parameter (isoperimetric lines) of the base surface. In addition, one curve is absent, and given the example, it is unclear as to where the missing curve will attach to the top surface. This example is tailor-made for the Planarize tool.
The Planarize tool works best when using a pre-defined construction plane. In this arrangement, the blend curve is pressured into the construction plane while maintaining the continuity of the reference geometry.
To execute this procedure, begin by creating the construction plane, then create a blend curve between the base surface and the top surface.
Make sure that the blend point(s) and the construction plane are both selected, then choose the Planarize tool. The result is that the blend point associated with the top surface moves to a position where the construction plane intersects with the top surface. The blend curve will then connect both surfaces by curvature and while maintaining a smooth planar character.
Information Provided by Blend Curves
To gain access to the information contained within a blend curve, use the Windows > Information > Information window menu item after selecting an associated blend point.
Primary options presented in the Information Window
Interpolation
This tool set allows the user to specify a direction type that determines how a blend curve passes through a blend point. For surface modelers, the BlendCrv Tools > Constraint Interpolation Direction > Blend constraint interp geometry tool is of particular interest. This tool aligns the blend curve to the reference geometry using a PARAMETRIC Curvature type that results in a similar curvature alignment to that achieved with the Object Edit > Align > Align tool.
The LOCATION setting is the Interpolation default for each new blend curve. The other Interpolation settings, DIRECTION and GEOMETRY, each offer a manipulator that can be accessed via the BlendCrv Tools > Blend curve edit tool.
The Interpolation type can be changed by using the highlighted buttons in the images below.
Three types of Interpolation
This setting forces the blend curve to pass through the blend point's location in space.
This setting forces the blend curve to pass through the blend point's location in space while travelling in a dictated world space direction.
This setting forces the blend curve to pass through a point on a curve or surface and travel in a direction relative to a curve or surface.
Connection
Blend curves attach to four geometry objects
Tangent type
The Tangent type option specifies a direction that dictates how a blend curve relates to blend point(s).
There are two Tangent types that are defined using the Constraint Interpolation Direction tool found in the blend curve toolbox.
Blend constraint directed (pt ray): sets an actual direction for the curve tangent. Use this Tangent type when the specific tangent direction at the location of the blend point is important.
Blend constraint parallel: sets a line along the curve that passes (in either direction) through the blend point. This option is easier to enforce and results in a better curve continuity.
Curvature type
This option is an important component of blend points when blend curves are attached to a surface.
To see the Curvature type in the Information Window, the blend point must be selected. If the Information Window returns a N/A value, select the Edit Tangent tool. The Curvature type will then be properly populated.
There are two curvature types available, PARAMETRIC and GEOMETRIC.
> PARAMETRIC
This type of curvature is similar in behavior to the Object Edit > Align > Align tool. In general, when attaching a blend curve with PARAMETRIC curvature to a surface, the result will appear similar to that achieved by aligning a blend curve to the isoparametric direction of a surface.
> GEOMETRIC
This type of curvature draws upon a plane constructed from the surface normal at the blend point and the tangent vector of the blend curve. By intersecting the plane with the surface, a section curve can be created that will allow for the alignment of the blend curve.
Continuity
This option defines the quality of the blend curve as it passes through the point in space associated with the blend point.
Three types of Continuity
Continuity types, G3 and G4, require the automatic insertion of additional spans to the blend curve. Usually, G2 Continuity is sufficient for meeting the requirements of a surface modeling project.
To set the Continuity type, the blend point must be selected. As detailed in the image below, use the highlighted tool found in the blend curve toolbox to change the Continuity type.
Blend Curves as Transition Curves
Auto Align at Surface Corners
When constraining a blend curve to surface corners it is expected that the blend curve will be aligned to the surface edges. To automatically constrain the blend curve, the option Auto Align at Surface Corners must be enabled in the option box for the BlendCrv Tools > Blend curve create tool. The blend points of the blend curve will automatically receive a PARAMETRIC constraint.
Without the automatic alignment of the blend points, the blend curve will be generated with a GEOMETRIC constraint approach at the blend points. The geometric alignment will differ from the shape of a curve that is simply aligned to the surface edges.
The image above shows two blend curves. The highlighted curve was created using the activated Auto Align at Surface Corners option while the other blend curve was created without the option.
Blend Curve Constraint to a Surface Edge
The Geometric Approach
To create a blend curve between the edges of two surfaces, but not at the corners, snap the blend points to the surface. The resulting planar blend curve will always contain geometry constrained blend points and will have the curvature continuity of the reference surface.
For a better understanding of the geometric curvature type, create a construction plane using three points snapped to the blend curve. Then, in the Control Panel, specify and calculate the sections in such a manner that the intersections between the surfaces and the construction plane appear as curves.
Both the intersection curves and the blend curve can be analyzed with the curve curvature tool. The curvature comb, as seen in the image below, details that the blend curve is planar and is curvature aligned to the two curves created by the intersection between the construction plane and both surfaces.
The Parametric Approach
Another option for aligning a blend curve to more than one surface is to use the parametric constraint that employs the U/V directions of the surfaces. The parametric method is very useful when creating transition surfaces that must follow the parameter flow of both surfaces. To achieve this behavior, select the blend points first, then choose the Geometry type by clicking the BlendCrv Tools > Constraint Interpolation Direction > Blend constraint interp geometry tool. Using the parametric method, a blend curve aligns to the appropriate isoperimetric line that represents the current U/V direction. In the Information Window, the Curvature type changes to PARAMETRIC.
Blend Curve Constraint to an ISO parameter
To create a blend curve constraint to an isoperimetric line, snap the blend points onto the isoperimetric lines. The resulting blend curve has a GEOMETRIC Curvature type. Because of this calculation method, the blend curve is not following the isoperimetric line.
To follow the isoperimetric line, select the blend points first then change the Curvature type to PARAMETRIC. The shape of the curve changes so that the blend curve properly aligns to the isoperimetric line.
Blend Curve Constraint to a Curve-On-Surface
To constrain a blend curve to a curve-on-surface, snap the blend points to the curve-on-surface. The blend point connected to the curve-on-surface has a GEOMETRY constraint.
When aligning a blend curve to a curve-on-surface, select the blend point that is constrained to the curve-on-surface, then click the GEOMETRY button. The Curvature type changes to PARAMETRIC and the blend curve aligns to the curve-on-surface.
Blend Curve Constraint to a Curve
When a blend point's lone attachment is to a curve, the Continuity type is always G0. This arrangement occurs when a blend curve is attached to an endpoint of a curve, or to an inner part of a curve.
Make sure to use the "Snap to Curve" mode (
+
) when attaching a blend point to a curve.
To align a blend curve to a curve, begin by selecting the blend point(s). As detailed in the image below, to change the Continuity setting, use the expanded tool set found in the blend curve toolbox.
By choosing the BlendCrv Tools > Constraint Continuity > Blend constraint G2 icon, the blend curve aligns to the curve.
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