1. Surface Continuity

2. Three Types of Continuity
Complex designs need to be built up from many surface patches. The relationship between these surface patches will give the design its character.
Positional Continuity
This is where surfaces meet at an angle—a sharp edge. Positional continuity is important if you need a ‘closed’ model that represents a solid object. This can be transferred successfully to another CAD system, have a volume calculated, or be tessellated to create a rapid prototype.
Tangent Continuity
This is the kind of continuity created by rounds or fillets. The join between the two surfaces is smooth, but, visually, you can see where one surface ends and the other one starts. There is a ‘highlight line’—the sort of line you would draw with a soft white pencil in a hand rendering.
Curvature Continuity
With curvature continuity, the join between the surfaces is smooth to touch and to see—it is hard to tell where one surface starts and the other ends.
G0, G1, G2
These are standard terms used for the continuity conditions.
Positional Continuity (G0)
Tangent Continuity (G1)
Curvature Continuity (G2)

3. Aligning Two Curves Object Edit>Align Positional (G0) Continuity1
The Align Tool
The Object Edit>Align tool can be used to align curves and surfaces with positional, tangent, or curvature continuity. A construction history relationship will be created so that continuity will be maintained while the curves or surfaces are modified.
The Align tool has been used in this example to show how CVs are used to achieve the three levels of continuity.
Understanding how CVs relate to continuity is important for trouble-shooting some of the advanced blending tools that you will use later in this course.
Continuity and CVs
Positional continuity requires the first CV of the aligned curve to be in the same position as the end CV of the curve being aligned to.
Tangent continuity requires the second CV of the aligned curve to have a straight-line relationship with the second CV of the curve being aligned to.
Curvature continuity requires the third CV of the aligned curve to be in a constrained relationship with the third CV of the curve being aligned to.
Positional (G0) Continuity
1 CV required 1 2
Tangent (G1) Continuity
2 CVs required 1 2
Curvature (G2) Continuity
3 CVs required 3

4. Evaluating Curve Continuity
Locators>Curve Curvature
Click on a curve or curves to display the curvature plot.
Positional
Comb plot at different angles
Continuity and CVs
Positional Continuity
The end points meet exactly, but the direction of each curve is different as shown by the ‘break’ in the comb plot.
Tangent Continuity
The end tangents of the curves are matched, meaning that the curves have the same direction at the point they meet.
A round or fillet on a straight edge is smooth
but clearly has a ‘jump’ in the curvature as it
transitions from flat to a radius.
Curvature Continuity
Just as the curve directions match, the radius value is also matched so the rate of change of direction is the same. This gives a smoother result.
Tangent
Comb plot aligned but different radius values R1 R2
Curvature
Comb plot aligned and radius value matched
R1=R2

5. Aligning Two Surfaces Object Edit>Align Positional (G0) Continuity
Align Tool Options
Positional (G0) Continuity
One row of CVs required 1
Tangent (G1) Continuity
Two rows of CVs required 1 2
Curvature (G2) Continuity
Three rows of CVs required 1 2 3

6. Evaluating Surface Continuity
Diagnostic Shading
Evaluating Surface Continuity Using ‘Zebra’ Stripes
Increase the quality of tessellation to ensure an accurate evaluation.
Positional
Zebra stripes are disjointed
Diagnostic Shading
The Diagnostic Shading in the Control Panel is designed to analyze the surface quality.
The ‘Zebra Stripe’ shader simulates the technique
used in automotive design where a model is
placed under strip lights. By changing your view,
you can assess the quality of the highlights.
In Alias, you will tumble the model under the
zebra stripe reflections to get the same effect.
Choose horizontal
or vertical stripes
as required to
assess your model.
Tangent
Zebra stripes are the same width but not smooth
Choose horizontal or vertical stripes
Curvature
Zebra stripes are smooth
Increase the number of stripes

7. Blending Between Two Curves
Degree 3 Curve
Single span
4 CVs
Degree 5 Curve
Single span
6 CVs
Surfaces:
Tangency at both ends: 4 CVs
Curvature at both ends:
six rows of CVs
Curvature at both ends: 6 CVs
Single-span degree 5 curves are recommended for blending curves.

8. Construction Options Preferences>Construction Options
Set the construction options before you start to model to define the accuracy of the continuity.
Position
Tangent
Curvature
Measuring Continuity
Evaluate>Continuity>Surface Continuity
Open the option window to set the level of continuity you want to check. Click Go, and then click on the join between two surfaces.
A green locator indicates that continuity has been achieved.
A yellow locator indicates a continuity problem.
Position mm
Tangent degrees
Curvature
Position mm
Tangent degrees
Curvature
Position mm
Tangent degrees
Curvature

9. When to Use Align Align doesn’t work successfully on adjacent edges:
Align works well on one or two (opposite) sides of a surface:
Using Align
There are two ways of using the Align tool:
Freeform Sculpting
When sculpting a fairly free shape—cosmetics bottles for example—Align can be used successfully to create the main shapes for the design.
Patching and Blending
To use the Align tool for patching and blending surfaces on all four sides, you will need to use advanced modeling techniques covered in a later course.
It is easier at this stage to use different
surface tools to achieve complex blends.
These will include the Surface Fillet,
Freeform Blend, Bi-Rail, and Square
surfaces.
Align doesn’t work on periodic surfaces
Align doesn’t work well on trimmed edges:
Heavy Parameterization
(but Align 2008 does)

10. Exercises EX1-Align-curve.wire EX2-Align-surface.wire
EX3-Align-two-curves.wire
EX4-Align-two-surfaces.wire
EX5-Align-eval.wire

11. Other Continuity Tools: Surface Fillet
Surfaces>Surface Fillet
(Tangent)
The basic operation of the Surface Fillet tool was covered in the ‘Intersecting and Trimming’ lesson.
You will typically use Circular (Tangent) or Curvature.
The same value will give different results:
In the same space, curvature continuity gives a softer connection but a tighter transition.
Constant Radius,
Tangent Continuity

12. Surface Fillet Options
Filleting Multiple Edges
The Surface Fillet tool will create a blend surface between one set of surfaces and another set.
Each set of surfaces must be tangent continuous for the fillet to work.
Max Surf Spans
If you are building a complex fillet,
you may get a warning about
Max Surf Spans.
If you are building a concept model,
you can ignore this warning.
If you are planning to export your
data to a CAD system, there may
be a problem with stitching the
surfaces together as a solid model.
Fixed Radius,
Variable Width
Fixed Width,
Variable Radius
Variable Fillets
Click on the blue line to add another radius value. Shift-click to remove.
Use the middle mouse button to change the position of the radius locator.

13. Surface Fillet Options
Recommended for best-quality layout of surfaces and regular hulls
Recommended for quick, tidy modeling
Flow Control and Continuity
Occasionally, choosing different flow control options can affect the continuity. To check, turn on the Continuity Check option in the Surface Fillet tool.
Green means continuity has been successfully achieved; yellow means there is a continuity break.

14. Other Continuity Tools: Freeform Blend
Surfaces>Multi-Surface Blend>Freeform Blend
Workflow:
Create a gap between different shaped surfaces
Create a Freeform Blend between the two edges
Creates a tangent or curvature blend between two edges
Shape
The Shape slider gives you some limited control over the blend shape.
It provides control over the ‘looseness’ or ‘tightness’ of the blend surface. If the value is greater than 1.0, the result is a blend that fits tighter to the corner of the input surfaces; if the value is less than 1.0, the result is a rounder blend that fits closer to the edges of the surface.
Chain Select will pick a series of edges.

15. Exercises EX6-Fillet-continuity.wire EX7-Fillet-options.wire
EX8-Freeform-hood.wire
EX9-Freeform-trunk.wire
EX10-Freeform-options.wire

16. Projects P1-Electric-toothbrush.wire P2-Phone-casing.wire
P3-Fillet-wheel.wire
P4-Electronic-gadget.wire