Primary Surfacing: Creating Complex Ball Corners with Curved Surfaces in Rhino (Episode 12)

thirtysixverts

Introduction to Advanced Ball Corners [0:00]

• This video is Episode 12 of the "Primary Surfacing" series, expanding on the ball corner basics introduced in Episode 10.
  • The focus is on creating ball corners when the primary surfaces have inherent curvature, rather than being flat planes.
  • The core approach remains identical to the easier cases, but the setup and refinement steps are more involved.
• The goal is to achieve soft, aesthetically pleasing corners, similar to those found in Subdivision (Sub-D) modeling.
  • A slightly different technique for generating the trimmed corner surface will be demonstrated.

Preparing Primary Surfaces [1:37]

• The process starts with simple primary surfaces that already possess curvature.
  • These surfaces are quickly created by point-editing a flat plane after increasing its degree.
• A sphere is introduced at the corner intersection point to serve as a sizing guide for the future blends.
  • The sphere is intersected with all surrounding primary surfaces.
• The primary surfaces are then split by iso-curve based on the intersection curves created by the sphere.
  • The sphere and any unwanted parts of the primary surfaces are deleted, leaving trimmed sections with open edges.

Creating Initial Blend Surfaces [3:06]

• Blend curves are generated between the newly trimmed edges of the primary surfaces.
  • When using Blend Curve, it is crucial to use the Curvature continuity option to influence the resulting surface shape.
  • Visualizing the curvature of the blend curves helps predict the final surface quality, an advantage over Rhino's BlendSurf command which lacks this real-time curvature feedback.
• Surface from Edge Curves is then used to create the actual blend surfaces from these blend curves.
  • The resulting blend surfaces should have appropriate point counts and degrees for clean geometry.
• The construction blend curves are deleted once the surfaces are created.

Matching Blend Surfaces for Continuity [5:22]

• The next step involves using Match Surf with the Multiple Matches option to ensure precise continuity between the blend surfaces and their adjacent primary surfaces.
  • The Curvature option should be selected for matching, along with Automatic for handling iso-curve directions.
• Analyzing the continuity is vital, using tools like VSR (Visual Surface Review) or traditional Zebra stripes.
  • Initial analysis might reveal areas with poor tangency or curvature breaks.
• To improve continuity, the degree of the blend surfaces might need to be increased (e.g., from 3 to 4, or higher).
  • After increasing the degree, the Match Surf command is run again to achieve better continuity values.
  • The goal is to achieve very low tangency breaks (e.g., in the ten-thousandths) and minimal curvature breaks.

Implementing an Alternative Trimmed Corner Method [8:38]

• A different approach for creating the central trimmed corner is introduced, contrasting with methods that explicitly move edges back.
  • Instead of directly creating a four-sided hole by moving edges, the existing blend surfaces are split further back by iso-curve.
  • This process effectively extends the boundary of the central corner area.

    

• The original central blend surface (if one was made) is deleted.
  • A new blend surface is then created using Surface from Edge Curves from the newly established, larger boundaries.
• This new central surface is matched on three of its sides to the surrounding blend surfaces, ignoring the edge that will eventually be trimmed.
  • Due to shared vertices and the desire for high continuity, the degree of this central surface might need to be increased multiple times (e.g., from 5x5 to 6x6, then 7x7) and Match Surf re-applied.
  • This iterative process is crucial for achieving satisfactory tangency and curvature on all matched edges.

    

    

Completing the Corner and Final Analysis [14:20]

• Once the central blend surface has good continuity on its three main edges, the final steps are performed.
  • The remaining central curve is pulled onto the surface, and the blend surface is split by this curve.
  • The excess portion of the surface and the curve are then deleted, leaving the final trimmed corner piece.
• The final, smallest surface piece (the actual ball corner) is matched to its surrounding edges.
  • During this step, it's important to use Merge Edge if any edges have inadvertently split during previous trimming operations, and then re-split them at precise endpoints.
• The matching is initially set for Tangency.
  • While attempting to achieve Curvature continuity for this final, small piece is possible by further increasing its degree and re-matching, it might not always be necessary depending on the design intent.
  • For non-Class A surfacing, good Tangency for this final trimmed edge is often sufficient, ensuring a smooth visual flow.
• A final check with Zebra stripes confirms the overall watertightness and smooth flow of light across the entire model.
  • The visual analysis should show continuous, flowing stripes, indicating a high-quality surface.