How to Create Clean and Editable Single-Span Surfaces in Rhino3D: A Comparison of Surfacing Commands

thirtysixverts

Introduction to Clean Surfaces in Rhino3D [0:00]

• Importance of Single-Span Surfaces
  • The video discusses the significance of creating single-span surfaces for maintaining high surface quality and facilitating accurate matching in Rhino3D.
  • Many Rhino commands unfortunately produce undesirable, overly dense surfaces.
• Test Case Setup
  • Two similar sets of input curves are prepared for testing various surfacing commands.
  • Right Example: Consists of degree 3 and degree 5 surfaces, which are identical with curvature-continuous degree 5 blends.
  • Left Example: A more challenging test case with mixed degrees (degree 5 and degree 3 on opposite edges) and a tangent blend on the inside edge, requiring more complex surface handling.

    

Sweep 1 Command Analysis [0:02:02]

• Initial Attempt (without Simple Sweep)
  • Applying Sweep 1 without the "Simple Sweep" option on the easy case (right example) results in an overly dense surface with 76 control points. This is deemed unacceptable for a primary surface.
  • The visible ISO curves indicate a complex surface topology unsuitable for clean modeling.

    

• Using Simple Sweep
  • Activating "Simple Sweep" produces a single-span surface, which is a step in the right direction.
  • However, the control points are still oddly distributed, and the surface exhibits a slight "rise" in the middle, making it less ideal.
  • It receives partial credit for being single-span and editable.

    

• Handling Mixed Degrees
  • On the more difficult (left) example with mixed degrees, Sweep 1 with "Simple Sweep" successfully bridges between degree 3 and degree 5 surfaces.
  • The output, while not perfect, is a single-span surface and considered better than the default Sweep 1 output, maintaining sculptability and matchability.

    

Sweep 2 Command Analysis [0:05:28]

• Easy Case (without Simple Sweep)
  • Applying Sweep 2 to the easy case without "Simple Sweep" again generates an excessively dense surface with a noticeable arch or rise in the middle.
  • This dense output is not suitable for a primary surface due to the difficulty in editing.

    

• Using Simple Sweep
  • With "Simple Sweep," Sweep 2 produces a single-span surface, maintaining degrees (e.g., degree 3 in one direction, degree 5 in another).
  • However, it moves control points upwards, resulting in a slightly distorted surface. While editable, it's considered slightly better than the non-simple sweep but still has issues.

    

• Limitations with Mixed Degrees and Reversed Rails
  • When attempting Sweep 2 on the mixed-degree case, the "Simple Sweep" option becomes unavailable.
  • The resulting surface is a "wacky", overly dense output with a rise in the middle, making it challenging to edit. This outcome is deemed a failure for primary surfacing, setting up future problems.

    

Network Surface Command Analysis (Strongly Discouraged) [0:08:37]

• High Point Density and Lack of Control
  • Network Surface generates surfaces that might visually appear decent but come with an extremely high number of control points (e.g., 160-169 points for a quad patch).
  • This excessive density makes the surface difficult to edit, sculpt, or match effectively.

    

• Tolerance-Based Generation vs. Exact Geometry
  • The command operates based on tolerance, meaning it approximates the input curves rather than creating exact geometry.
  • This tolerance-based approach can lead to issues like wrinkles, non-watertight models, and compatibility problems when exporting to other CAD systems.

    

• Methodological "Box Canyon"
  • The presenter strongly advises against using Network Surface as a primary surfacing tool, likening its use to entering a "methodological box canyon" where the modeling process becomes increasingly problematic.
  • It encourages a "curves in, surfaces out" workflow that often leads to poor surface quality.
  • This method is a significant contributor to common modeling issues like wrinkles and non-watertight geometry.

Patch Command Analysis (Strongly Discouraged) [0:13:02]

• Trimmed Edges and High Point Density
  • The Patch command creates surfaces where all four edges are trimmed, increasing the risk of issues when transferring models.
  • It also generates surfaces with extremely high control point counts (e.g., 169 points for a simple patch), similar to Network Surface.

    

• Inaccurate Edge Generation and Curvature Problems
  • An input curve that is a smooth, single-span degree 5 curve can become a highly dense and irregular trimmed edge when used with Patch.
  • Curvature analysis reveals internal discontinuities and "garbagey curvature," which propagates to any surfaces matched to it, compromising overall model quality.

    

    

• Consequences of Poor Primary Surfacing
  • Using Patch or Network Surface for primary surfaces leads to a cascading "infection" of problems throughout the model, as subsequent matched surfaces inherit these issues.
  • This results in models with high point density, tangency issues, and wrinkles, making them difficult to work with and ensure watertight integrity.
  • The presenter strongly advises against using Patch if surface quality is a priority.

Demonstration of Bad Workflow Consequences (Network Surface Example) [0:18:26]

• Exaggerated Point Density
  • A simple "soft corner of a box" shape, which could be made from three clean surfaces with a total of 16 control points, is intentionally created using Network Surface by connecting multiple curves.
  • This results in a single surface with an astonishing 264 control points, demonstrating the severe inefficiency and complexity introduced by inappropriate command usage.

    

• Curvature and Continuity Issues
  • The curvature graph of this Network Surface output is revealed to be "super garbagey," exhibiting severe discontinuities and even concavity where the surface should be flat or convex.
  • This poor curvature directly translates to problems like wrinkles, tangency issues, and difficulty achieving proper continuity in later stages of modeling.

    

Recommended Commands for "Clean" Surfaces [0:23:01]

• 1. Surface From Edge Curves (SrfPt) [0:23:09]
  • Primary Tool for Four-Sided Holes: This command is highlighted as the go-to tool for creating surfaces in any four-sided hole.
  • Elegant and Exact Output: It produces beautiful, single-span surfaces that precisely follow the input curves without unwanted distortions or rises.

    

  • Sculpt and Match Workflow: The process involves first creating the surface with the correct point count and degree, then using the MatchSrf command to achieve desired tangency or curvature continuity. This separation of form creation and continuity matching allows for a more flexible and editable workflow.

    

  • Editability: The resulting single-span surfaces have a manageable number of control points (e.g., 6 points for a degree 5 edge), making them highly editable and easy to match with other surfaces to ensure watertight models.

    

• 2. Loft Command [0:32:28]
  • Ideal for Specific Geometries: Loft is excellent for creating forms defined by profiles, such as wings, boat hulls, or similar flowing shapes.
  • Clean Surface Generation: It reliably creates clean, single-span surfaces. It also intelligently elevates the degree of lower-degree input curves to match higher-degree ones, ensuring seamless transitions.

    

  • "Loose" Option for Control: The "Loose" option in Loft is particularly useful. Instead of forcing the surface to pass through the input curves, it uses the curves to define the surface's control points. This allows for creating complex, elegant forms like elliptical wings while maintaining optimal surface quality.

    

    

Conclusion and Best Practices [0:37:09]

• Practice and Expectation Setting: Users are encouraged to consistently practice creating clean, single-span primary surfaces. Setting this expectation for oneself is crucial for developing good modeling habits.
• Focus on Design Intent: The key takeaway is that the specific command used to initially create a surface is less important than whether that command generates a surface with the correct point count, degree, and distribution to express the design idea.
• Avoid "Curves In, Surfaces Out" Mindset: Freeing oneself from the rigid "curves in, surfaces out" workflow prevents getting bogged down by command specifics and opens up more creative and effective surfacing solutions.
• Importance of Patch Layout and Degree Choice: If a single-span surface cannot express the desired design, the problem usually lies in either poor patch layout or an incorrect choice of surface degree.