The problem
Class-A surfaces are the visible exterior surfaces whose reflections define perceived quality. Achieving them requires disciplined surface structure, low-degree single-span patches where possible, and precise continuity, verified by objective diagnostics rather than eye alone. Many students can make a shape but not a Class-A one, and few can show measured evidence of quality.
Objectives
- Choose a test area with a feature line, a blend and a corner, for example a wing, bonnet and lamp junction.
- Plan surface topology with patch layout, isoparm flow and boundary continuity targets.
- Build surfaces with controlled degree and spans, using Align and blends to reach G2 or G3.
- Verify continuity numerically and visually with diagnostic tools.
- Compare structures and report a method with quantitative quality measures.
What you need
Tools and toolboxes
- Autodesk Alias (Surface or AutoStudio) — Surface construction with controlled degree and spans, Align, Freeform Blend and Surface Fillet.
- Alias Evaluate tools — Continuity checks, curvature combs, deviation measurement and cross sections.
- Diagnostic Shading — Zebra, highlight and curvature shading (for example Gaussian and mean curvature) for visual quality checks.
- Autodesk VRED (optional) — Real-time reflection reviews in a lighting environment.
- IGES/STEP export — Delivery of surfaces for engineering and for quality checks in another system.
Software
- Autodesk Alias 2024 or later
- VRED (optional)
Extra parts and data
- A styling model or scan of a reference form you create — A starting form to rebuild as Class-A; use your own concept, not an existing product.
Key concepts
- G0 to G3 continuity
- G0 means boundaries touch, G1 tangency matches, G2 curvature matches and G3 the rate of curvature change matches, giving progressively smoother reflections.
- Surface structure
- The layout of patches, their degrees and spans, and how isoparms flow. A clean structure is easier to control and modify.
- Curvature comb
- A plot of curvature along a curve or isoparm. A smooth comb without spikes or reversals indicates a fair curve.
- Highlight lines
- Reflections of linear light sources across the surface; evenly spaced, smooth highlights crossing boundaries without breaks indicate quality.
Step-by-step guide
- 1
Choose the test area and targets
Select a region of your own concept containing a feature line, a soft blend and a three-way corner, such as where a front wing meets the bonnet near the lamp. Define targets: G2 across the main blend, G3 on the most visible boundary and G1 at the feature line with a small controlled radius. Record positional and angular tolerances taken from your course or typical studio practice.
Checkpoint: A target table listing each boundary, its continuity target and tolerances.
- 2
Plan surface topology
Sketch the patch layout over the region, aiming for four-sided patches with isoparms that follow the main flow of the form. Decide where the corner patch sits and how each blend runs out into the neighbouring surfaces. Plan the degree and spans of every patch, for example degree 5 to 7 single-span patches on primary surfaces, to keep control over continuity with few CVs.
Checkpoint: A patch layout drawing with the degree and span plan for each patch.
- 3
Build primary curves
Create the primary curves with low degree and single spans where possible, turning on curvature combs as you build them. Make each comb change smoothly with no unintended inflections or spikes, and keep the comb's peak where the design wants the tightest curvature. If a comb looks wrong, rebuild the curve with fewer CVs rather than adding CVs to fight the problem.
Checkpoint: Every primary curve has a smooth curvature comb with no unintended inflection.
- 4
Build primary surfaces
Construct the large surfaces with Square or Rail Surface from the primary curves, keeping their degree and spans as planned and building them oversize so they can be trimmed later. Check isoparm flow in wireframe and adjust CV rows so they stay evenly spaced and do not bunch. Do not trim yet; trimming hides structure problems that are easier to fix now.
Checkpoint: Primary surfaces with the planned degree and spans and evenly flowing isoparms.
- 5
Align boundaries
Use Align to match neighbouring surfaces at G1, G2 or G3 as targeted, choosing which side moves and how many CV rows are affected. After each align, check that the moved surface keeps its shape away from the boundary using the curvature comb on its isoparms. Use the continuity check to measure position, tangent and curvature deviation along the whole boundary, not just at its ends.
Checkpoint: Continuity check results meet the tolerances for each aligned boundary.
- 6
Build blends and corner
Create the main soft blend with Freeform Blend or Surface Fillet set to G2 or G3, then build the corner patch so it matches all three neighbours. Corners are usually the hardest part; prefer a four-sided patch with careful CV control over a trimmed n-sided fill. Check each side of the corner patch separately with the continuity check and fix one side at a time.
Checkpoint: The corner patch meets its continuity targets on all sides, shown by continuity checks.
- 7
Check visually
Use Diagnostic Shading in zebra and highlight modes from several view angles, and curvature shading such as Gaussian or mean curvature to see how curvature is distributed. Look for breaks at boundaries, wobbles and flat spots inside patches. Review the region in VRED under a studio light environment if available. Record before and after images of each fix for the report.
Checkpoint: Zebra and highlight lines cross every target boundary smoothly, documented with screenshots.
- 8
Compare G2 and G3
For the most visible boundary, build one version at G2 and one at G3 from the same starting surfaces. Compare continuity check numbers, curvature combs across the boundary and highlight appearance under identical lighting and camera. Discuss whether the visual difference justifies the extra control effort and CV complexity, and ask a peer to judge the two blind for a simple check.
Checkpoint: A side-by-side comparison with measured continuity values and highlight images.
- 9
Quantify quality
Build a table for every boundary with measured positional, tangent and curvature deviations, the patch degrees and spans and the number of CVs used. Add the deviation of the final surfaces from the original concept form using the Evaluate deviation tools, so you can show the rebuild kept the design intent. This turns a visual judgement of quality into numbers that can be compared.
Checkpoint: A quality table with all boundaries within tolerance and deviation from the concept form stated.
- 10
Export and report
Export the surfaces to STEP or IGES and reimport them into another system, such as SOLIDWORKS, to check that boundaries stay closed and continuity is preserved, using Zebra Stripes and Deviation Analysis there. Write a report with targets, the topology plan, the method, the quality table, the G2 versus G3 comparison and lessons learned for the next Class-A region. Archive the native Alias file with its layers.
Checkpoint: Reimported surfaces knit without gaps and a report with quantitative quality evidence.
Testing & validation
- Measured continuity meets tolerance at every target boundary.
- Highlight and zebra checks show smooth flow across boundaries.
- Curvature combs are fair on primary curves and isoparms.
- Exported surfaces reimport and knit without gaps.
Expected results
A Class-A quality surface set for a complex body region with documented topology, measured continuity, highlight evidence and a G2 versus G3 comparison.
Research angle for your M.Tech thesis
Class-A quality is often judged subjectively. A study that links measured continuity values, curvature distribution and highlight-line metrics to expert ratings of perceived quality would be useful. Compare surface structures, such as single-span high-degree versus multi-span lower-degree patches, for controllability and quality, and make the evaluation method reproducible.
Take it further
- Apply the method to a full body side.
- Study the effect of patch degree on control and quality.
- Build a checklist that a studio could use for Class-A reviews.
- Compare with subdivision-to-NURBS workflows.
Viva and review questions
What is the difference between G2 and G3?
G2 matches curvature at the boundary, G3 also matches the rate of change of curvature. G3 gives smoother highlight transitions but needs more careful control.
Why prefer single-span patches?
They have fewer CVs and no internal knots, so curvature is smooth and easier to control when aligning to neighbours.
How did you prove the surfaces are Class-A quality?
With measured continuity values within tolerance at each boundary, fair curvature combs and smooth highlight and zebra lines, all documented.
Learn it at DIYguru
These DIYguru courses and programmes teach what this project needs.