Almost every 3D CAD tool follows one of two philosophies. Parametric (history-based) modeling records every step you take — sketch, extrude, fillet — into an ordered feature tree that can be replayed when a dimension changes. Direct modeling skips the recording: you edit the geometry itself, immediately.
Parametric modeling in a nutshell
In tools like SolidWorks, Fusion 360, Onshape, and FreeCAD, the model is a program: change the "hole diameter = 6 mm" parameter and the whole part rebuilds. Strengths:
- Design intent is captured — relationships survive edits.
- Part families and configurations are easy (one model, many sizes).
- Engineering changes late in a project can propagate automatically.
Weaknesses: the feature tree is fragile ("rebuild errors"), the learning curve is steep, and every edit requires understanding how the model was built.
Direct modeling in a nutshell
In tools like Shapr3D and OpenShape3D, you grab a face and move it. Strengths:
- Immediate, intuitive editing — ideal for touch screens and Apple Pencil.
- No rebuild failures; edits are local and predictable.
- Works on imported geometry with no history.
Weaknesses: no automatic propagation of a dimension change through a chain of features, and complex mechanical intent must be maintained by hand.
The middle ground: constrained sketches + direct solids
A popular hybrid keeps 2D parametrics (sketch constraints and driving dimensions) while treating the 3D solids directly. You get exact, dimension-driven profiles — "this slot is 25.4/2 mm wide, centered, symmetric" — and fast, fearless push/pull editing in 3D. This is the approach OpenShape3D takes, with a full 2D constraint solver (coincident, tangent, symmetric, driving dimensions with live degree-of-freedom feedback) feeding a direct-modeling workflow on the OpenCASCADE kernel.
How to choose
| If you… | Lean toward |
|---|---|
| Design product families driven by spreadsheets of dimensions | Parametric |
| Sketch concepts on an iPad, iterate fast, 3D print one-offs | Direct |
| Edit STL/OBJ or STEP files from other people | Direct |
| Need full assemblies with mates and motion | Parametric |
Many designers use both: direct tools for exploration, parametric tools for production. Starting with a free direct modeler like OpenShape3D is a low-risk way to learn solid modeling fundamentals that transfer to any CAD system.