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How it works

Reading this page is optional, but it explains why Seamless CAD behaves the way it does. Most surprises people report turn out to be consequences of the design described here.

Blender does not compute the shapes

Blender's own modelling tools operate on polygon meshes. Seamless CAD does not. Shapes are computed by OpenCASCADE (OCCT), an industrial B-Rep kernel, which runs as a separate process alongside Blender. Blender receives the result and draws it.

This has consequences worth knowing:

  • The add-on ships an executable (cad_server.exe). This is the kernel, not a network service — it is why the add-on is currently Windows-only.
  • The first Start Seamless CAD takes a moment because the process is starting.
  • Surfaces are exact. A cylinder is a mathematical cylinder, not a many-sided prism. What you see in the viewport is a mesh approximation of that exact surface, generated for display only.
  • Blender's mesh editing tools do not apply to CAD shapes. To get a real Blender mesh you must Bake to Mesh.

The proxy and the result

Each entry in the Feature Tree has a lightweight proxy object in the viewport. The proxy is what you select and move; it carries position, rotation and scale.

When you move a proxy with G / R / S, Blender tells the add-on the proxy moved, the kernel recomputes, and the drawn result follows. This is why standard Blender navigation and transform keys work normally — they are acting on ordinary Blender objects.

The Feature Tree is a program, not a layer stack

The Feature Tree is evaluated top to bottom. Each entry's Operation (Base / Add (Fuse) / Subtract (Cut) / Intersect (Common)) says how it combines with the accumulated result of everything above it.

This means order is meaningful, and changing it changes the result. A fillet placed before a boolean rounds a different edge than the same fillet placed after it.

Base is worth singling out: it discards the accumulated result and starts fresh. An entry set to Base partway down the tree effectively cuts the history in two. The Property Editor offers Separate Previous to New Part at that point, which splits the discarded portion into its own part rather than leaving it inert.

Rollback

The pin icon at the right of each Feature Tree row sets a rollback point. Evaluation stops there; rows below are greyed out and ignored.

Use it to inspect an intermediate state, or to stop a long history from being recomputed while you edit an early operation. Unpin to restore full evaluation.

It does not let you insert an operation into the middle. New entries are always appended to the end of the tree, including while a pin is set — where they land below the pin and are not evaluated. See The Feature Tree and parts.

Some controls are disabled while a rollback point hides them — Edit Sketch is one. The panel says "Hidden by rollback point" rather than silently doing nothing.

Face and edge identity — why Cleanup is manual

Operations like Fillet, Chamfer, Shell and Draft do not store "the edge at these coordinates." They store a reference to a specific face or edge identity in the kernel's topology. That is what makes them survive when you go back and change a dimension: the fillet still refers to the same edge, even though the edge has moved.

This is also the reason Cleanup (Unify) is opt-in and never applied automatically. Merging coplanar faces destroys the identities of the faces it merges. Any fillet or offset referring to one of them loses its target. It is a useful operation before export, and a destructive one in the middle of a live history, so the decision is left to you.

The same mechanism explains why selection happens in a dedicated Selection Mode rather than through Blender's normal selection: you are picking kernel topology, not mesh elements.

Display quality is separate from real quality

The Linear and Curvature settings control how finely the exact surface is converted into display triangles. They affect only what you see. They do not change the model, and they do not change STEP export, which writes the exact surfaces.

Bake to Mesh is the one place display quality matters for output, which is why Use High Quality Bake exists as a separate setting — you can work at a coarse, responsive display quality and still bake finely.

The Fast Modifier Preview and Live Boolean Preview options go further: during a drag they show an approximate result computed by a faster path. The result you get when you release is always computed exactly. If a preview ever looks wrong, that is what these options are; turn them off.

See also