How to Use Geometric Constraints in BricsCAD

Quick answer: Geometric constraints fall into three categories, relations between two entities, fixed angles, and fixed positions. Applying one actively adjusts the entity’s position to satisfy it, and displays an icon in a constraint bar next to the entity. Tools to create and manage them live on the 2D Constraints toolbar, the Parametric ribbon tab, and the Parametric menu.

Types of Geometric Constraints

• Relations between two entities: coincident, concentric, collinear, parallel, perpendicular, tangent, smooth, symmetric, equal.
• Fixed angle: horizontal, vertical.
• Fixed location: fix.

What Happens When You Apply One

Applying a geometric constraint doesn’t just flag a relationship, it actively adjusts the entity’s position to satisfy that constraint. An icon appears next to the entity showing which constraint was applied; apply several to the same entity, and their icons join together in a single constraint bar.

Creating Geometric Constraints

Tools live on the 2D Constraints toolbar, the Parametric ribbon tab, and the 2D Constraints flyout of the Parametric menu.

Controlling Constraint Bar Visibility

Constraint bars are hidden by default whenever a drawing is closed and reopened. Tools to control their display live on the Geometric Constraint toolbar, the Parametric ribbon tab, and the Show/Hide 2D Constraints flyout on the Parametric menu, all running CONSTRAINTBAR with a specific option:

• Show/Hide: prompts you to select which entities to toggle.
• Show All: shows every geometric constraint bar in the drawing.
• Hide All: hides every geometric constraint bar in the drawing.

Note: Octave’s own documentation describes these two options as showing and hiding “dimensional” constraints, which looks like wording copied from the dimensional-constraints page rather than an intentional statement, since CONSTRAINTBAR controls geometric constraint bars specifically. The list above reflects what the command actually does.

Two related settings:

• CONSTRAINTBARDISPLAY: 1 displays constraint bars as geometric constraints get added; 2 displays otherwise-hidden constraint bars when their entities are selected. (The source text describes option 2 as revealing “dimensional” constraints, but this variable governs geometric constraint bars, so that’s read here as the same wording mix-up as above.)
• With SELECTIONPREVIEW set to 1 or 3, or whenever the Quad is active, hovering over a constrained entity shows a small blue constraint glyph.

Repositioning Constraint Bars

By default, a constraint bar sits near the entity’s midpoint and keeps that relative position as the entity moves.

To relocate one: click and hold on the constraint bar, drag it to a new spot, and release. It stays there, relative to the entity, until reset.

To reset it back to the default position: click the Show/Hide Geometric Constraint tool, or run CONSTRAINTBAR, select the entities, right-click or press Enter, then choose Reset (or type R) when prompted.

Working with Individual Constraints

• Inspecting one: hover over its icon. A tooltip names the constraint type, the associated entity or entities highlight, and the matching icon on any other affected entity’s constraint bar highlights too.
• Deleting one constraint: hover its icon, right-click, and choose Delete. Only that constraint is removed, its icon disappears from every constraint bar it appeared on.
• Hiding one entity’s constraint bar: hover over the bar and click its Close (x) button.

Deleting All Constraints from a Selection

To strip every constraint from a group of entities at once: click the Delete 2D Constraints tool, or run DELCONSTRAINT, select the entities, then right-click to finish. Every dimensional and geometric constraint on that selection is removed.

Practical Examples

• Coincident constraints can join a line’s endpoint to two circles, keeping them connected however the drawing changes.
• Autoconstrain: if endpoints or centre points already coincide when you run GCCOINCIDENT, its Autoconstrain option applies coincident constraints to them automatically, rather than requiring you to select each pair manually.
• Driving a shape from one dimension: applying coincident constraints between a pentagon’s vertices and a circle, plus equal constraints between one side and the other four, means the circle’s radius alone controls the pentagon’s overall size.
• Keeping tangent lines in bounds: coincident constraints at the endpoints of tangent lines stop them extending past their tangent points on the circles.
• Centering one line between two others: coincident constraints force a line’s endpoints onto two guide lines and its midpoint onto a third, while perpendicular constraints between the guide lines and that line keep the third line permanently centred between the other two.

Frequently Asked Questions

Does applying a geometric constraint move existing geometry? Yes. It actively adjusts the entity’s position to satisfy the constraint, it isn’t just a passive label.

What’s the difference between deleting one constraint and using DELCONSTRAINT? Right-clicking a single constraint icon and choosing Delete removes just that one. DELCONSTRAINT removes every constraint, dimensional and geometric, from your selected entities at once.

Can BricsCAD automatically detect and constrain points that already coincide? Yes, the Autoconstrain option of GCCOINCIDENT applies coincident constraints automatically to endpoints or centre points that are already touching.

Why did my constraint bars disappear after reopening the drawing? That’s expected; they’re hidden by default every time a drawing is closed and reopened. Use CONSTRAINTBAR (Show All, Hide All, or Show/Hide) to bring them back.