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The ideas the rest of the manual assumes - construction vs. node graph, exact geometry, associativity, and the compass-and-straightedge law.

A handful of ideas run through everything in Locus. Understanding them once makes the rest of the manual click.

Locus has two ways to make geometry, and they're complementary:

• Construction (Modeling window): you build objects by hand, each defined relative to others: a midpoint of two points, a tangent to a circle, an intersection of two lines. It's direct and visual.
• Node graph (Nodes window): you describe geometry as a flow of data through nodes. It's parametric and repeatable: one slider can drive a hundred copies.

Neither replaces the other. You typically draw a motif by construction and then repeat and vary it in the node graph.

Locus color-codes every kind of data, and the same color means the same type everywhere you look - so once the palette clicks, you can read the whole interface at a glance.

It starts in the menus. A tool's icon is tinted by what it produces: point tools are blue, curve tools (segments, lines, circles) are green, and the region tools (the booleans) are orange.

Pick a tool and that color carries straight through. The node it creates wears the color on its header; its output socket is the same color; and any wire leaving it is drawn in that color too.

So a green header, a green socket, and a green wire are all saying the same thing: this is a curve. Sockets add a shape on top of the color - a colorblind-safe backup - so you can still tell types apart without relying on color alone. See the full palette on Data types, and how wires read on Sockets & wires.

Color tells you a point's type. Its shape tells you two things at once: how the point was created, and whether you can drag it.

• Filled disc. You placed it yourself, on empty canvas. Nothing holds it, so drag it anywhere.
• Ring with a dot. You placed it on something: a circle, a segment, an arc. It is still yours to move, but only along its host. One degree of freedom instead of two.
• Hollow ring. You did not place it at all. The construction worked it out, from a crossing, a midpoint, a tangent, so there is nothing to drag. Move what it was built from and it follows.

The full set of point kinds is on Points.

Objects remember how they were defined. A perpendicular bisector knows it bisects a particular segment; move the segment and the bisector moves. In the node graph, changing an upstream value re-evaluates everything downstream. You rarely edit geometry directly - you edit the definitions, and the geometry follows.

All geometry is analytic - real lines and arcs with exact math behind them, not polylines pretending to be curves. The booleans (union / intersect / difference) return true arcs, so paths stay smooth and corners meet cleanly at any zoom.

The Modeling canvas follows one design law:

Nothing in the construction that a compass and straightedge couldn't do.

No typing in an angle, no protractor, no numerically-placed vertices. This is a deliberate constraint that keeps constructions honest and, for pattern work, exact.

Locus works internally in millimeters - the real-world size of the finished piece. Where measurements surface, they're at that real-world scale.

The two windows exchange geometry through two nodes:

• Reference takes a model object and feeds it into the node graph.
• Realize takes the node graph's geometry and makes it real in the model.

Everything you build ultimately flows across this bridge. See Bridging Modeling & Nodes.

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