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compass straightedge
Home > Compass & straightedge
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The paper-geometry move: draw a full line or circle, tick only the crossings you care about, and let the rest disappear.
Classical compass-and-straightedge constructions clutter up fast - every step leaves a whole line or circle on the page when all you needed was one crossing. Locus's straightedge-and-compass tools keep the crossings and tidy the hosts away.
The idea: draw a construction line or circle, click the intersections worth keeping to drop ticks on them, then finish - the full host retires into a hidden helper group, leaving just your ticks.
All three live in the Draw menu (and the command palette):
- Straightedge: a construction line through two points.
- Compass: a construction circle from a center and a point on it.
- Compass: Radius - a construction circle from a center and a size source (an existing circle, arc, or segment sets the radius). Select the finished circle and the status bar names its source, with that object faintly highlighted - see Compass: Radius.
- Draw the host. Pick a tool and place its points - the host previews dashed, the same look as a shape mid-draw.
- Tick the crossings. Click where the host crosses other geometry. Each click drops a short tick mark centered on that crossing.
- Finish with Esc or Enter. The host - its line/circle, its defining points, and every crossing you ticked - retires into a standing hidden helper group ("Hidden Lines" / "Hidden Circles"). Only the ticks stay visible. Points that were already in your drawing before you started are left where they are - only the scaffolding the construction itself created is tucked away.
Ticking marks a crossing; striking lays down the actual arc or segment you're constructing. In tick mode a click drops a tick - but a drag strikes a real object locked to the host: a struck arc along a compass circle, a struck segment along a straightedge line. It's the productive half of the classical compass-and-straightedge move.
On paper, once the compass point is planted you don't stop to switch tools - you drop the ticks you want and strike the full arcs you want in the same pass, all off that one setting of the compass. Locus keeps them unified for exactly that reason: ticking and striking aren't separate modes or separate steps, just a click versus a drag on the same live host.
- Drag along the host instead of clicking. The preview rides the host - an arc bowing along the compass circle, a segment lying on the straightedge - with a dot at each end marking where it will anchor.
- Ends snap to what's already there. Each end prefers, in order: a point already on the host, then a crossing of the host with other geometry, then a fresh point that slides along it - and both ends get the same treatment. The first end (where you press down) snaps exactly like the second (where you release), so a strike that starts right on an existing point or crossing is honest about it from the first pixel, not just once it's finished.
- The compass keeps its radius. A struck arc takes the host circle's own center, so its radius is the compass radius - nothing to set, nothing to keep in sync. It winds the way you drag, not always the short way around.
An end you create by dragging is a glider on the host - a real, editable point, not throwaway scaffolding. It hides in place when you strike, tidying away with the rest of the construction, but you can reveal it from the Layers panel and drag it along its host to re-place the strike later. An end you reuse - an existing point or crossing - stays visible. A struck object is ordinary geometry: it styles, layers (reading as struck arc / struck seg), and exports like any arc or segment, it can itself be ticked, and selecting it lights up its whole host system just as a tick does.
A click and a drag are told apart on release, so a small wobble still counts as a click - and a plain click still just ticks.
A tick isn't a special annotation - it's an ordinary short segment (straightedge) or arc (compass) whose endpoints ride the host. So a tick styles, layers, exports, and deletes like any other segment or arc, and it stays live on its host: move what the host was built from and the tick follows.
In the Layers panel a tick reads as tick (segment) or tick (arc) so you can tell it from ordinary geometry.
Click any one piece of a construction - a tick, a struck arc, even the retired host - and Locus lights up the whole system it belongs to. The host circle (or line) is revealed and drawn in blue, together with the ticks and strikes built on it, while the exact piece you clicked is drawn in red on top. It answers two questions at a glance: what all moves together as one construction, and which mark you actually have hold of.
This still works when a struck object's ends aren't scaffolding of its own - a strike that reused an existing point, or landed on a crossing that's also the anchor of some other tick or strike, lights up the same whole chain either way. You don't lose the system view just because two constructions happen to share a point.
While a construction is selected, invoke its tool again - Compass for a circle system, Straightedge for a line system - and Locus reopens that system instead of starting a new one: the retired host reappears and you drop back into tick-and-strike mode, free to add more ticks and struck arcs to it. Finish again to tuck it away.
Snapping to a hidden host
Even after the host is hidden, it stays a live snap target - you can still snap new points onto it and onto its crossings. That's what lets you keep building off a construction you've tucked away. (Ordinary hidden objects don't snap; only these construction hosts do, the same way guides stay snappable.)
- Resize: select one or more ticks and press Up / Down to grow or shrink them. Shift moves ten times as far, Option a tenth. It's one undo step.
- Add more ticks: with any part of a tick system selected, invoke the matching tool again (Straightedge for a line system, Compass for a circle) - Locus reveals the retired host and drops you back into ticking mode instead of starting fresh.
- Selecting a tick isolates its system: click a tick and everything related - its anchor, its sibling ticks, the host and the host's points - fades back so you see just that construction, with the selected tick drawn opaque on top.
The default tick length is set in Preferences - separate lengths for compass and straightedge ticks.
Caution
Confirm against the build This is a young feature. The tool names, the menu they sit under, and the tick-adjustment keys are current as of writing - confirm against your version if something doesn't match.
< Previous: Ellipses | Next: Rectangles & squares >
- Circles & arcs | Lines, rays & segments
- Preferences: tick length