Align and stretch a reference bond or edge

Implementation notes and historical checks. Open the visual guide →.

Reference align/stretch turns a drawing so that one bond or shape edge points exactly where you want, rotates copies around a shared center, and changes one bond’s length while the rest of the molecule stays rigid.

Use it

Align or rotate

1. Select both ends of a bond, or select a shape with a straight edge.
2. Open Properties → Transform → Reference: align / stretch.
3. Choose the bond or edge under Reference.
4. Under Moves, choose Connected fragment to move the whole molecule, or Selected objects to move only complete selected fragments and drawing objects.
5. Under Pivot, choose Center of moved objects, Reference start, Reference midpoint or Pinned point. Pin selected center pins the center of the current selection; X and Y show it in points.
6. Choose Horizontal or Vertical, or type an angle in Direction and choose To angle. 0° points right, 90° down, 180° left and −90° up.
7. To turn by a relative angle, type it in Turn and choose Rotate. Copy + rotate keeps the original and selects the rotated copy.

For a fourfold layout, pin one center and make copies of the original fragment at 90°, 180° and 270°.

Stretch one bond

1. Select the bond that links two parts of a molecule and open the same section.
2. Type a length in Length and choose Set length, or choose Drag to stretch and drag the moving end along the bond.
3. The line next to Swap fixed end names the fixed end, for example Length 14.40 pt · C (start) stays fixed. Swap fixed end fixes the other end instead.
4. After Drag to stretch, choose Done in the context row to return to Select.

Everything beyond the moving end moves with it and keeps its shape. Each alignment, rotation, copy or stretch is one Undo step.

Limits

• A partial molecule is rejected; select the whole fragment or use Connected fragment.
• Ring bonds cannot be stretched alone, because the rest of the ring would change. Bonds with other fixed attachments are rejected too.
• Stretching never merges nearby atoms or fuses rings. The shortest length is 0.1 pt.
• The pinned center and unapplied settings are not saved in the file.
• Copies that share a point are not bonded to each other. Join them explicitly.

Implementation notes

Horizontal and Vertical rotate the reference to the nearest matching axis, using its actual angle. To angle uses a directed angle. These operations accept arbitrary orientations; they do not round the reference to a 15° grid. A wedge’s reference is its bond axis, retaining its endpoint order, taper and stereochemistry. Alignment rotates the drawing in its plane and leaves projection depth unchanged. An unchanged alignment records no Undo step.

Center of moved objects uses the stable center of all objects in the chosen scope. The pin’s X/Y fields are publication points in the drawing’s coordinate system. The pin stays fixed across edits and selection changes until it is changed or another file is opened. The existing numeric Rotate field and keyboard commands keep their relative rotation behavior. The ordinary rotation handle remains free by default; Shift snaps that gesture to 15° steps.

Drag to stretch keeps the exact original bond direction, even at angles such as 17.3°. The square marker is the fixed end and the round marker the moving end. Drag the moving end or its branch; perpendicular pointer motion is ignored. All atoms downstream of the moving end translate together, retaining their internal lengths and angles. Dragging clamps the length at 0.1 pt, so it cannot cross zero. Semantic attachment nodes are rejected. Use ordinary selection movement for broader geometry changes.

Escape or loss of focus cancels a stretch gesture, and release outside the canvas leaves the drawing unchanged. Switching tabs or replacing the drawing also ends stretch mode; choose the reference again in the new drawing. .rsk files keep the resulting coordinates, IDs, marks and chemistry; SVG/PDF/PNG and supported editable exchange use the same resulting drawing. The pinned center and the reference settings are editor state, not document metadata.

Reproduce the examples

cargo run --locked --example reference_geometry_qa -- /tmp/reference-geometry

The generator produces editable inputs, eight renderer cases and a combined gallery in SVG, PNG and PDF. The quarter-turn example contains separate carbocycles to demonstrate construction geometry; it is not a porphyrin identity reference. The desktop review includes real control captures, saved files and an independent coordinate audit. It distinguishes the checked align/stretch interactions from renderer-only common-center examples.