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ADR-0035 — An angle and a position for every shape ​

Status: Accepted (Product Owner's decisions of the F04 refinement, 2026-10-11; model chosen by the agent at SP-007) — partly supersedes ADR-0003 (the position of any shape may be fractional), ADR-0032 and ADR-0033 (where a free shape is drawn) and ADR-0034 (on a turned shape a coordinate typed is an integer in the drawing only, and the point is stored un-turned).

Context ​

F04 turns shapes by an integer angle (shapes.md §1). The Product Owner decided: about the center of the shape's box, clockwise, kept from 0 to 359; width and height stay those before turning (Product Owner decision, as in Figma); the position shown is the x and y of the straight box, as Inkscape, the true bounding box of the SVG, moved by typing x and y; editing a turned shape keeps it in place.

Today a regular polygon has no position (its box starts at 0, 0) and a free shape has one only through its stored points (the top-left of their box). A turned shape whose box changes would jump, its center moving with the box: something must store where the shape is.

Decision ​

  • Every shape (free shape, regular polygon) stores an angle, an integer from 0 to 359, and a position x, y: the top-left of its box before it turns, in the SVG's coordinates. A typed angle is kept as ((a % 360) + 360) % 360 (DERIV-turned-shape step 1).
  • The drawing is the straight shape in its box placed at x, y, then turned by the angle about the center of the box, clockwise on screen (DERIV-turned-shape steps 2–4). At 0° it is drawn as it is, bit for bit.
  • The straight box (the smallest upright rectangle around the drawn outline, arcs included, DERIV-arc-bounding-box) is derived, never stored. The x and y shown and typed are its top-left; typing them moves the position by the difference.
  • The stored position is not the straight box's top-left on purpose: a corner radius or an anchor changed on a turned shape changes its straight box, and the shape must not move.
  • Editing a turned shape places its new box so that the shape stays in place: its turned top-left corner for a width or height typed, the points not moved for a point typed, the drawing for a conversion (DERIV-turned-shape step 5).
  • Fractions are allowed in the stored position of any shape, a regular polygon included, as in a free shape (ADR-0034): the position after editing a turned shape is computed with cos and sin. This partly supersedes ADR-0003 ("the model stores integers only: positions…"). The user types integers.
  • A free shape's drawn box is placed at its position instead of the top-left o of its stored points: the drawn point is b + w · (p − o) / n on each axis (b the position, n the span of the stored points). o is only the reference of the stretch; it no longer places the shape. When b = o the earlier formula is used as it is, so that the drawing stays bit for bit (ADR-0033). This partly supersedes ADR-0032 and ADR-0033, where the stored top-left placed the shape.
  • On a turned shape, a coordinate typed is an integer in the drawing: the point is stored un-turned, fractional, and drawn back within a rounding error (about 1e-16 of the size), written at 5 decimals (Q10). The field shows the drawn value at 5 decimals, so the integer typed reads back as typed.

Consequences ​

  • Earlier shapes are the angle 0 and the position 0, 0 (regular polygon) or the top-left of their points (free shape): their drawings do not change, which the earlier tests check.
  • The view of the playground fits the straight box, which may have negative coordinates.
  • ADR-0003's "no accumulated numerical drift across edits" no longer holds for a turned shape: each width or point typed recomputes its position with cos and sin, and errors of about 1e-16 of the size may add up over edits. Accepted: they stay far below the 5 decimals written (Q10), as for the stretch of a free shape; at 0° nothing is computed, so a straight shape keeps no drift.
  • The rotation mode of E02 (a movable center) changes the angle and the position, never the shape: one angle only, as the Product Owner wished.
  • Instances in views still turn by quarter turns only (ADR-0008).

References ​

ADR-0003, ADR-0008, ADR-0032, ADR-0033, ADR-0034; DERIV-turned-shape, DERIV-arc-bounding-box; REF-SVG11-TRANSFORM, REF-CSS-TRANSFORMS, REF-INKSCAPE-SELECTOR, REF-FIGMA-ROTATION.