ADR-0031 — A free shape keeps the fractional vertices of a converted polygon

Status: Accepted (Product Owner's refinement of F08, 2026-10-11) — partly superseded by ADR-0034 (only the coordinate typed is an integer)

Context

Every input of the model is an integer, and derived geometry is fractional and never written back (ADR-0003). A regular polygon stores integers (corners, width, height, radii, anchor); its vertices are derived, fractional (25, 6.69873 for a hexagon in 100 × 100).

F08 turns a regular polygon into a free shape whose vertices are stored, so that they can be moved one by one. The Product Owner asked that converting does not move the shape, and that only the points moved afterwards land on integers. Rounding at the conversion would move a triangle's apex by up to half a unit and break its symmetry: an apex at x 0.5 cannot stay in the middle once rounded.

Options:

Option	The shape at conversion	Integers in the model
Round every vertex to an integer	moves by up to 0.5	always
Keep the derived vertices, exact	unchanged	not for the converted vertices, until they move

Decision

A free shape stores its vertices as numbers: a vertex obtained by converting a polygon, or added in the middle of an edge, keeps its fractional coordinates; a vertex typed or moved by the user is an integer, as every input is (ADR-0003). Its radii stay integers ≥ 0 (ADR-0029). A vertex added in the middle of an edge is exactly the midpoint (REF-MATHWORLD-MIDPOINT), fractional when needed, so that the edge does not change.

Consequences

• ADR-0003 is partly superseded on one point, its rule "the model stores integers only": the vertices of a free shape may be fractional; the user still types only integers. ADR-0003's status says so.
• Written with at most 5 decimals in the output (Q10), as every derived coordinate.
• A free shape is never converted back into a regular polygon (as in Inkscape, REF-INKSCAPE-OBJECT-TO-PATH).

References

ADR-0003, ADR-0029, REF-INKSCAPE-OBJECT-TO-PATH, REF-MATHWORLD-MIDPOINT