editor

ADR-0003 — Integer model, derived geometry ​

Status: Accepted — partly superseded by ADR-0031 and ADR-0034 (the points and size of a free shape may be fractional after any edit) and ADR-0035 (the position of any shape may be fractional)

Context ​

The Product Owner places and sizes everything with integers only, without decimals. Yet several geometric quantities are mathematically non-integer:

  • fillet setbacks r / tan(θ/2)
  • vertices of regular polygons (cos, sin)
  • intersection points

Decision ​

  • The model stores integers only: positions, sizes, radii, widths, number of corners.
  • Geometry is derived from the model at each render, as floats.
  • A derived value is never written back into the model, unless an ADR states an explicit rule (e.g. destructive boolean + snap rounding).
  • Robust orientation predicates; named EPSILON constant for comparisons.
  • Fixed-precision SVG output (SVG_DECIMALS).

Consequences ​

  • No accumulated numerical drift across edits.
  • Shapes are reproducible exactly.
  • Operations creating new vertices (destructive booleans) require a rounding rule (Q2).

References ​

REF-SHEWCHUK-1997, REF-HOBBY-1999