OneCAD-Draft/docs/viewport-dimensions.md

Dimensions through paper-space viewports

Paper-space point commands can snap to displayed model geometry through a planar, orthographic layout viewport. The acquired result retains the original model coordinate, its paper coordinate, the viewport frame, and feature identity. Ordinary selection continues to operate on the active drawing space.

DIMLINEAR, DIMALIGNED, DIMANGULAR, DIMRADIUS, and DIMDIAMETER use this acquisition context. The supported object-pick modes also query viewport geometry. Snapping and object picking share viewport draw order, visibility, and clipping rules. Object picking uses the resident interaction index and applies block instance transforms. Nonuniformly transformed circles and circular arcs are declined as radial object picks because their displayed geometry is elliptical.

Input and measurement rules

• Free and typed sheet points measure paper units, including points inside a viewport rectangle.
• Measuring points must all belong to the same space and, when applicable, the same viewport. An incompatible pick leaves the command at its current step.
• Dimension-line and text placement do not change the measurement's space.
• Paper geometry takes priority over viewport geometry at an object pick.
• The final dimension style is applied before computing measurement scaling. Length dimensions keep paper-space definition points and a negative DIMLFAC override containing the effective user factor multiplied by 1 / viewport scale. Angular measurements do not use this length factor.
• DIMASSOC=0 uses the same compensated value before exploding the annotation.

Persistent measurement data

OCS_VIEWPORT_MEASUREMENT XData stores three values: version 1 (integer16), the positive user factor (real), and viewport compensation (real). This keeps the user's unit conversion separate from viewport scale. The ordinary DIMLFAC override remains sufficient for displaying the saved measurement in other CAD readers. The data survives DXF/DWG saves and undo/redo.

The dependent association change uses this metadata when viewport scale changes. This creation change alone creates nonassociative viewport dimensions and leaves the drawing's DIMASSOC setting unchanged. Direct model/paper associations retain their existing behavior.

Validation

src/app/viewport_dimension_tests.rs exercises snap-engine acquisition, command commits for all five commands, object picks, input retries, active positive and negative style factors, exploded dimensions, undo/redo, DXF/DWG persistence, clipping, overlapping viewports, pan, scale, and twist. These are headless tests; they do not establish visual acceptance in the desktop UI.