Dimensions through layout viewports

In paper space, snap to model geometry displayed through a planar, orthographic viewport. DIMLINEAR, DIMALIGNED, DIMANGULAR, DIMRADIUS, and DIMDIAMETER also support their usual object-pick modes through these viewports.

Measurement

Model geometry supplies the viewport scale. A 100-unit model length displayed at 1:10 measures 100, although it occupies 10 paper units.
A paper-space centerline or free sheet point can be combined with model geometry through one viewport, in either pick order. The paper-space span is measured using that viewport's scale.
Sheet-only dimensions measure paper units, even inside a viewport rectangle.
Model picks from a second viewport are rejected without advancing the command. Dimension-line and text placement do not change the measurement scale.
The current dimension style's unit conversion is preserved. Angular dimensions do not use length compensation. DIMASSOC=0 explodes the compensated dimension.

Snapping and object picking respect visibility, clipping, and viewport draw order. Paper geometry has priority in object picks. Ordinary selection uses the active drawing space. Object picks reuse the resident interaction index and resolve nested block transforms.

Saved scale

Length dimensions store paper-space definition points and a negative DIMLFAC override: user unit factor multiplied by 1 / viewport scale. OCS_VIEWPORT_MEASUREMENT XData stores version 1 (integer16), the positive user factor (real), and viewport compensation (real). Keeping the factors separate allows compensation to change without losing the user's unit conversion. Both the override and XData survive DXF/DWG saves and undo/redo.

Associations

With DIMASSOC=2, acquired features retain their viewport, block-instance path, source entity, and snap feature. Intersections retain both source chains. Source edits and viewport changes update the dimension and its compensation, preserving style overrides and user-positioned text on the sheet.

A sheet centerline stays on the sheet; a model endpoint follows its viewport. Moving the viewport therefore changes the distance between them. Undo/redo and DXF/DWG saves preserve the references.

Properties shows association status. If a required reference is broken or cannot be resolved, the entire dimension keeps its last valid geometry and reference records. A successful refresh regenerates its displayed and exported geometry.

Limits

Unsupported cases preserve saved dimensions:

Perspective or oblique views; missing or malformed reference chains.
Array instances without an addressable instance index, nesting beyond eight levels, and radial measurements of nonuniformly scaled circles or arcs.
Intersections without two identifiable source chains inside a batched block.
Ambiguous imported endpoint, midpoint, or quadrant markers. A stored point can validate a feature; it cannot establish an otherwise unknown identity.
Tangency without a verified solution, or two perpendicular/tangent references requiring a coupled solve.
Imported viewport scaling without OCS scale metadata or ACAD_DIMASSOC_CALC_DIMLFAC identifying the saved compensation.

Tests

Run cargo test --locked --lib viewport_dimension. Tests cover the five commands, mixed sheet/model input, styles, clipping, object picks, source/viewport edits, snap features, nested paths, intersections, preserved overrides, undo/redo, and DXF/DWG persistence. The synthetic association fixture is in tests/fixtures/dim_assoc/.

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