Isovist / Viewshed

Scale

Pixels per metre

Areas and lengths are reported in metres using this plan scale (default 50 px = 1 m). It rescales the readout only — the drawing is unchanged.

Draw

Drag inside the room to add a partition wall or a rectangular obstacle. You can grab and drag the red viewpoint at any time. The isovist recomputes live.

Display

Show sight rays Metre grid Mark visible corners

Export

Saves the plan, the shaded isovist and a metrics caption. SVG is vector (true plan units); PNG is a high-resolution raster.

About

An isovist is the region visible from one location. Rays are cast to every corner — plus a sliver to either side so they slip past — and stopped at the nearest wall. The hit points, sorted by angle, are the visibility polygon. Drift is the distance from the viewpoint to that polygon's centroid: small means you sit at the visual centre, large means most of what you see is off to one side.

Sources & method

What this computes. The visibility polygon by angular ray casting to the scene's corners: from the viewpoint, one ray is aimed at every vertex of every wall, obstacle and the boundary, plus two slivers ±0.0001 rad to either side (so rays slip past a corner into the space behind it). Each ray is clipped to its nearest wall by an exact parametric line–segment intersection — the resolution is set by the geometry's corners, not a fixed angular grid. The hits, sorted by angle, trace the polygon. Metrics: area by the shoelace formula; perimeter; compactness = 4πA/P² (the isoperimetric quotient, 1.0 = a circle — akin to Benedikt's “circularity”); the longest sight line; and drift, the distance from the viewpoint to the polygon's area centroid.

Sources. Benedikt, M.L. (1979) “To take hold of space: isovists and isovist fields”, Environment and Planning B 6(1), 47–65 — doi:10.1068/b060047 — the paper that defined the isovist and its measures for architecture. On the computation: visibility polygon (angular-sweep construction).

Scope. A 2-D plan slice: every wall is treated as opaque, full-height and infinitely thin, with no transparency or 3-D occlusion — a study of spatial structure, not a lighting or sightline code check. Areas and lengths are true to the plan-scale setting above.