Compute the area (2D) or volume (3D) of an isovist, optionally with a distance falloff.

Input parameters:

1. Ry → Isovist rays. Use the Rays output of an Isovist 2D or Isovist 3D component.
2. Wt → Distance falloff applied to each ray's contribution.
   • 0 → Block
   • 1 → Ramp
   • 2 → Decay
   • 3 → Smooth
3. Tr → Transition distance where the falloff begins to take effect. Ignored for Block.
4. Nm → If true, divide by the maximum possible extent at the sample horizon so the result is a unitless score. If false, report the absolute area or volume metrics.

Output parameters:

1. Et → Isovist area (2D) or volume (3D). With Normalise on, a unitless openness score in $[0,1]$; otherwise an absolute measure.