Nearest obstruction distance per height row through a forward corridor.
The corridor; see ObstructionProbeOptions.
One distance (metres from the origin) per row of the scanned band,
index 0 = the LOWEST row, row i centred at
baseM + (i + 0.5) × (topM − baseM) / rows; NaN where that row is clear.
null when the layer cannot answer (not loaded, hidden, no GL).
Structural interface of a layer that can answer "is there something solid in front of me?" — the horizontal counterpart of SurfaceSampleable.
A top-down height probe cannot see a wall: a wall's side faces are edge-on to an overhead pass and rasterise nothing, and its top face is above the walker's cut, so an overhead probe reads the floor slab running underneath it — a doorway and a wall return the same height. Genuine wall blocking therefore needs a second, FORWARD pass; see
buildForwardFrame.