Handoff: Document the multiple-return-fraction gap (AI3D-337)

2026-09-01 · next agent session · repo 3dai.iolabs.imageanalyzer.rasterizer, branch feature/zrange-extra-channels

Summary

Continuation target: write a small, durable documentation note about the missing multiple-return-fraction channel in the AI3D-337 z-channel work. The channel cannot be produced because the LAS NumberOfReturns / ReturnNumber fields are stripped upstream during LAS → npz export; the rasterizer's input *_run3_points.npz carries only points, scan_angle, intensity, red, green, blue.

Miro's assessment (authoritative, include it in the doc): this gap is not a concern for the Z diff plots. Their main purpose is (a) checking whether lidar passes are aligned on the pavement parts and (b) seeing the edges of the pavement — both served by the existing channels. The gap should nevertheless be documented so nobody re-discovers it or silently assumes the channel exists.

Current state

Branch feature/zrange-extra-channels (5 commits, all AI3D-337, clean tree at handoff time, HEAD 65b9308) adds TopdownRasterizer.compute_z_channels: per-cell cell-relative z-histogram → z_bimodality, z_lowest_layer_top, z_lowest_layer_jump, n_points. Additive; legacy process_tile byte-identical.
The gap is already mentioned in two places, but neither carries Miro's "not a concern for the intended use" assessment:
- Commit message of 9843585 ("multiple-return-fraction channel NOT produced … Documented as follow-up.").
- Docstring of compute_z_channels — src/iolabs_image_analyzer_rasterizer/rasterization/topdown_rasterizer.py:1473-1478 ("See the branch validation report / module follow-ups.").
Background for the doc author: multi-return lidar points come from permeable/edge structures (vegetation, guardrail edges, sign posts); bare pavement gives 1-of-1 returns. A per-cell fraction of multi-return points was proposed in the AI3D-337 research doc (§ referenced as "s2" in the commit) as a discriminator to separate "high z-spread because guardrail/vegetation" from pavement. Recomputing it here is impossible — the information is physically absent from the npz input; only an upstream export change (Step 3 of iolabs-point-cloud-segmentation-trajectory) could restore it.

Task & next steps

Read the existing mentions (docstring around topdown_rasterizer.py:1473, commit 9843585 message) so the new note links to them instead of duplicating.
Write the documentation note. Suggested home: a short section in README.md or a new docs/ note (repo has docs/plans/ only; a docs/known-gaps.md or similar is fine — pick what fits repo conventions, keep it brief). Content:
- What the channel would be (per-cell fraction of points with NumberOfReturns > 1) and why it was proposed (vegetation/guardrail vs. pavement discrimination, AI3D-337 research doc).
- Why it can't be produced: NumberOfReturns/ReturnNumber stripped during LAS → npz export upstream (Step 3, iolabs-point-cloud-segmentation-trajectory); *_run3_points.npz keys are points, scan_angle, intensity, red, green, blue.
- Impact assessment (Miro, 2026-09-01): acceptable. The Z diff plots' purpose is lidar-pass alignment on pavement and pavement-edge visibility; the multiple-return channel is not needed for that. Document as a known limitation, not a blocker.
- What it would take to add later: extend the upstream npz export to carry the return fields, then a straightforward extra per-cell scatter channel here.
Optionally update the docstring's trailing pointer ("See the branch validation report / module follow-ups.") to point at the new doc location.
Commit on feature/zrange-extra-channels, message prefixed AI3D-337:, one concise line, no co-author trailer (Miro's convention).

Key artifacts

Handle	What
`/home/ai/dev/3dai.iolabs.imageanalyzer.rasterizer`	Repo (WSL2). Branch `feature/zrange-extra-channels`, main branch `master`.
`src/iolabs_image_analyzer_rasterizer/rasterization/topdown_rasterizer.py:1473`	Existing docstring mention of the gap inside `compute_z_channels`.
`git show 9843585`	Commit that introduced the z-channels and first recorded the gap.
`scripts/run_zchannels.py`	QA panel renderer (intensity \| z_spread \| J \| B \| top) — the "Z diff plots" context.
AI3D-337	Jira issue; the research doc (§2) that proposed the channel lives with it. Not in this repo.
`iolabs-point-cloud-segmentation-trajectory`, Step 3	Upstream pipeline whose LAS → npz export strips the return fields. Any real fix lands there.

Verification

Nothing run for this handoff beyond git log/grep; docs-only change ahead, but run pytest before committing if the docstring is touched (123 tests passing on branch per commit message).

Risks & open questions

Doc location is the only real decision. Miro didn't specify where; README section vs. docs/ note vs. expanding the docstring alone. Any is acceptable — prefer one discoverable place plus pointers, not three copies.
Commit 9843585 says "See the branch validation report" — no such report file exists in the repo (only docs/plans/perspective-multisegment-20260707.html). Don't chase it; treat the new note as the canonical record.
The exact upstream export code was not inspected in this session — the "stripped upstream" claim comes from the commit message and NPZ key list. If precision matters, verify the Step 3 export in iolabs-point-cloud-segmentation-trajectory before asserting it in the doc.