# Topotek KHP20S30 camera calibration (factory-sheet approximation)

**Task**: AZ-702_khp20s30_calibration
**Name**: Provide a calibration JSON for the Topotek KHP20S30 nadir camera (factory-sheet approximation)
**Description**: Compute and commit a `CameraCalibrationArtifact` JSON for the Derkachi camera (Topotek KHP20S30) from manufacturer factory data. Replaces the `adti26.json` placeholder that AC-3 currently uses. Documents the residual error vs a per-unit checkerboard refinement.
**Complexity**: 1 point
**Dependencies**: None
**Component**: input_data / shared_helpers
**Tracker**: AZ-702
**Epic**: AZ-696

## Problem

`_docs/00_problem/input_data/flight_derkachi/camera_info.md` states the
Topotek KHP20S30 intrinsics are unknown. `tests/e2e/replay/conftest.py`
(line 50–56) substitutes `tests/fixtures/calibration/adti26.json` as a
placeholder. AC-3 (≤ 100 m horizontal error for 80 % of ticks) is
`@xfail` until a real calibration ships.

The cheapest reasonable starting point is a factory-sheet approximation
— compute `K` from the manufacturer's published focal length + sensor
geometry, accept the 1–3 % focal-length residual as a documented
budget, and let AC-3 either PASS or honestly FAIL with the residual
attributed.

## Outcome

- A calibration JSON `khp20s30_factory.json` exists in the Derkachi
  input directory, parses against the project's
  `CameraCalibrationArtifact` schema, and documents the acquisition
  method as `factory_sheet`.
- `camera_info.md` is updated to reference the new calibration + the
  residual budget + the deferral handle (`AZ-XXX_checkerboard_refinement`).
- AZ-699 (T3) uses this calibration as its `--camera-calibration` input.

## Scope

### Included
- Source manufacturer factory data for the Topotek KHP20S30 (sensor: 1/2.8" CMOS, 2.13 MP, 1920×1080; lens focal length, FOV, pixel pitch).
- Compute `K = [[fx, 0, cx], [0, fy, cy], [0, 0, 1]]` from `fx = fy = focal_length_mm × (image_width_px / sensor_width_mm)`.
- Set distortion to `[0, 0, 0, 0, 0]` (factory-sheet approximation).
- Set `body_to_camera_se3` to identity-down (nadir; camera-z = aircraft-down).
- Set `acquisition_method = "factory_sheet"`.
- Write `_docs/00_problem/input_data/flight_derkachi/khp20s30_factory.json`.
- Update `_docs/00_problem/input_data/flight_derkachi/camera_info.md`.
- New unit test under `tests/unit/calibration/` asserting the JSON parses and matches the documented inputs.

### Excluded
- Physical checkerboard calibration (needs hardware).
- PnP-from-tlog back-computation (deferred follow-up).
- Updating `adti26.json` or other test fixtures.

## Acceptance Criteria

**AC-1: Calibration JSON parses**
Given the new `khp20s30_factory.json`
When loaded by the project's calibration parser (same schema as `adti26.json`)
Then it parses without error and all fields are populated

**AC-2: Doc updated**
Given `camera_info.md` before
When the calibration is committed
Then `camera_info.md` says "factory-sheet approximation; per-unit checkerboard refinement deferred — see <future-task>" and lists the residual budget

**AC-3: Unit test snapshot**
Given the new JSON
When the unit test runs
Then it asserts `fx == fy` (square pixels), `cx ≈ width/2`, `cy ≈ height/2`, distortion all zero

**AC-4: T3 consumes this calibration**
Given AZ-699's `test_derkachi_real_tlog.py`
When it runs
Then it loads `khp20s30_factory.json` as `--camera-calibration` (no longer the `adti26.json` placeholder)

## Non-Functional Requirements

**Compatibility**
- JSON schema MUST be identical to existing calibration fixtures (`adti26.json`) — no schema changes in this task.

## Unit Tests

| AC Ref | What to Test | Required Outcome |
|--------|-------------|-----------------|
| AC-1 | JSON loads via existing parser | Object populated |
| AC-3 | Field values match factory inputs | fx == fy, cx/cy at centre, zero distortion |

## Blackbox Tests

| AC Ref | Initial Data/Conditions | What to Test | Expected Behavior | NFR References |
|--------|------------------------|-------------|-------------------|----------------|
| AC-4 | T3 test pointed at new JSON | T3 launches without calibration parse error | Test starts cleanly | Compat |

## Constraints

- MUST follow the calibration contract in `_docs/02_document/contracts/shared_helpers/descriptor_normaliser.md` (or wherever the camera-calibration schema lives).
- MUST be a single committed JSON — no generator script with side effects.

## Risks & Mitigation

**Risk 1: Factory data unavailable at required precision**
- *Risk*: Topotek does not publish the exact focal length / sensor width to the precision needed.
- *Mitigation*: Document the gap; ship with the best-available estimate; flag in `camera_info.md` so T3 surfaces the uncertainty in its failure message.

**Risk 2: Residual error exceeds AC-3 budget**
- *Risk*: 1–3 % focal-length error may push horizontal error past 100 m at 1 km AGL.
- *Mitigation*: That's the honest finding. T3 reports it. A follow-up task can pursue checkerboard refinement if needed.