scripts/holicity/add_coord_head.py

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""
Stamp a spatial reference (EPSG) into the headers of LAS/LAZ point clouds.

Role in the pipeline:
    After HoliCity (London) tiles have been translated into real-world projected
    coordinates, some files still lack a CRS recorded in their LAS header. This
    utility writes the target EPSG into each file's header WITHOUT modifying any
    point coordinates, so the tiles are correctly tagged for QGIS / satellite
    imagery alignment before tiling.

Behavior:
    - Recursively scans a directory for .las/.laz files (skipping macOS "._*"
      AppleDouble sidecar files).
    - For each file, first attempts to write the CRS with laspy
      (header.add_crs, falling back to header.epsg).
    - If laspy cannot write the SRS, falls back to the PDAL CLI
      (`pdal translate ... --writers.las.a_srs=EPSG:<code>`), which embeds the
      SRS into the LAS header while leaving coordinates unchanged.
    - Verifies each result by reading back the header EPSG and reprojecting the
      bbox center to WGS84 for a printed sanity check.

Inputs:  directory of .las/.laz files (CLI: -d/--dir).
Outputs: either overwritten files (--overwrite) or copies with a suffix
         (default *_srs.las, configurable via --suffix), each carrying TARGET_EPSG.
External tools: laspy, pyproj, and PDAL (`pdal translate`) as a fallback.
"""

import os
import sys
import argparse
import subprocess
from pathlib import Path

import laspy
from pyproj import CRS, Transformer

TARGET_EPSG = 27700  # OSGB36 / British National Grid (commonly used for London)

def is_mac_dot_underscore(p: Path) -> bool:
    return p.name.startswith("._")

def try_write_epsg_with_laspy(in_path: Path, out_path: Path, epsg: int) -> bool:
    """Write the CRS using laspy first; return True on success."""
    las = laspy.read(str(in_path))
    ok = False
    # Option A: add_crs (laspy 2.3+)
    try:
        las.header.add_crs(CRS.from_epsg(epsg))
        ok = True
    except Exception:
        pass
    # Option B: write header.epsg directly (works on some versions)
    if not ok:
        try:
            las.header.epsg = int(epsg)
            ok = True
        except Exception:
            ok = False
    if ok:
        las.write(str(out_path))
    return ok

def try_write_epsg_with_pdal(in_path: Path, out_path: Path, epsg: int) -> bool:
    """Fall back to PDAL to write the SRS into the LAS header; coordinates unchanged."""
    try:
        cmd = [
            "pdal", "translate", str(in_path), str(out_path),
            "-f", "writers.las",
            f"--writers.las.a_srs=EPSG:{epsg}",
            "--writers.las.compression=false"
        ]
        subprocess.check_call(cmd, stdout=subprocess.DEVNULL, stderr=subprocess.STDOUT)
        return True
    except Exception:
        return False

def center_wgs84(path: Path, epsg: int):
    """Read the bbox center and reproject to WGS84, for printed verification only."""
    with laspy.open(str(path)) as f:
        hdr = f.header
        mins = getattr(hdr, "mins", getattr(hdr, "min", (0,0,0)))
        maxs = getattr(hdr, "maxs", getattr(hdr, "max", (0,0,0)))
        cx = (mins[0] + maxs[0]) * 0.5
        cy = (mins[1] + maxs[1]) * 0.5
        tr = Transformer.from_crs(f"EPSG:{epsg}", "EPSG:4326", always_xy=True)
        lon, lat = tr.transform(cx, cy)
        return (lon, lat), (cx, cy), getattr(hdr, "epsg", None)

def process_file(p: Path, overwrite: bool, keep_suffix: str):
    out_path = p if overwrite else p.with_name(p.stem + keep_suffix)
    # Try laspy first
    ok = try_write_epsg_with_laspy(p, out_path, TARGET_EPSG)
    method = "laspy"
    # Then fall back to PDAL
    if not ok:
        ok = try_write_epsg_with_pdal(p, out_path, TARGET_EPSG)
        method = "pdal"

    if not ok:
        print(f"[FAIL] {p.name}: failed to write EPSG (neither laspy nor pdal available)")
        return

    # Read back to verify
    (lon, lat), (cx, cy), epsg_now = center_wgs84(out_path, TARGET_EPSG)
    print(f"[OK]   {p.name} -> {out_path.name}  via {method} "
          f"| EPSG: {epsg_now} | center_xy=({cx:.3f},{cy:.3f}) | WGS84=({lon:.6f},{lat:.6f})")

def main():
    ap = argparse.ArgumentParser(description="Batch-write EPSG into LAS/LAZ headers (without changing coordinates) and print a center-point verification.")
    ap.add_argument("-d", "--dir", default=".", help="Directory to scan (recursive)")
    ap.add_argument("--overwrite", action="store_true", help="Overwrite the original files (default writes *_srs.las)")
    ap.add_argument("--suffix", default="_srs.las", help="Output suffix when not overwriting (default _srs.las)")
    args = ap.parse_args()

    root = Path(args.dir).resolve()
    if not root.exists():
        print(f"Directory does not exist: {root}", file=sys.stderr); sys.exit(1)

    files = [p for p in root.rglob("*")
             if p.is_file()
             and p.suffix.lower() in (".las", ".laz")
             and not is_mac_dot_underscore(p)]

    if not files:
        print("No .las/.laz files found (._* filtered out)"); return

    print(f"Target EPSG: {TARGET_EPSG}  | Scan directory: {root}\n")
    for p in sorted(files):
        try:
            process_file(p, overwrite=args.overwrite, keep_suffix=args.suffix)
        except Exception as e:
            print(f"[ERR]  {p.name}: {e}")

if __name__ == "__main__":
    main()