Stretch: carry over source EXIF/XMP when no JSON sidecar is present

Stretching an already-embedded thermal folder (e.g. Streifen_exif/thermal,
which has tagged PNGs but no .json sidecars) produced bare 8-bit PNGs: the
old code skipped frames with no sidecar, so nothing was embedded.

_embed_exif now always copies the source image's EXIF/XMP via -tagsFromFile
and overlays JSON-sidecar tags when available, so metadata transfers whether
you stretch the raw session (sidecars) or already-embedded PNGs (source EXIF).
GPS verification only fires when there was metadata to embed; otherwise it
logs a clear note instead of a false "full disk" error.

Verified: raw session and embedded-no-sidecar both yield 0 frames missing GPS.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CLAUDE.md

@@ -39,7 +39,7 @@ whether you run from the session's parent or from a grouping folder.
 | File | What it does | Output |
 |------|--------------|--------|
 | `embed_metadata.py` | Copies each image and writes its JSON sidecar into the copy's EXIF/XMP (GPS, true camera orientation, UTC time, lens/exposure, full JSON in `UserComment`). | `<session>_exif/` (mirrors layout; originals untouched) |
-| `stretch_thermal.py` | Rescales all thermal frames to one session-wide brightness window → 8-bit, with a palette (inferno/ironbow/gray). Embeds the sidecar EXIF into the output too. | `<session>/thermal_stretched/` |
+| `stretch_thermal.py` | Rescales all thermal frames to one session-wide brightness window → 8-bit, with a palette (inferno/ironbow/gray). Carries metadata into each output: from the JSON sidecar if present, otherwise copied from the source image's own EXIF/XMP (so stretching already-embedded PNGs keeps GPS/orientation). | `<session>/thermal_stretched/` |
 | `session_map.py` | Shared geometry: trigger points + oblique footprints; also the camera-orientation math used by `embed_metadata`. Not run directly. | — |
 | `firemapper_gui.py` | Tkinter GUI wrapping all of the above. Tabs: **1. Embed**, **2. Thermal Stretch**, **3. Map**. | — |
 

stretch_thermal.py

@@ -164,33 +164,43 @@ def _embed_exif(pngs, out_dir: Path, exiftool=None, log=None):
     except BaseException as e:  # noqa: BLE001  (ensure_exiftool may sys.exit)
         log(f"  ! skipping EXIF embed - exiftool unavailable ({e})")
         return
-    arg_lines, dsts = [], []
+    arg_lines, dsts, srcs = [], [], []
+    have_sidecars = False
     for p in pngs:
-        sidecar = p.with_suffix(".json")
-        if not sidecar.exists():
-            continue
-        try:
-            data = json.loads(sidecar.read_text(encoding="utf-8"))
-        except Exception:  # noqa: BLE001
-            continue
         dst = out_dir / (p.stem + ".png")
+        # 1) copy any metadata already on the SOURCE image (covers stretching
+        #    already-embedded PNGs that have no JSON sidecar next to them);
+        # 2) overlay/refresh from the JSON sidecar when it exists (raw session).
+        seg = ["-overwrite_original", "-tagsFromFile", str(p), "-EXIF:all", "-XMP:all"]
+        sidecar = p.with_suffix(".json")
+        if sidecar.exists():
+            try:
+                data = json.loads(sidecar.read_text(encoding="utf-8"))
+                seg += embed.build_tags(data, {}, None, "gps")
+                have_sidecars = True
+            except Exception:  # noqa: BLE001
+                pass
+        seg += [str(dst), "-execute"]
+        arg_lines += seg
         dsts.append(dst)
-        arg_lines += ["-overwrite_original", *embed.build_tags(data, {}, None, "gps"),
+        srcs.append(p)
-                      str(dst), "-execute"]
     if not arg_lines:
         return
     argfile = out_dir / "_exif_args.txt"
     argfile.write_text("\n".join(arg_lines) + "\n", encoding="utf-8")
     cmd = [et, "-m", "-charset", "UTF8", "-charset", "filename=UTF8", "-@", str(argfile)]
-    samples = {dsts[0], dsts[len(dsts) // 2], dsts[-1]}   # spot-check first/mid/last
+    idx = sorted({0, len(dsts) // 2, len(dsts) - 1})       # spot-check first/mid/last
 
-    def gps_written():
+    def has_gps(path):
-        return all(subprocess.run([et, "-s", "-s", "-s", "-GPSLatitude", str(s)],
+        return bool(subprocess.run([et, "-s", "-s", "-s", "-GPSLatitude", str(path)],
-                                  capture_output=True, text=True).stdout.strip() for s in samples)
+                                   capture_output=True, text=True).stdout.strip())
+
+    # only expect GPS in the output if there was any to embed (sidecar or source EXIF)
+    expect_gps = have_sidecars or any(has_gps(srcs[i]) for i in idx)
 
     log("Embedding GPS / orientation EXIF into the stretched frames ...")
     proc = subprocess.run(cmd, capture_output=True, text=True)
-    if proc.returncode == 0 and not gps_written():
+    if expect_gps and proc.returncode == 0 and not all(has_gps(dsts[i]) for i in idx):
         log("  ! GPS not present after first pass - retrying once ...")
         proc = subprocess.run(cmd, capture_output=True, text=True)
     argfile.unlink(missing_ok=True)
@@ -198,13 +208,18 @@ def _embed_exif(pngs, out_dir: Path, exiftool=None, log=None):
     err = (proc.stderr or "").strip()
     if err:
         log(err)
-    if proc.returncode != 0 or not gps_written():
+    if proc.returncode != 0 or (expect_gps and not all(has_gps(dsts[i]) for i in idx)):
         raise RuntimeError(
             "Could not embed GPS/EXIF into the stretched PNGs (exiftool exit "
             f"{proc.returncode}). The usual cause is a FULL DISK - free up space and re-run. "
             "The stretched images were written; only the embedded metadata is missing."
             + (f"\nexiftool: {err}" if err else ""))
-    log(f"EXIF embedded into {len(dsts)} stretched frames.")
+    if not expect_gps:
+        log("  note: no JSON sidecars and the source images carry no GPS - the stretched "
+            "PNGs have no location metadata. Stretch the raw session (with .json sidecars), "
+            "or run Embed first.")
+    else:
+        log(f"EXIF embedded into {len(dsts)} stretched frames.")
 
 
 def stretch_session(session, out_dir=None, colormap="inferno", lo=None, hi=None,