4-seasons: validate JPEGs with ffprobe, add optional grey-frame filter

The previous size check caught empty files but not corrupt JPEGs with
a partial header (common first frame after a camera reconnect).
Replace with ffprobe codec detection — if ffprobe can't identify a
video stream in the file it's skipped with a clear "corrupt" message.

Also adds SEASONS_GREY_STDDEV_MIN (default 0 = off): when enabled,
frames with near-zero pixel standard deviation (uniform grey = bad
RTSP signal) are silently dropped before encoding.

https://claude.ai/code/session_01C4jbd3waXG3eKZYbGUjLUQ

4-seasons.sh

@@ -87,17 +87,36 @@ cleanup() { rm -f "$temp_file" "$temp_video" 2>/dev/null || true; }
 trap cleanup EXIT
 
 mapfile -t all_images < <(find "$image_dir" -type f -name "*.jpg" | sort)
-images=()
-bad=0
+
+# Validate each frame: skip empty files, corrupt JPEGs, and optionally grey/uniform frames.
+# SEASONS_GREY_STDDEV_MIN=0 (default) disables the grey filter.
+# Set to e.g. 5 in sky-cam.conf to drop uniform grey frames (bad-signal captures).
+GREY_MIN="${SEASONS_GREY_STDDEV_MIN:-0}"
+images=(); bad=0; grey=0
 for img in "${all_images[@]}"; do
-    if [ -r "$img" ] && [ -s "$img" ]; then
-        images+=("$img")
-    else
-        echo "WARNING: skipping unreadable/empty frame: $img"
-        ((bad++)) || true
+    # Empty or unreadable
+    if [ ! -r "$img" ] || [ ! -s "$img" ]; then
+        echo "WARNING: skipping empty/missing frame: $(basename "$img")"
+        ((bad++)) || true; continue
     fi
+    # Corrupt JPEG — ffprobe can't identify any video stream
+    if ! ffprobe -v quiet -select_streams v:0 \
+            -show_entries stream=codec_name -of csv=p=0 "$img" 2>/dev/null \
+            | grep -q .; then
+        echo "WARNING: skipping corrupt frame: $(basename "$img")"
+        ((bad++)) || true; continue
+    fi
+    # Optional: skip uniform-grey bad-signal frames (low pixel standard deviation)
+    if [ "$GREY_MIN" != "0" ] && command -v convert &>/dev/null; then
+        stddev=$(convert "$img" -colorspace Gray -format "%[fx:int(standard_deviation*255)]" info: 2>/dev/null || echo "255")
+        if [ "${stddev:-255}" -lt "$GREY_MIN" ]; then
+            ((grey++)) || true; continue
+        fi
+    fi
+    images+=("$img")
 done
-[ "$bad" -gt 0 ] && echo "WARNING: skipped $bad bad frame(s) out of ${#all_images[@]} found"
+[ "$bad"  -gt 0 ] && echo "WARNING: skipped $bad corrupt/empty frame(s) of ${#all_images[@]}"
+[ "$grey" -gt 0 ] && echo "INFO: skipped $grey grey/uniform frame(s) (SEASONS_GREY_STDDEV_MIN=${GREY_MIN})"
 
 for img in "${images[@]}"; do
     printf "file '%s'\nduration %s\n" "$img" "$INTERVAL"

sky-cam.conf

@@ -255,6 +255,16 @@ CRF_MONTAGE_FINAL=22     # montage-mvt.sh           — final archive montage (H
 ENCODE_PRESET=slow       # ffmpeg preset for all encodes (slow/medium/fast)
 AUDIO_BITRATE=192k       # montage-mvt.sh           — music track on movement montages
 
+# ── Frame validation (4-seasons.sh) ──────────────────────────────────────────
+# Corrupt or empty frames (e.g. first frame after a camera reconnect) are always
+# skipped automatically via ffprobe.
+#
+# SEASONS_GREY_STDDEV_MIN: also skip uniform-grey frames caused by a bad RTSP
+# signal.  These are valid JPEGs but contain a near-solid grey image.
+# Value is the minimum pixel standard deviation on a 0–255 scale.
+# 0 = disabled (default).  Try 5–10 if your camera produces grey frames on reconnect.
+SEASONS_GREY_STDDEV_MIN=0
+
 # ── Montage attribution overlay ───────────────────────────────────────────────
 # A translucent bar across the top of each movement montage, naming the
 # music source ("The Four Seasons — Antonio Vivaldi ...").