From 788c2288530a52016cebfe5cd1d7a6c18e94dd8f Mon Sep 17 00:00:00 2001
From: leether <leether@outlook.com>
Date: Sun, 7 Jun 2026 16:10:30 +0800
Subject: [PATCH] fix(concat): two critical ffmpeg filter_complex bugs
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Bug 1: Empty filter name when fade_in=fade_out=0
  Generated: [0:v]setpts=PTS-STARTPTS,[v0]
  ffmpeg parsed ',' as filter separator → [v0] treated as filter name
  Fix: [0:v]setpts=PTS-STARTPTS[v0] (no comma between filter and output pad)

Bug 2: Audio stream lost in _normalize_video
  -an deleted original audio, but need_audio logic skipped recovery
  when source had audio (has_audio=True → need_audio=False).
  Result: normalized videos had no audio → acrossfade 'matches no streams'.
  Fix: replace -an with -c:a copy to preserve original audio stream.

Both bugs caused filter_complex path to fail with exit code 8.
Fast path was unaffected.

Closes: concat-engine-ffmpeg-bugs
---
 core/concat_engine.py | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/core/concat_engine.py b/core/concat_engine.py
index fcb46fb..5eff436 100644
--- a/core/concat_engine.py
+++ b/core/concat_engine.py
@@ -155,7 +155,7 @@ def _normalize_video(
                 "-c:v", self.config.video_codec,
                 "-pix_fmt", self.config.pixel_format,
                 "-crf", str(self.config.crf),
-                "-an",  # 先不处理音频
+                "-an",  # 图片无音频
                 output_path,
             ]
         else:
@@ -183,9 +183,9 @@ def _normalize_video(
                 "-vf", filter_str,
                 "-t", str(target_duration),
                 "-c:v", self.config.video_codec,
+                "-c:a", "copy",  # 保留原始音频流（filter_complex 路径需要）
                 "-pix_fmt", self.config.pixel_format,
                 "-crf", str(self.config.crf),
-                "-an",  # 先不处理音频
                 output_path,
             ]
 
@@ -346,22 +346,20 @@ def _build_filter_complex(
             duration = clip["duration"]
 
             # 视频 fade
-            video_filters = [f"[{i}:v]setpts=PTS-STARTPTS"]
+            video_filters = ["setpts=PTS-STARTPTS"]
             if fade_in > 0:
                 video_filters.append(f"fade=t=in:st=0:d={fade_in}")
             if fade_out > 0:
                 video_filters.append(f"fade=t=out:st={duration - fade_out}:d={fade_out}")
-            video_filters.append(f"[v{i}]")
-            filter_parts.append(",".join(video_filters))
+            filter_parts.append(f"[{i}:v]{','.join(video_filters)}[v{i}]")
 
             # 音频 fade
-            audio_filters = [f"[{i}:a]asetpts=PTS-STARTPTS"]
+            audio_filters = ["asetpts=PTS-STARTPTS"]
             if fade_in > 0:
                 audio_filters.append(f"afade=t=in:st=0:d={fade_in}")
             if fade_out > 0:
                 audio_filters.append(f"afade=t=out:st={duration - fade_out}:d={fade_out}")
-            audio_filters.append(f"[a{i}]")
-            filter_parts.append(",".join(audio_filters))
+            filter_parts.append(f"[{i}:a]{','.join(audio_filters)}[a{i}]")
 
         # 链式应用 transition
         video_chain = "v0"
@@ -453,7 +451,13 @@ def _concat_effect_path(
             output_video,
         ]
 
-        subprocess.run(cmd, capture_output=True, check=True)
+        result = subprocess.run(cmd, capture_output=True, check=False)
+        if result.returncode != 0:
+            print(f"[ConcatEngine] ffmpeg failed with code {result.returncode}")
+            print(f"[ConcatEngine] stderr: {result.stderr.decode('utf-8', errors='replace')[:2000]}")
+            raise subprocess.CalledProcessError(
+                result.returncode, cmd, output=result.stdout, stderr=result.stderr
+            )
         return Path(output_video)
 
     # ═══════════════════════════════════════════════════════