feat: build_crossfade_merge_command — chained acrossfade / concat

core/ffmpeg.py

@@ -244,6 +244,35 @@ def build_audio_clip_command(input_path: str, start: float, duration: float,
     ]
 
 
+def build_crossfade_merge_command(clips: list[str], crossfade: float,
+                                  out_path: str) -> list[str]:
+    """ffmpeg command that concatenates *clips* in order into *out_path*,
+    crossfading each join by *crossfade* seconds (0 = butt-join via concat).
+    Re-encoded per the output extension."""
+    if not clips:
+        raise ValueError("no clips to merge")
+    ext = os.path.splitext(out_path)[1].lower()
+    codec = _AUDIO_CODEC_BY_EXT.get(ext, [])
+    cmd = [_bin("ffmpeg"), "-y"]
+    for c in clips:
+        cmd += ["-i", c]
+    if len(clips) == 1:
+        return cmd + ["-vn", *codec, out_path]
+    if crossfade > 0:
+        # Chain acrossfade: [0][1]->[a1]; [a1][2]->[a2]; …; last label = [out].
+        # default acrossfade curve is 'tri' (linear); per-join curves deferred
+        parts, prev = [], "0"
+        for i in range(1, len(clips)):
+            label = "out" if i == len(clips) - 1 else f"a{i}"
+            parts.append(f"[{prev}][{i}]acrossfade=d={round(crossfade, 3)}[{label}]")
+            prev = label
+        fc = ";".join(parts)
+    else:
+        inputs = "".join(f"[{i}]" for i in range(len(clips)))
+        fc = f"{inputs}concat=n={len(clips)}:v=0:a=1[out]"
+    return cmd + ["-filter_complex", fc, "-map", "[out]", *codec, out_path]
+
+
 def detect_hw_encoders() -> list[str]:
     """Probe ffmpeg for available H.264 hardware encoders.
 

tests/test_utils.py

@@ -1,6 +1,6 @@
 import tempfile, os, json
 from main import build_export_path, format_time, build_ffmpeg_command, build_sequence_dir, build_audio_extract_command, resolve_keyframe, apply_keyframes_to_jobs
-from core.ffmpeg import build_audio_clip_command
+from core.ffmpeg import build_audio_clip_command, build_crossfade_merge_command
 from core.annotations import build_annotation_json_path, upsert_clip_annotation
 from main import ProcessedDB
 
@@ -101,6 +101,45 @@ def test_audio_clip_command_appends_filter_chain():
     assert i < len(cmd) - 1 and cmd[-1] == "/o/a.wav"
 
 
+def test_merge_single_clip_reencodes():
+    cmd = build_crossfade_merge_command(["/a.wav"], 0.5, "/o/out.mp3")
+    assert cmd[0] == "ffmpeg"
+    assert cmd.count("-i") == 1
+    assert "libmp3lame" in cmd            # codec by out ext
+    assert "acrossfade" not in " ".join(cmd)
+    assert "-map" not in cmd
+    assert "-filter_complex" not in cmd
+    assert cmd[-1] == "/o/out.mp3"
+
+def test_merge_two_clips_acrossfade():
+    cmd = build_crossfade_merge_command(["/a.wav", "/b.wav"], 0.5, "/o/out.wav")
+    assert cmd.count("-i") == 2
+    fc = cmd[cmd.index("-filter_complex") + 1]
+    assert "[0][1]acrossfade=d=0.5" in fc
+    assert "[out]" in fc
+    assert cmd[cmd.index("-map") + 1] == "[out]"
+    assert "pcm_s16le" in cmd              # codec by out ext (multi-clip path)
+
+def test_merge_three_clips_chains():
+    cmd = build_crossfade_merge_command(["/a.wav", "/b.wav", "/c.wav"], 1.0, "/o/o.wav")
+    fc = cmd[cmd.index("-filter_complex") + 1]
+    assert fc.count("acrossfade=d=1.0") == 2      # two joins
+    assert "[0][1]acrossfade=d=1.0[a1]" in fc
+    assert "[a1][2]acrossfade=d=1.0[out]" in fc
+    assert cmd[cmd.index("-map") + 1] == "[out]"
+
+def test_merge_zero_crossfade_uses_concat():
+    cmd = build_crossfade_merge_command(["/a.wav", "/b.wav"], 0.0, "/o/o.wav")
+    fc = cmd[cmd.index("-filter_complex") + 1]
+    assert "concat=n=2:v=0:a=1" in fc
+    assert "acrossfade" not in fc
+
+def test_merge_empty_raises():
+    import pytest
+    with pytest.raises(ValueError):
+        build_crossfade_merge_command([], 0.5, "/o/o.wav")
+
+
 # --- ProcessedDB ---
 
 def test_db_add_and_get_markers():