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.