feat: 视频调速引擎（快进/慢放） (#294)

apps/worker/video_processing/speed_engine.py

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+"""视频调速引擎 — 基于 FFmpeg setpts + atempo 的速度调整能力。
+
+支持：
+- 0.25x ~ 4x 变速范围
+- 视频调速（setpts）
+- 音频调速（atempo，多级串联处理超范围值）
+- 音调修正（pitch_correct，默认开启）
+- 边界自动钳制，不阻断渲染
+"""
+
+from dataclasses import dataclass
+from typing import Optional
+
+# ─── 常量 ───────────────────────────────────────────────
+MIN_SPEED = 0.25
+MAX_SPEED = 4.0
+DEFAULT_SPEED = 1.0
+
+# atempo 单级有效范围
+_ATEMPO_MIN = 0.5
+_ATEMPO_MAX = 2.0
+
+
+@dataclass
+class SpeedConfig:
+    """调速配置。
+
+    Attributes:
+        speed: 播放速度，0.25~4.0，1.0 为原速
+        pitch_correct: 是否保持音调（默认 True，用 atempo 时间拉伸算法）
+    """
+
+    speed: float = DEFAULT_SPEED
+    pitch_correct: bool = True
+
+    @classmethod
+    def parse(cls, data: Optional[dict]) -> "SpeedConfig":
+        """从 dict 解析配置，无效值回退到默认。"""
+        if not data or not isinstance(data, dict):
+            return cls()
+
+        speed = data.get("speed", DEFAULT_SPEED)
+        if not isinstance(speed, (int, float)):
+            speed = DEFAULT_SPEED
+
+        pitch_correct = data.get("pitch_correct", True)
+        if not isinstance(pitch_correct, bool):
+            pitch_correct = True
+
+        config = cls(speed=float(speed), pitch_correct=pitch_correct)
+        config.clamp()
+        return config
+
+    def clamp(self) -> None:
+        """将速度钳制到合法范围。"""
+        if self.speed <= 0:
+            self.speed = DEFAULT_SPEED
+        elif self.speed < MIN_SPEED:
+            self.speed = MIN_SPEED
+        elif self.speed > MAX_SPEED:
+            self.speed = MAX_SPEED
+
+    @property
+    def is_original(self) -> bool:
+        """是否原速（无需调速）。"""
+        return abs(self.speed - 1.0) < 1e-6
+
+
+class SpeedEngine:
+    """调速引擎 — 生成 FFmpeg 调速滤镜链。
+
+    用法：
+        engine = SpeedEngine()
+        video_filter = engine.build_video_filter(config)
+        audio_filter = engine.build_audio_filter(config)
+        new_duration = engine.adjust_duration(duration, config)
+    """
+
+    def build_video_filter(self, config: SpeedConfig) -> str:
+        """生成视频调速滤镜字符串。
+
+        返回 setpts 滤镜表达式，原速时返回空字符串。
+        """
+        if config.is_original:
+            return ""
+        # setpts=PTS/speed — speed>1 加速，speed<1 减速
+        return f"setpts=PTS/{config.speed:.4f}"
+
+    def build_audio_filter(self, config: SpeedConfig) -> str:
+        """生成音频调速滤镜字符串。
+
+        atempo 单级范围 0.5~2.0，超出范围时自动多级串联：
+        - 0.25x → atempo=0.5,atempo=0.5
+        - 4x → atempo=2.0,atempo=2.0
+        - 0.3x → atempo=0.5,atempo=0.6
+        - 3x → atempo=2.0,atempo=1.5
+
+        原速时返回空字符串。
+        """
+        if config.is_original:
+            return ""
+
+        speed = config.speed
+        stages: list[float] = self._split_atempo_stages(speed)
+        return ",".join(f"atempo={s:.4f}" for s in stages)
+
+    @staticmethod
+    def _split_atempo_stages(speed: float) -> list[float]:
+        """将速度拆分为多级 atempo 串联，每级都在 [0.5, 2.0] 范围内。"""
+        if _ATEMPO_MIN <= speed <= _ATEMPO_MAX:
+            return [speed]
+
+        stages: list[float] = []
+        remaining = speed
+
+        # 加速场景（speed > 2.0）
+        if speed > _ATEMPO_MAX:
+            while remaining > _ATEMPO_MAX:
+                stages.append(_ATEMPO_MAX)
+                remaining /= _ATEMPO_MAX
+            stages.append(remaining)
+
+        # 减速场景（speed < 0.5）
+        else:
+            while remaining < _ATEMPO_MIN:
+                stages.append(_ATEMPO_MIN)
+                remaining /= _ATEMPO_MIN
+            stages.append(remaining)
+
+        return stages
+
+    def adjust_duration(self, original_duration: float, config: SpeedConfig) -> float:
+        """计算调速后的时长。
+
+        加速 → 时长变短；减速 → 时长变长。
+        """
+        if config.is_original or original_duration <= 0:
+            return original_duration
+        return original_duration / config.speed
+
+    def build_clip_speed_filter(
+        self,
+        speed: float,
+        pitch_correct: bool = True,
+    ) -> tuple[str, str, SpeedConfig]:
+        """便捷方法：从单一 speed 值生成视频+音频滤镜。
+
+        返回 (video_filter, audio_filter, config)。
+        """
+        config = SpeedConfig(speed=speed, pitch_correct=pitch_correct)
+        config.clamp()
+        return (
+            self.build_video_filter(config),
+            self.build_audio_filter(config),
+            config,
+        )
+
+    @staticmethod
+    def resolve_clip_speed(
+        clip_config: dict,
+        global_speed: float = DEFAULT_SPEED,
+    ) -> float:
+        """从 clip config 中解析 playback_speed，0 或缺失则使用全局速度。"""
+        speed = clip_config.get("playback_speed", 0) if clip_config else 0
+        if not isinstance(speed, (int, float)) or speed <= 0:
+            return global_speed
+        return float(speed)