diff --git a/apps/worker/video_processing/sticker_engine_pure.py b/apps/worker/video_processing/sticker_engine_pure.py
new file mode 100755
index 000000000..1fc70487d
--- /dev/null
+++ b/apps/worker/video_processing/sticker_engine_pure.py
@@ -0,0 +1,534 @@
+"""贴纸引擎纯逻辑模块.
+
+所有函数均为纯函数，不调用 FFmpeg、不操作文件。
+便于单元测试，也方便被其他模块复用。
+"""
+
+from __future__ import annotations
+
+from typing import Any, Optional
+
+# ── 安全类型转换 ──────────────────────────────────────────────────────────────
+
+
+def safe_float(val: Any) -> Optional[float]:
+    """安全转换为 float.
+
+    Args:
+        val: 任意值
+
+    Returns:
+        float 值，失败返回 None
+    """
+    if val is None:
+        return None
+    try:
+        result = float(val)
+        if result != result:  # NaN check
+            return None
+        return result
+    except (ValueError, TypeError):
+        return None
+
+
+def safe_int(val: Any, default: int = 0) -> int:
+    """安全转换为 int.
+
+    Args:
+        val: 任意值
+        default: 默认值
+
+    Returns:
+        int 值，失败返回默认值
+    """
+    if val is None:
+        return default
+    try:
+        result = int(float(val))
+        return result
+    except (ValueError, TypeError):
+        return default
+
+
+def safe_bool(val: Any) -> bool:
+    """安全转换为 bool.
+
+    Args:
+        val: 任意值
+
+    Returns:
+        bool 值
+    """
+    if isinstance(val, bool):
+        return val
+    if val is None:
+        return False
+    if isinstance(val, str):
+        return val.lower() in ("true", "1", "yes", "on")
+    return bool(val)
+
+
+# ── 尺寸估算 ──────────────────────────────────────────────────────────────────
+
+
+def estimate_sticker_size(
+    canvas_w: int,
+    canvas_h: int,
+    scale: float = 1.0,
+    fixed_width: Optional[int] = None,
+    fixed_height: Optional[int] = None,
+) -> tuple[int, int]:
+    """估算贴纸尺寸.
+
+    如果指定了固定宽高，直接使用；否则按画布的 30% * scale 估算。
+
+    Args:
+        canvas_w: 画布宽度
+        canvas_h: 画布高度
+        scale: 缩放比例
+        fixed_width: 固定宽度（可选）
+        fixed_height: 固定高度（可选）
+
+    Returns:
+        (估算宽度, 估算高度)
+    """
+    if fixed_width and fixed_height:
+        return (fixed_width, fixed_height)
+
+    base_ratio = 0.3
+    est_w = int(canvas_w * base_ratio * scale) if not fixed_width else fixed_width
+    est_h = int(canvas_h * base_ratio * scale) if not fixed_height else fixed_height
+
+    return (max(1, est_w), max(1, est_h))
+
+
+def estimate_text_size(
+    text: str,
+    font_size: int,
+) -> tuple[int, int]:
+    """估算文字贴纸尺寸.
+
+    粗略估算：宽度 = 字数 * 字号 * 0.6，高度 = 字号 * 1.4
+
+    Args:
+        text: 文字内容
+        font_size: 字号
+
+    Returns:
+        (估算宽度, 估算高度)
+    """
+    if not text:
+        return (0, 0)
+    est_w = int(len(text) * font_size * 0.6)
+    est_h = int(font_size * 1.4)
+    return (max(1, est_w), max(1, est_h))
+
+
+# ── 时间计算 ──────────────────────────────────────────────────────────────────
+
+
+def calculate_fade_out_start(
+    start_time: float,
+    duration: float,
+    fade_out: float,
+) -> float:
+    """计算淡出开始时间.
+
+    Args:
+        start_time: 开始时间（秒）
+        duration: 持续时长（秒）
+        fade_out: 淡出时长（秒）
+
+    Returns:
+        淡出开始时间（秒），最小为 0
+    """
+    if fade_out <= 0 or duration <= 0:
+        return 0.0
+    fade_start = start_time + duration - fade_out
+    return max(0.0, fade_start)
+
+
+def calculate_end_time(start_time: float, duration: float) -> float:
+    """计算结束时间.
+
+    Args:
+        start_time: 开始时间（秒）
+        duration: 持续时长（秒）
+
+    Returns:
+        结束时间（秒）
+    """
+    if duration <= 0:
+        return start_time
+    return start_time + duration
+
+
+def has_time_range(duration: float) -> bool:
+    """是否有时间范围限制.
+
+    Args:
+        duration: 持续时长（秒）
+
+    Returns:
+        duration > 0 时返回 True
+    """
+    return duration > 0
+
+
+# ── 滤镜组件构建 ──────────────────────────────────────────────────────────────
+
+
+def build_scale_filter(
+    width: Optional[int] = None,
+    height: Optional[int] = None,
+    scale: float = 1.0,
+) -> Optional[str]:
+    """构建缩放滤镜.
+
+    优先使用固定宽高，否则按比例缩放。
+    scale=1.0 且无固定尺寸时返回 None。
+
+    Args:
+        width: 固定宽度（可选）
+        height: 固定高度（可选）
+        scale: 缩放比例
+
+    Returns:
+        scale 滤镜字符串，不需要缩放时返回 None
+    """
+    if width and height:
+        return f"scale={width}:{height}"
+    if scale != 1.0:
+        return f"scale=iw*{scale}:ih*{scale}"
+    return None
+
+
+def build_opacity_filter(opacity: float) -> Optional[str]:
+    """构建透明度滤镜.
+
+    Args:
+        opacity: 不透明度 0.0~1.0
+
+    Returns:
+        colorchannelmixer 滤镜字符串，完全不透明时返回 None
+    """
+    if opacity >= 1.0:
+        return None
+    safe_opacity = max(0.0, min(1.0, opacity))
+    return f"colorchannelmixer=aa={safe_opacity}"
+
+
+def build_image_fade_filters(
+    start_time: float,
+    duration: float,
+    fade_in: float = 0.0,
+    fade_out: float = 0.0,
+) -> list[str]:
+    """构建图片贴纸淡入淡出滤镜列表.
+
+    使用 FFmpeg fade 滤镜（alpha 通道）。
+
+    Args:
+        start_time: 开始时间（秒）
+        duration: 持续时长（秒）
+        fade_in: 淡入时长（秒）
+        fade_out: 淡出时长（秒）
+
+    Returns:
+        fade 滤镜字符串列表
+    """
+    filters: list[str] = []
+
+    if fade_in > 0:
+        filters.append(f"fade=in:st={start_time}:d={fade_in}:alpha=1")
+
+    if fade_out > 0 and duration > 0:
+        fade_out_start = calculate_fade_out_start(start_time, duration, fade_out)
+        filters.append(f"fade=out:st={fade_out_start}:d={fade_out}:alpha=1")
+
+    return filters
+
+
+def build_enable_expr(
+    start_time: float,
+    duration: float,
+) -> str:
+    """构建 enable 表达式（时间范围）.
+
+    Args:
+        start_time: 开始时间（秒）
+        duration: 持续时长（秒）
+
+    Returns:
+        enable 表达式字符串（包含开头的冒号），无时间限制时返回空字符串
+    """
+    if duration <= 0:
+        return ""
+    end_time = calculate_end_time(start_time, duration)
+    return f":enable='between(t,{start_time},{end_time})'"
+
+
+# ── drawtext 文字贴纸相关 ─────────────────────────────────────────────────────
+
+
+def escape_drawtext_text(text: str) -> str:
+    """转义 drawtext 中的特殊字符.
+
+    转义冒号和单引号。
+
+    Args:
+        text: 原始文字
+
+    Returns:
+        转义后的文字
+    """
+    result = text.replace(":", "\\:")
+    result = result.replace("'", "\\'")
+    return result
+
+
+def build_drawtext_alpha_expr(
+    start_time: float,
+    duration: float,
+    fade_in: float = 0.0,
+    fade_out: float = 0.0,
+) -> str:
+    """构建 drawtext 的 alpha 淡入淡出表达式.
+
+    drawtext 没有直接的 fade 滤镜，用 alpha 表达式模拟。
+
+    Args:
+        start_time: 开始时间（秒）
+        duration: 持续时长（秒）
+        fade_in: 淡入时长（秒）
+        fade_out: 淡出时长（秒）
+
+    Returns:
+        alpha 表达式字符串，无淡入淡出时返回 "1"
+    """
+    parts: list[str] = []
+
+    if fade_in > 0:
+        fade_in_end = start_time + fade_in
+        parts.append(f"if(lt(t,{fade_in_end}),(t-{start_time})/{fade_in},1)")
+
+    if fade_out > 0 and duration > 0:
+        fade_out_start = calculate_fade_out_start(start_time, duration, fade_out)
+        end_time = calculate_end_time(start_time, duration)
+        parts.append(f"if(gt(t,{fade_out_start}),({end_time}-t)/{fade_out},1)")
+
+    if not parts:
+        return "1"
+
+    return "*".join(parts)
+
+
+def build_stroke_params(
+    stroke_width: int = 0,
+    stroke_color: str = "black",
+) -> list[str]:
+    """构建 drawtext 描边参数.
+
+    Args:
+        stroke_width: 描边宽度（0 表示无描边）
+        stroke_color: 描边颜色
+
+    Returns:
+        描边参数列表
+    """
+    if stroke_width <= 0:
+        return []
+    return [
+        f"borderw={stroke_width}",
+        f"bordercolor={stroke_color}",
+    ]
+
+
+def build_shadow_params(
+    shadow_alpha: float = 0.0,
+    shadow_x: int = 2,
+    shadow_y: int = 2,
+    shadow_color: str = "black",
+) -> list[str]:
+    """构建 drawtext 阴影参数.
+
+    Args:
+        shadow_alpha: 阴影透明度（0 表示无阴影）
+        shadow_x: 阴影 X 偏移
+        shadow_y: 阴影 Y 偏移
+        shadow_color: 阴影颜色
+
+    Returns:
+        阴影参数列表
+    """
+    if shadow_alpha <= 0:
+        return []
+    safe_alpha = max(0.0, min(1.0, shadow_alpha))
+    return [
+        f"shadowx={shadow_x}",
+        f"shadowy={shadow_y}",
+        f"shadowcolor={shadow_color}@{safe_alpha}",
+    ]
+
+
+# ── 贴纸排序与过滤 ────────────────────────────────────────────────────────────
+
+
+def sort_stickers_by_z_index(
+    stickers: list[dict[str, Any]],
+) -> list[dict[str, Any]]:
+    """按 z_index 排序贴纸.
+
+    z_index 小的在底层，大的在上层。
+    相同 z_index 保持原顺序（稳定排序）。
+
+    Args:
+        stickers: 贴纸配置列表
+
+    Returns:
+        排序后的贴纸列表
+    """
+    return sorted(stickers, key=lambda s: safe_int(s.get("z_index"), 10))
+
+
+def filter_enabled_stickers(
+    stickers: list[dict[str, Any]],
+) -> list[dict[str, Any]]:
+    """过滤出启用的贴纸.
+
+    Args:
+        stickers: 贴纸配置列表
+
+    Returns:
+        启用的贴纸列表
+    """
+    result = []
+    for s in stickers:
+        enabled = s.get("enabled", True)
+        if safe_bool(enabled):
+            result.append(s)
+    return result
+
+
+def count_sticker_types(
+    stickers: list[dict[str, Any]],
+) -> dict[str, int]:
+    """统计各类型贴纸数量.
+
+    Args:
+        stickers: 贴纸配置列表
+
+    Returns:
+        类型计数字典
+    """
+    counts: dict[str, int] = {}
+    for s in stickers:
+        stype = s.get("type", "image")
+        counts[stype] = counts.get(stype, 0) + 1
+    return counts
+
+
+# ── overlay 滤镜构建 ──────────────────────────────────────────────────────────
+
+
+def build_overlay_position(
+    pos_x: float,
+    pos_y: float,
+) -> str:
+    """构建 overlay 位置参数.
+
+    Args:
+        pos_x: X 坐标
+        pos_y: Y 坐标
+
+    Returns:
+        overlay 位置字符串 "x:y"
+    """
+    return f"{pos_x:.0f}:{pos_y:.0f}"
+
+
+def build_pre_filter_label(idx: int) -> str:
+    """构建贴纸预处理后的标签名.
+
+    Args:
+        idx: 贴纸索引
+
+    Returns:
+        滤镜标签字符串
+    """
+    return f"sticker_{idx}_scaled"
+
+
+# ── 验证函数 ──────────────────────────────────────────────────────────────────
+
+
+def validate_image_sticker(sticker: dict[str, Any]) -> tuple[bool, list[str]]:
+    """验证图片贴纸配置.
+
+    Args:
+        sticker: 贴纸配置字典
+
+    Returns:
+        (是否合法, 错误信息列表)
+    """
+    errors: list[str] = []
+
+    # 图片路径
+    image_path = sticker.get("image_path", "")
+    if not image_path and not sticker.get("asset_id"):
+        errors.append("图片贴纸需要 image_path 或 asset_id")
+
+    # 透明度范围
+    opacity = safe_float(sticker.get("opacity", 1.0))
+    if opacity is not None and (opacity < 0 or opacity > 1):
+        errors.append("opacity 必须在 0~1 之间")
+
+    # 缩放比例
+    scale = safe_float(sticker.get("scale", 1.0))
+    if scale is not None and scale <= 0:
+        errors.append("scale 必须大于 0")
+
+    # 时间参数
+    duration = safe_float(sticker.get("duration", 0))
+    if duration is not None and duration < 0:
+        errors.append("duration 不能为负数")
+
+    start_time = safe_float(sticker.get("start_time", 0))
+    if start_time is not None and start_time < 0:
+        errors.append("start_time 不能为负数")
+
+    return (len(errors) == 0, errors)
+
+
+def validate_text_sticker(sticker: dict[str, Any]) -> tuple[bool, list[str]]:
+    """验证文字贴纸配置.
+
+    Args:
+        sticker: 贴纸配置字典
+
+    Returns:
+        (是否合法, 错误信息列表)
+    """
+    errors: list[str] = []
+
+    # 文字内容
+    text = sticker.get("text", "")
+    if not text:
+        errors.append("文字贴纸需要 text 内容")
+
+    # 字号
+    font_size = safe_int(sticker.get("font_size", 36))
+    if font_size <= 0:
+        errors.append("font_size 必须大于 0")
+
+    # 颜色
+    font_color = sticker.get("font_color", "white")
+    if not font_color:
+        errors.append("font_color 不能为空")
+
+    # 时间参数
+    duration = safe_float(sticker.get("duration", 0))
+    if duration is not None and duration < 0:
+        errors.append("duration 不能为负数")
+
+    return (len(errors) == 0, errors)
diff --git a/tests/unit/test_sticker_engine_pure.py b/tests/unit/test_sticker_engine_pure.py
new file mode 100755
index 000000000..f945f5192
--- /dev/null
+++ b/tests/unit/test_sticker_engine_pure.py
@@ -0,0 +1,780 @@
+"""贴纸引擎纯逻辑单元测试."""
+
+from __future__ import annotations
+
+import pytest
+from video_processing.sticker_engine_pure import (
+    build_drawtext_alpha_expr,
+    build_enable_expr,
+    build_image_fade_filters,
+    build_opacity_filter,
+    build_overlay_position,
+    build_pre_filter_label,
+    build_scale_filter,
+    build_shadow_params,
+    build_stroke_params,
+    calculate_end_time,
+    calculate_fade_out_start,
+    count_sticker_types,
+    escape_drawtext_text,
+    estimate_sticker_size,
+    estimate_text_size,
+    filter_enabled_stickers,
+    has_time_range,
+    safe_bool,
+    safe_float,
+    safe_int,
+    sort_stickers_by_z_index,
+    validate_image_sticker,
+    validate_text_sticker,
+)
+
+# ─────────────────────────────────────────────────────────────────────────────
+# 安全类型转换测试
+# ─────────────────────────────────────────────────────────────────────────────
+
+
+class TestSafeFloat:
+    """safe_float 测试."""
+
+    def test_int_input(self):
+        """整数输入."""
+        assert safe_float(42) == 42.0
+
+    def test_float_input(self):
+        """浮点数输入."""
+        assert safe_float(3.14) == 3.14
+
+    def test_string_number(self):
+        """字符串数字."""
+        assert safe_float("3.14") == 3.14
+
+    def test_string_int(self):
+        """字符串整数."""
+        assert safe_float("100") == 100.0
+
+    def test_none_input(self):
+        """None 输入."""
+        assert safe_float(None) is None
+
+    def test_invalid_string(self):
+        """无效字符串."""
+        assert safe_float("abc") is None
+
+    def test_empty_string(self):
+        """空字符串."""
+        assert safe_float("") is None
+
+    def test_zero(self):
+        """零值."""
+        assert safe_float(0) == 0.0
+
+    def test_negative(self):
+        """负值."""
+        assert safe_float(-5.5) == -5.5
+
+
+class TestSafeInt:
+    """safe_int 测试."""
+
+    def test_int_input(self):
+        """整数输入."""
+        assert safe_int(42) == 42
+
+    def test_float_input(self):
+        """浮点数输入（截断）."""
+        assert safe_int(3.7) == 3
+
+    def test_string_number(self):
+        """字符串数字."""
+        assert safe_int("42") == 42
+
+    def test_none_input(self):
+        """None 输入用默认值."""
+        assert safe_int(None) == 0
+
+    def test_none_custom_default(self):
+        """None 输入自定义默认值."""
+        assert safe_int(None, default=10) == 10
+
+    def test_invalid_string(self):
+        """无效字符串."""
+        assert safe_int("abc") == 0
+
+    def test_negative(self):
+        """负值."""
+        assert safe_int(-5) == -5
+
+    def test_zero(self):
+        """零值."""
+        assert safe_int(0) == 0
+
+
+class TestSafeBool:
+    """safe_bool 测试."""
+
+    def test_true_bool(self):
+        """True."""
+        assert safe_bool(True) is True
+
+    def test_false_bool(self):
+        """False."""
+        assert safe_bool(False) is False
+
+    def test_none(self):
+        """None -> False."""
+        assert safe_bool(None) is False
+
+    def test_string_true(self):
+        """字符串 true."""
+        assert safe_bool("true") is True
+
+    def test_string_yes(self):
+        """字符串 yes."""
+        assert safe_bool("yes") is True
+
+    def test_string_one(self):
+        """字符串 1."""
+        assert safe_bool("1") is True
+
+    def test_string_false(self):
+        """字符串 false."""
+        assert safe_bool("false") is False
+
+    def test_int_one(self):
+        """整数 1 -> True."""
+        assert safe_bool(1) is True
+
+    def test_int_zero(self):
+        """整数 0 -> False."""
+        assert safe_bool(0) is False
+
+    def test_empty_list(self):
+        """空列表 -> False."""
+        assert safe_bool([]) is False
+
+
+# ─────────────────────────────────────────────────────────────────────────────
+# 尺寸估算测试
+# ─────────────────────────────────────────────────────────────────────────────
+
+
+class TestEstimateStickerSize:
+    """贴纸尺寸估算测试."""
+
+    def test_default_scale(self):
+        """默认 scale=1.0."""
+        w, h = estimate_sticker_size(1000, 1000)
+        assert w == 300  # 1000 * 0.3 * 1.0
+        assert h == 300
+
+    def test_custom_scale(self):
+        """自定义缩放."""
+        w, h = estimate_sticker_size(1000, 1000, scale=0.5)
+        assert w == 150
+        assert h == 150
+
+    def test_fixed_width_height(self):
+        """固定宽高."""
+        w, h = estimate_sticker_size(1000, 1000, fixed_width=200, fixed_height=100)
+        assert w == 200
+        assert h == 100
+
+    def test_scale_2x(self):
+        """2倍缩放."""
+        w, h = estimate_sticker_size(800, 600, scale=2.0)
+        assert w == 480  # 800 * 0.3 * 2
+        assert h == 360  # 600 * 0.3 * 2
+
+    def test_zero_canvas(self):
+        """零画布尺寸，返回最小 1."""
+        w, h = estimate_sticker_size(0, 0)
+        assert w >= 1
+        assert h >= 1
+
+
+class TestEstimateTextSize:
+    """文字尺寸估算测试."""
+
+    def test_normal_text(self):
+        """普通文字."""
+        w, h = estimate_text_size("Hello", 36)
+        assert w == int(5 * 36 * 0.6)
+        assert h == int(36 * 1.4)
+
+    def test_empty_text(self):
+        """空文字."""
+        w, h = estimate_text_size("", 36)
+        assert w == 0
+        assert h == 0
+
+    def test_large_font(self):
+        """大字号."""
+        w, h = estimate_text_size("A", 72)
+        assert w == int(1 * 72 * 0.6)
+        assert h == int(72 * 1.4)
+
+    def test_chinese_chars(self):
+        """中文字符."""
+        w, h = estimate_text_size("你好世界", 48)
+        assert w == int(4 * 48 * 0.6)
+        assert h == int(48 * 1.4)
+
+
+# ─────────────────────────────────────────────────────────────────────────────
+# 时间计算测试
+# ─────────────────────────────────────────────────────────────────────────────
+
+
+class TestCalculateFadeOutStart:
+    """淡出开始时间计算测试."""
+
+    def test_normal_case(self):
+        """正常情况."""
+        assert calculate_fade_out_start(10, 30, 2) == pytest.approx(38.0)
+
+    def test_no_fade_out(self):
+        """无淡出."""
+        assert calculate_fade_out_start(10, 30, 0) == 0.0
+
+    def test_negative_fade_out(self):
+        """负淡出."""
+        assert calculate_fade_out_start(10, 30, -1) == 0.0
+
+    def test_zero_duration(self):
+        """零时长."""
+        assert calculate_fade_out_start(10, 0, 2) == 0.0
+
+    def test_fade_out_longer_than_duration(self):
+        """淡出超过时长，返回 0."""
+        # start=10, dur=5, fade=10 -> 10+5-10 = 5 > 0
+        assert calculate_fade_out_start(10, 5, 10) == pytest.approx(5.0)
+
+    def test_fade_out_starts_before_zero(self):
+        """淡出开始时间在 0 之前，钳制到 0."""
+        # start=0, dur=3, fade=5 -> 0+3-5 = -2 -> 0
+        assert calculate_fade_out_start(0, 3, 5) == 0.0
+
+
+class TestCalculateEndTime:
+    """结束时间计算测试."""
+
+    def test_normal_case(self):
+        """正常情况."""
+        assert calculate_end_time(10, 30) == 40.0
+
+    def test_zero_duration(self):
+        """零时长."""
+        assert calculate_end_time(10, 0) == 10.0
+
+    def test_negative_duration(self):
+        """负时长."""
+        assert calculate_end_time(10, -5) == 10.0
+
+    def test_zero_start(self):
+        """零开始."""
+        assert calculate_end_time(0, 100) == 100.0
+
+
+class TestHasTimeRange:
+    """时间范围判断测试."""
+
+    def test_positive_duration(self):
+        """正时长."""
+        assert has_time_range(30) is True
+
+    def test_zero_duration(self):
+        """零时长."""
+        assert has_time_range(0) is False
+
+    def test_negative_duration(self):
+        """负时长."""
+        assert has_time_range(-5) is False
+
+
+# ─────────────────────────────────────────────────────────────────────────────
+# 滤镜构建测试
+# ─────────────────────────────────────────────────────────────────────────────
+
+
+class TestBuildScaleFilter:
+    """缩放滤镜构建测试."""
+
+    def test_fixed_width_height(self):
+        """固定宽高."""
+        result = build_scale_filter(width=200, height=100)
+        assert result == "scale=200:100"
+
+    def test_scale_only(self):
+        """仅缩放."""
+        result = build_scale_filter(scale=0.5)
+        assert result == "scale=iw*0.5:ih*0.5"
+
+    def test_no_scaling_needed(self):
+        """无需缩放."""
+        result = build_scale_filter(scale=1.0)
+        assert result is None
+
+    def test_scale_2x(self):
+        """2倍缩放."""
+        result = build_scale_filter(scale=2.0)
+        assert result == "scale=iw*2.0:ih*2.0"
+
+    def test_fixed_overrides_scale(self):
+        """固定宽高优先于 scale."""
+        result = build_scale_filter(width=100, height=50, scale=0.5)
+        assert result == "scale=100:50"
+
+
+class TestBuildOpacityFilter:
+    """透明度滤镜构建测试."""
+
+    def test_partial_opacity(self):
+        """部分透明."""
+        result = build_opacity_filter(0.5)
+        assert result == "colorchannelmixer=aa=0.5"
+
+    def test_fully_opaque(self):
+        """完全不透明."""
+        result = build_opacity_filter(1.0)
+        assert result is None
+
+    def test_fully_transparent(self):
+        """完全透明."""
+        result = build_opacity_filter(0.0)
+        assert result == "colorchannelmixer=aa=0.0"
+
+    def test_opacity_above_1_clamped(self):
+        """超过 1 被钳制."""
+        result = build_opacity_filter(1.5)
+        assert result is None
+
+    def test_opacity_below_0_clamped(self):
+        """低于 0 被钳制."""
+        result = build_opacity_filter(-0.5)
+        assert result == "colorchannelmixer=aa=0.0"
+
+
+class TestBuildImageFadeFilters:
+    """图片淡入淡出滤镜测试."""
+
+    def test_fade_in_only(self):
+        """仅淡入."""
+        result = build_image_fade_filters(10, 30, fade_in=1.0)
+        assert len(result) == 1
+        assert "fade=in:st=10:d=1.0:alpha=1" in result[0]
+
+    def test_fade_out_only(self):
+        """仅淡出."""
+        result = build_image_fade_filters(10, 30, fade_out=2.0)
+        assert len(result) == 1
+        assert "fade=out" in result[0]
+        assert "st=38.0" in result[0]  # 10 + 30 - 2 = 38
+
+    def test_fade_in_and_out(self):
+        """淡入+淡出."""
+        result = build_image_fade_filters(0, 10, fade_in=1.0, fade_out=1.0)
+        assert len(result) == 2
+        assert "fade=in" in result[0]
+        assert "fade=out" in result[1]
+
+    def test_no_fade(self):
+        """无淡入淡出."""
+        result = build_image_fade_filters(10, 30)
+        assert len(result) == 0
+
+    def test_zero_duration_no_fade_out(self):
+        """零时长不生成淡出."""
+        result = build_image_fade_filters(10, 0, fade_out=1.0)
+        assert len(result) == 0
+
+
+class TestBuildEnableExpr:
+    """enable 表达式构建测试."""
+
+    def test_normal_duration(self):
+        """正常时长."""
+        result = build_enable_expr(10, 30)
+        assert "between(t,10,40" in result
+        assert "enable" in result
+
+    def test_zero_duration(self):
+        """零时长返回空."""
+        result = build_enable_expr(10, 0)
+        assert result == ""
+
+    def test_negative_duration(self):
+        """负时长返回空."""
+        result = build_enable_expr(10, -5)
+        assert result == ""
+
+    def test_zero_start(self):
+        """从零开始."""
+        result = build_enable_expr(0, 100)
+        assert "t,0,100" in result
+
+
+# ─────────────────────────────────────────────────────────────────────────────
+# drawtext 相关测试
+# ─────────────────────────────────────────────────────────────────────────────
+
+
+class TestEscapeDrawtextText:
+    """文字转义测试."""
+
+    def test_no_special_chars(self):
+        """无特殊字符."""
+        assert escape_drawtext_text("Hello") == "Hello"
+
+    def test_colon_escaped(self):
+        """冒号转义."""
+        assert escape_drawtext_text("a:b") == "a\\:b"
+
+    def test_quote_escaped(self):
+        """单引号转义."""
+        assert escape_drawtext_text("it's") == "it\\'s"
+
+    def test_multiple_special_chars(self):
+        """多个特殊字符."""
+        assert escape_drawtext_text("a:b:c'd") == "a\\:b\\:c\\'d"
+
+    def test_empty_string(self):
+        """空字符串."""
+        assert escape_drawtext_text("") == ""
+
+
+class TestBuildDrawtextAlphaExpr:
+    """drawtext alpha 表达式测试."""
+
+    def test_no_fade(self):
+        """无淡入淡出."""
+        assert build_drawtext_alpha_expr(10, 30) == "1"
+
+    def test_fade_in_only(self):
+        """仅淡入."""
+        result = build_drawtext_alpha_expr(10, 30, fade_in=2.0)
+        assert "if(lt(t,12.0)" in result
+        assert "(t-10)/2.0" in result
+
+    def test_fade_out_only(self):
+        """仅淡出."""
+        result = build_drawtext_alpha_expr(10, 30, fade_out=3.0)
+        assert "if(gt(t,37" in result
+        assert "-t)/3.0" in result
+
+    def test_fade_in_and_out(self):
+        """淡入+淡出（相乘）."""
+        result = build_drawtext_alpha_expr(0, 10, fade_in=1.0, fade_out=1.0)
+        assert "*" in result
+        assert result.count("if(") == 2
+
+    def test_zero_duration_no_fade_out(self):
+        """零时长不生成淡出."""
+        result = build_drawtext_alpha_expr(10, 0, fade_out=1.0)
+        assert result == "1"
+
+
+class TestBuildStrokeParams:
+    """描边参数测试."""
+
+    def test_no_stroke(self):
+        """无描边."""
+        result = build_stroke_params(0)
+        assert len(result) == 0
+
+    def test_with_stroke(self):
+        """有描边."""
+        result = build_stroke_params(2, "red")
+        assert len(result) == 2
+        assert "borderw=2" in result
+        assert "bordercolor=red" in result
+
+    def test_negative_width(self):
+        """负宽度."""
+        result = build_stroke_params(-1)
+        assert len(result) == 0
+
+
+class TestBuildShadowParams:
+    """阴影参数测试."""
+
+    def test_no_shadow(self):
+        """无阴影."""
+        result = build_shadow_params(0)
+        assert len(result) == 0
+
+    def test_with_shadow(self):
+        """有阴影."""
+        result = build_shadow_params(0.5, 3, 4, "black")
+        assert len(result) == 3
+        assert "shadowx=3" in result
+        assert "shadowy=4" in result
+        assert "shadowcolor=black@0.5" in result
+
+    def test_shadow_alpha_clamped(self):
+        """透明度钳制."""
+        result = build_shadow_params(1.5)
+        assert "shadowcolor=black@1.0" in result[2]
+
+
+# ─────────────────────────────────────────────────────────────────────────────
+# 贴纸排序与过滤测试
+# ─────────────────────────────────────────────────────────────────────────────
+
+
+class TestSortStickersByZIndex:
+    """贴纸排序测试."""
+
+    def test_sorted_by_z_index(self):
+        """按 z_index 排序."""
+        stickers = [
+            {"z_index": 20, "name": "top"},
+            {"z_index": 5, "name": "bottom"},
+            {"z_index": 10, "name": "middle"},
+        ]
+        result = sort_stickers_by_z_index(stickers)
+        assert result[0]["name"] == "bottom"
+        assert result[1]["name"] == "middle"
+        assert result[2]["name"] == "top"
+
+    def test_same_z_index_preserves_order(self):
+        """相同 z_index 保持原顺序."""
+        stickers = [
+            {"z_index": 10, "name": "first"},
+            {"z_index": 10, "name": "second"},
+        ]
+        result = sort_stickers_by_z_index(stickers)
+        assert result[0]["name"] == "first"
+        assert result[1]["name"] == "second"
+
+    def test_empty_list(self):
+        """空列表."""
+        assert sort_stickers_by_z_index([]) == []
+
+    def test_default_z_index_10(self):
+        """无 z_index 默认 10."""
+        stickers = [
+            {"z_index": 5, "name": "low"},
+            {"name": "default"},
+        ]
+        result = sort_stickers_by_z_index(stickers)
+        assert result[0]["name"] == "low"
+        assert result[1]["name"] == "default"
+
+
+class TestFilterEnabledStickers:
+    """启用贴纸过滤测试."""
+
+    def test_all_enabled(self):
+        """全部启用."""
+        stickers = [{"enabled": True}, {"enabled": True}]
+        assert len(filter_enabled_stickers(stickers)) == 2
+
+    def test_mixed(self):
+        """混合."""
+        stickers = [
+            {"enabled": True, "name": "a"},
+            {"enabled": False, "name": "b"},
+            {"enabled": True, "name": "c"},
+        ]
+        result = filter_enabled_stickers(stickers)
+        assert len(result) == 2
+        assert result[0]["name"] == "a"
+
+    def test_default_enabled(self):
+        """默认启用."""
+        stickers = [{"name": "a"}]
+        result = filter_enabled_stickers(stickers)
+        assert len(result) == 1
+
+    def test_empty_list(self):
+        """空列表."""
+        assert filter_enabled_stickers([]) == []
+
+
+class TestCountStickerTypes:
+    """贴纸类型统计测试."""
+
+    def test_mixed_types(self):
+        """混合类型."""
+        stickers = [
+            {"type": "image"},
+            {"type": "text"},
+            {"type": "image"},
+        ]
+        counts = count_sticker_types(stickers)
+        assert counts["image"] == 2
+        assert counts["text"] == 1
+
+    def test_default_type(self):
+        """默认 image."""
+        stickers = [{}]
+        counts = count_sticker_types(stickers)
+        assert counts["image"] == 1
+
+    def test_empty_list(self):
+        """空列表."""
+        assert count_sticker_types([]) == {}
+
+
+# ─────────────────────────────────────────────────────────────────────────────
+# overlay 相关测试
+# ─────────────────────────────────────────────────────────────────────────────
+
+
+class TestBuildOverlayPosition:
+    """overlay 位置构建测试."""
+
+    def test_integer_position(self):
+        """整数位置."""
+        assert build_overlay_position(100, 200) == "100:200"
+
+    def test_float_position_rounded(self):
+        """浮点取整."""
+        assert build_overlay_position(100.6, 200.4) == "101:200"
+
+    def test_zero_position(self):
+        """零位置."""
+        assert build_overlay_position(0, 0) == "0:0"
+
+    def test_negative_position(self):
+        """负位置."""
+        assert build_overlay_position(-10, -20) == "-10:-20"
+
+
+class TestBuildPreFilterLabel:
+    """预处理标签构建测试."""
+
+    def test_normal_idx(self):
+        """正常索引."""
+        assert build_pre_filter_label(3) == "sticker_3_scaled"
+
+    def test_zero_idx(self):
+        """零索引."""
+        assert build_pre_filter_label(0) == "sticker_0_scaled"
+
+
+# ─────────────────────────────────────────────────────────────────────────────
+# 验证函数测试
+# ─────────────────────────────────────────────────────────────────────────────
+
+
+class TestValidateImageSticker:
+    """图片贴纸验证测试."""
+
+    def test_valid_with_image_path(self):
+        """有 image_path，合法."""
+        ok, errors = validate_image_sticker({"image_path": "/a.png"})
+        assert ok is True
+        assert len(errors) == 0
+
+    def test_valid_with_asset_id(self):
+        """有 asset_id，合法."""
+        ok, errors = validate_image_sticker({"asset_id": "123"})
+        assert ok is True
+
+    def test_missing_image_source(self):
+        """缺图片来源."""
+        ok, errors = validate_image_sticker({})
+        assert ok is False
+        assert any("image_path" in e or "asset_id" in e for e in errors)
+
+    def test_opacity_out_of_range(self):
+        """透明度超范围."""
+        ok, errors = validate_image_sticker(
+            {
+                "image_path": "/a.png",
+                "opacity": 1.5,
+            }
+        )
+        assert ok is False
+        assert any("opacity" in e for e in errors)
+
+    def test_negative_scale(self):
+        """负缩放."""
+        ok, errors = validate_image_sticker(
+            {
+                "image_path": "/a.png",
+                "scale": -0.5,
+            }
+        )
+        assert ok is False
+        assert any("scale" in e for e in errors)
+
+    def test_negative_duration(self):
+        """负时长."""
+        ok, errors = validate_image_sticker(
+            {
+                "image_path": "/a.png",
+                "duration": -10,
+            }
+        )
+        assert ok is False
+        assert any("duration" in e for e in errors)
+
+    def test_multiple_errors(self):
+        """多个错误."""
+        ok, errors = validate_image_sticker(
+            {
+                "opacity": 1.5,
+                "duration": -1,
+                "start_time": -5,
+            }
+        )
+        assert ok is False
+        assert len(errors) >= 3
+
+
+class TestValidateTextSticker:
+    """文字贴纸验证测试."""
+
+    def test_valid(self):
+        """合法配置."""
+        ok, errors = validate_text_sticker(
+            {
+                "text": "Hello",
+                "font_size": 36,
+                "font_color": "white",
+            }
+        )
+        assert ok is True
+        assert len(errors) == 0
+
+    def test_empty_text(self):
+        """空文字."""
+        ok, errors = validate_text_sticker({"text": ""})
+        assert ok is False
+        assert any("text" in e for e in errors)
+
+    def test_zero_font_size(self):
+        """零字号."""
+        ok, errors = validate_text_sticker(
+            {
+                "text": "Hi",
+                "font_size": 0,
+            }
+        )
+        assert ok is False
+        assert any("font_size" in e for e in errors)
+
+    def test_empty_font_color(self):
+        """空颜色."""
+        ok, errors = validate_text_sticker(
+            {
+                "text": "Hi",
+                "font_color": "",
+            }
+        )
+        assert ok is False
+        assert any("font_color" in e for e in errors)
+
+    def test_negative_duration(self):
+        """负时长."""
+        ok, errors = validate_text_sticker(
+            {
+                "text": "Hi",
+                "duration": -5,
+            }
+        )
+        assert ok is False
+        assert any("duration" in e for e in errors)