feat(bilibili-analyzer): 支持字幕区域裁剪

- 新增 --crop-ratio 参数，提取底部1/N区域（默认1/4）
- 新增 --no-crop 参数，禁用裁剪提取完整帧
- 优化字幕识别效率，只处理字幕区域

skills/bilibili-analyzer/SKILL.md

@@ -118,7 +118,7 @@ python3 -m venv .venv
 ### 使用方法
 
 ```bash
-# 基本用法
+# 基本用法（默认裁剪底部1/4，用于提取字幕）
 dotnet run scripts/prepare.cs "<视频URL>" -o <输出目录>
 
 # 示例
@@ -126,6 +126,12 @@ dotnet run scripts/prepare.cs "https://www.bilibili.com/video/BV1xx411c7mD" -o .
 
 # 长视频（降低帧率）
 dotnet run scripts/prepare.cs "https://www.bilibili.com/video/BV1xx411c7mD" -o ./output --fps 0.5
+
+# 提取全帧（不裁剪）
+dotnet run scripts/prepare.cs "https://www.bilibili.com/video/BV1xx411c7mD" -o ./output --no-crop
+
+# 调整裁剪比例（底部1/2）
+dotnet run scripts/prepare.cs "https://www.bilibili.com/video/BV1xx411c7mD" -o ./output --crop-ratio 2
 ```
 
 ### 参数说明
@@ -137,6 +143,8 @@ dotnet run scripts/prepare.cs "https://www.bilibili.com/video/BV1xx411c7mD" -o .
 | `--fps` | 每秒提取帧数 | 1.0 |
 | `--similarity` | 相似度阈值（0-1），超过此值的相邻帧会被去重 | 0.80 |
 | `--no-dedup` | 禁用相似帧去重 | false |
+| `--crop-ratio` | 裁剪比例（提取底部1/N） | 4（底部1/4） |
+| `--no-crop` | 禁用裁剪，提取完整帧 | false |
 | `--video-only` | 只下载视频，不提取帧 | false |
 | `--frames-only` | 只提取帧（需已有video.mp4） | false |
 

skills/bilibili-analyzer/scripts/prepare.cs

@@ -27,6 +27,13 @@ if (!double.TryParse(fpsStr, NumberStyles.Float, CultureInfo.InvariantCulture, o
     Console.WriteLine($"[ERROR] Invalid fps value: {fpsStr}");
     Environment.Exit(1);
 }
+var cropRatioStr = GetArgValue(args, "--crop-ratio") ?? "4";
+if (!int.TryParse(cropRatioStr, out var cropRatio) || cropRatio < 2 || cropRatio > 10)
+{
+    Console.WriteLine($"[ERROR] Invalid crop-ratio value: {cropRatioStr} (must be 2-10)");
+    Environment.Exit(1);
+}
+var noCrop = args.Contains("--no-crop");
 var similarityStr = GetArgValue(args, "--similarity") ?? "0.80";
 if (!double.TryParse(similarityStr, NumberStyles.Float, CultureInfo.InvariantCulture, out var similarityThreshold))
 {
@@ -51,6 +58,7 @@ Console.WriteLine($"Output: {outputDir}");
 Console.WriteLine($"FPS: {fps}");
 Console.WriteLine($"Similarity Threshold: {similarityThreshold:P0}");
 Console.WriteLine($"Deduplication: {(noDedup ? "Disabled" : "Enabled")}");
+Console.WriteLine($"Crop: {(noCrop ? "Disabled" : $"Bottom 1/{cropRatio}")}");
 Console.WriteLine(new string('=', 50));
 
 // Download video
@@ -71,7 +79,7 @@ if (!videoOnly)
         Environment.Exit(1);
     }
 
-    if (!await ExtractFramesAsync(videoPath, imagesDir, fps))
+    if (!await ExtractFramesAsync(videoPath, imagesDir, fps, !noCrop, cropRatio))
     {
         Environment.Exit(1);
     }
@@ -237,9 +245,9 @@ HttpClient CreateHttpClient()
     return client;
 }
 
-async Task<bool> ExtractFramesAsync(string videoPath, string outputDir, double fps)
+async Task<bool> ExtractFramesAsync(string videoPath, string outputDir, double fps, bool useCrop, int cropRatio)
 {
-    Console.WriteLine($"[INFO] Extracting frames (fps={fps})");
+    Console.WriteLine($"[INFO] Extracting frames (fps={fps}, crop={(useCrop ? $"bottom 1/{cropRatio}" : "disabled")})");
 
     var ffmpeg = FindExecutable("ffmpeg", "ffmpeg.exe");
     if (ffmpeg == null)
@@ -258,10 +266,20 @@ async Task<bool> ExtractFramesAsync(string videoPath, string outputDir, double f
     {
         Console.WriteLine($"[INFO] Running ffmpeg: {ffmpeg}");
 
+        // Build filter string
+        var filter = $"fps={fps.ToString(CultureInfo.InvariantCulture)}";
+        if (useCrop)
+        {
+            // Crop bottom portion using FFmpeg's crop filter
+            // crop=w=iw:h=ih/{cropRatio}:x=0:y=ih-h/ih-{cropRatio}
+            // This crops the bottom 1/cropRatio of the frame
+            filter = $"fps={fps.ToString(CultureInfo.InvariantCulture)},crop=iw:ih/{cropRatio}:0:ih-ih/{cropRatio}";
+        }
+
         var psi = new ProcessStartInfo
         {
             FileName = ffmpeg,
-            Arguments = $"-i \"{videoPath}\" -vf \"fps={fps.ToString(CultureInfo.InvariantCulture)}\" -q:v 2 -y \"{outputPattern}\"",
+            Arguments = $"-i \"{videoPath}\" -vf \"{filter}\" -q:v 2 -y \"{outputPattern}\"",
             RedirectStandardOutput = true,
             RedirectStandardError = true,
             UseShellExecute = false,
@@ -576,6 +594,8 @@ Options:
   --fps <value>          Frames per second (default: 1.0)
   --similarity <value>   Similarity threshold for deduplication (default: 0.80)
   --no-dedup             Disable frame deduplication
+  --crop-ratio <value>  Crop to bottom 1/N of frame (default: 4, N=4 means bottom 1/4)
+  --no-crop              Disable cropping (extract full frame)
   --video-only           Only download video, skip frame extraction
   --frames-only          Only extract frames (requires existing video.mp4)
   -h, --help             Show this help
@@ -586,6 +606,14 @@ Examples:
   dotnet run prepare.cs ""https://www.bilibili.com/video/BV1xx411c7mD"" -o ./output
   dotnet run prepare.cs ""https://www.bilibili.com/video/BV1xx411c7mD"" --similarity 0.85
   dotnet run prepare.cs ""https://www.bilibili.com/video/BV1xx411c7mD"" --no-dedup
+  dotnet run prepare.cs ""https://www.bilibili.com/video/BV1xx411c7mD"" --crop-ratio 4
+  dotnet run prepare.cs ""https://www.bilibili.com/video/BV1xx411c7mD"" --no-crop
+
+Cropping (for subtitle extraction):
+  By default, only the bottom 1/4 of the frame is extracted (--crop-ratio 4)
+  This is useful for capturing subtitles without the main video content
+  Use --no-crop to extract the full frame
+  Use --crop-ratio 2 for bottom half, --crop-ratio 8 for bottom 1/8, etc.
 
 Deduplication:
   Consecutive frames with similarity >= threshold will be deduplicated.