Introduction

Bilibili provides Danmaku in an XML format that mpv cannot natively render. Furthermore, Bilibili’s AI subtitles often use non-standard language tags (like ai-zh), which can lead to “null” tracks or rendering errors in older mpv kernels (like the one used in IINA).

Thankfully, yt-dlp allows us to extract and download these Danmaku and AI subtitles; danmaku2ass(deveoped by @m13253) provides a brilliant solution to convert XML files to ASS subtitles.

Objective

Encode and mux assets for video files ensuring compatibility with iOS 26+ and macOS 26+ (Tahoe).

This includes:

• Embedding subtitles in Apple’s tx3g format.
• Incorporating chapter markers with both text labels and thumbnail images.

1. Chapter Metadata Specification

To ensure chapters are recognized, the metadata file must follow the FFMETADATA header format with timestamps in nanoseconds (or based on timebase).