Convert VOB Subtitles to WebVTT or ASS Files

VOB subtitles found on DVDs are not text-based; they are multiplexed bitmap images known as subpictures (VobSub). Because they are graphical overlays rather than encoded text, converting them into modern, scalable formats like WebVTT (.vtt) or Advanced SubStation Alpha (.ass) requires software equipped with Optical Character Recognition (OCR). This article covers the primary software tools and workflows available to extract these bitmap streams, run them through an OCR engine, and export them as WebVTT or ASS subtitle files.

1. Subtitle Edit

Subtitle Edit is the industry standard open-source desktop tool for converting bitmap subtitles to text formats. It natively opens .vob files, .ifo files, and extracted .sub/.idx (VobSub) streams.

• How it works: Subtitle Edit uses either an internal OCR engine (based on character image comparison) or Tesseract OCR to read the bitmaps and convert them into editable text.
• Output Capabilities: Once the OCR process is complete and spelling errors are reviewed, you can directly choose WebVTT or Advanced SubStation Alpha from the "Save as" format dropdown. It automatically calculates the precise start, end, and duration timestamps derived from the DVD stream.

2. FFmpeg and VobSub2SRT (CLI Workflow)

For headless systems, Linux environments, or batch processing, command-line utilities provide automated conversion pathways.

• Demuxing with FFmpeg: FFmpeg can extract the raw VobSub stream from a VOB or MKV file without re-encoding: ffmpeg -i input.vob -map 0:s:0 -c:s copy output.idx This generates an .idx index file and a .sub bitmap package.
• VobSub2SRT: This is a command-line tool that leverages Tesseract to scan the .sub/.idx files and generate plain text subtitles.
• Final Conversion to WebVTT or ASS: After generating the intermediate text file with VobSub2SRT, FFmpeg can instantly repackage it into your desired format:
  • For WebVTT: ffmpeg -i output.srt output.vtt
  • For ASS: ffmpeg -i output.srt output.ass

3. BDSup2Sub + Aegisub

If you need highly customized, styled ASS subtitles (often used for complex styling, positioning, and animation), a specialized multi-tool workflow is ideal.

• BDSup2Sub: This Java-based utility processes DVD VobSub streams and allows you to inspect, repair time codes, and export the bitmaps to formats like BD-SUP (Blu-ray format) or individual PNG frames with accompanying XML timing data.
• Aegisub: Once the text has been extracted via an OCR tool like Subtitle Edit, Aegisub is the premier editor for formatting the output into the ASS specification. It allows fine control over font styles, margins, z-indexing, and karaoke effects that standard WebVTT files cannot replicate.

4. CCExtractor

While primarily engineered for extracting closed captions (CEA-608/708), CCExtractor also includes functionality to detect and process DVD bitmap tracks on video inputs. It includes built-in OCR capabilities that can decode subpicture frames directly from raw VOB transport streams into standard text outputs without requiring third-party GUI software.