Yes — set the source language in advanced options so the recognizer loads the right model. Matching the language to the WebM audio noticeably improves the Image transcript quality.
Yes — a transcript Image is real text, so you can edit, correct, search, and copy from it. That is the point of turning a WebM video into a document rather than leaving it as a media file.
Can I convert several WebM videos to Image at once?
Can I get subtitles instead of a plain Image transcript?
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For caption-style output, use the dedicated subtitle tool which emits SRT / VTT timed to the video. This WebM to Image flow focuses on a document — a plain transcript or a frame contact sheet.