Subtitles in digital video exist in several distinct formats, each with different capabilities, and can be integrated with video in fundamentally different ways. Understanding subtitle formats and the distinction between soft and hard subtitles is essential for content creators who need their text to display correctly across diverse platforms and devices.

SRT (SubRip Text) is the most widely used subtitle format due to its simplicity. An SRT file is a plain text document containing numbered sequential entries, each with a timestamp range and the corresponding text. The timestamp format is hours:minutes:seconds,milliseconds, and entries are separated by blank lines. SRT supports basic formatting like bold and italic through HTML-like tags, but has no built-in support for positioning, font specification, or color. Its simplicity makes it universally compatible but limited in presentation options.

WebVTT (Web Video Text Tracks) is the modern web standard for subtitle files, designed specifically for HTML5 video. VTT files share a similar structure to SRT but add significant capabilities: CSS-like styling with the ::cue pseudo-element, positioning through line and position settings, vertical text support for East Asian languages, and region definitions that constrain where subtitles can appear. WebVTT also supports metadata cues for non-visual timed data. The format is natively supported by all modern web browsers through the HTML5 track element.

ASS (Advanced SubStation Alpha) is the most feature-rich subtitle format, popular in anime fansubbing and advanced subtitle design. ASS supports full typographic control including font face, size, color, outline, shadow, rotation, animation, and complex positioning. Subtitles can include karaoke timing, fade effects, drawing commands, and even banner scrolling. This power comes at the cost of complexity and limited platform support outside of dedicated media players like VLC and MPC-HC.

The distinction between soft subtitles and hard subtitles is fundamental. Soft subtitles (also called closed captions in broadcast terminology) are stored as a separate data stream within the video container or as an external file. The video player reads these text streams and renders them as an overlay during playback. Viewers can toggle soft subtitles on or off, change languages if multiple tracks are provided, and the player may allow customization of appearance. However, soft subtitles depend entirely on the player supporting the subtitle format, and many platforms (particularly social media) strip or ignore subtitle tracks.

Hard subtitles (also called burned-in or open subtitles) are rendered directly onto the video frames during encoding, becoming permanent pixels in the image data. The text rendering process involves rasterizing the subtitle text into a bitmap using the specified font, size, and style, then compositing this bitmap onto each video frame where the subtitle is active. Anti-aliasing ensures smooth text edges, and outline or shadow effects improve readability against varying backgrounds. Because the text is part of the video image itself, hard subtitles are guaranteed to display on every platform and player, cannot be disabled by viewers, and survive re-encoding, format conversion, and platform processing. The trade-off is that they cannot be turned off, cannot be searched or indexed as text data, and changing them requires re-processing the entire video.