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srt / vtt / ass Subtitle Formats ​

SRT (SubRip Subtitle), VTT (WebVTT), and ASS (Advanced SubStation Alpha) are 3 very common subtitle formats. The following details each subtitle format, its attributes, and settings.

SRT Subtitle Format ​

SRT is a simple and widely used subtitle format with the extension .srt, especially popular in video players and subtitle editors. Its basic structure includes subtitle number, timestamp, and subtitle text. Subtitle attributes cannot be directly defined through SRT (e.g., color, font), and usually rely on the player's default settings or external style files to control.

SRT Format Structure ​

Each subtitle block in an SRT file is arranged in the following format:

  1. Subtitle Number (incrementing line by line)
  2. Timestamp (display start and end times, accurate to milliseconds)
  3. Subtitle Content (can contain multiple lines of text)
  4. A blank line (to separate subtitle blocks)

SRT Example ​

plaintext
1
00:00:01,000 --> 00:00:04,000
Hello my friend!

2
00:00:05,000 --> 00:00:08,000
The weather is nice today, what do you think.

Detailed Explanation ​

  • Subtitle Number: Each subtitle block has a unique number, incrementing sequentially. The number starts from 1 and must be an integer.

    • Example: 1

  • Timestamp: The format is HH:MM:SS,mmm, where HH is hours, MM is minutes, SS is seconds, and mmm is milliseconds. The timestamp consists of two times separated by -->, with a space on each side of the symbol, indicating the start and end times of the subtitle.

    • Example: 00:00:01,000 --> 00:00:04,000

  • Subtitle Content: The subtitle text can contain one or more lines, displayed on the video. SRT does not support formatting text, such as color, font size, etc. These must be defined through player settings or additional style files.

    • Example: Hello my friend!

SRT Format Limitations ​

  • No Text Formatting: Cannot directly set colors, fonts, etc., requiring style adjustments through the player or other tools.

VTT Subtitle Format ​

WebVTT (Web Video Text Tracks) is a subtitle format for HTML5 video elements, designed specifically for online video. It is more powerful than the SRT format, supporting attributes such as styles, annotations, multiple languages, and location information. The subtitle file format suffix is .vtt. However, it cannot be directly embedded in video and must be referenced in the <video> element of HTML.

VTT Format Structure ​

VTT files are similar to SRT, but with more features. VTT files start with WEBVTT followed by 1 blank line, and use the . dot symbol instead of , to separate seconds and milliseconds.

VTT Example ​

plaintext
WEBVTT

1
00:00:01.000 --> 00:00:04.000
Hello, <b>friends!</b>

2
00:00:05.000 --> 00:00:08.000
The rain today is <i>very, very heavy</i>.

Detailed Explanation ​

  • WEBVTT Declaration: All VTT files must start with WEBVTT to declare their file format.

    • Example: WEBVTT

  • Subtitle Number: The subtitle number is optional, unlike the SRT format where it is required. Its function is to distinguish the order of each subtitle segment, but it can be omitted in VTT.

  • Timestamp: The format is HH:MM:SS.mmm, where HH is hours, MM is minutes, SS is seconds, and mmm is milliseconds. Use a . English period dot to separate seconds and milliseconds instead of ,. The timestamp consists of two times separated by -->, also with a space on each side.

    • Example: 00:00:01.000 --> 00:00:04.000

  • Subtitle Content: The subtitle text can contain HTML tags to format the text, such as bold (<b>), italics (<i>), underline (<u>), etc.

    • Example:
      plaintext
      Hello, <b>friends!</b>

Other Features Supported by VTT ​

  1. Styles (CSS):

    • VTT supports text style adjustments through CSS, such as color, font size, position, etc. Styles can be defined in HTML via the <style> tag or external CSS files.
    • Example:
      plaintext
      <c.red>Hello friends!</c>
      Defining .red { color: red; } in HTML will display Hello, world! in red.

  2. Location Information:

    • VTT supports setting the specific position of subtitles through attributes such as position, line, etc.
    • Example:
      plaintext
      00:00:01.000 --> 00:00:04.000 position:90% line:10%

  3. Annotations:

    • VTT supports adding comments to the file, with comments starting with NOTE.
    • Example:
      plaintext
      NOTE This line is a comment and will not be displayed.

  4. Multi-language Support:

    • VTT can support multi-language subtitles through metadata or the HTML5 <track> tag.

Advantages of the VTT Format ​

  • Text Formatting: Supports HTML tags for simple text formatting, such as bold, italics, etc.
  • Styles and Positioning: Styles and positions of subtitles can be set through CSS.
  • Annotations and Metadata: Supports adding annotation information without affecting subtitle display.
  • Web Compatibility: Designed specifically for HTML5 video, suitable for Web environments.

SRT vs VTT ​

FeatureSRTVTT
File HeaderNoneWEBVTT followed by 1 blank line
Timestamp FormatHH:MM:SS,mmm, comma separating seconds and millisecondsHH:MM:SS.mmm, period separating seconds and milliseconds
Text Formatting SupportNot supportedSupports HTML tags, such as <b>, <i>
Subtitle NumberRequiredOptional
Style and Position SupportRelies on player or external style filesBuilt-in CSS style support, supports location information
AnnotationsNot supportedSupports NOTE annotations
Advanced Features SupportedBasic subtitle features onlySupports Karaoke, annotations, styles, etc.
Usage ScenariosLocal video files, simple subtitle displayHTML5 video, online subtitles, complex subtitle display
Embedded in videoEmbeddable in video filesCannot be embedded in video, can only be used in <video> elements of web pages

VTT (WebVTT) subtitle format cannot be directly embedded into MP4 files, but you can associate VTT files with MP4 videos through the HTML5 <track> tag. When you open the MP4 in a browser, these associated subtitles can be displayed normally.

Playing MP4 in a Browser Using VTT Subtitles ​

In HTML5, you can load an MP4 video through the <video> element and associate VTT subtitles to the video using the <track> element.

HTML Example:

html
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title></title>
</head>
<body>
    <video controls width="600">
        <source src="video.mp4" type="video/mp4">
        <track src="subtitles.vtt" kind="subtitles" srclang="zh" label="Simplified Chinese">
        Your browser does not support the video tag.
    </video>
</body>
</html>

HTML Element Explanation

  • <video>: Used to embed video files. The controls attribute allows users to control video playback (play/pause, etc.).
  • <source>: Defines the path and type of the video file, using MP4 here.
  • <track>: Defines the subtitle file. The src attribute points to the path of the VTT file, kind="subtitles" indicates that it is a subtitle, srclang specifies the language of the subtitle (zh for Chinese), and label gives the subtitle track a descriptive label.

Place the HTML file and the related video and subtitle files in the same directory. Then, open the HTML file (e.g., index.html) through a browser, and you will see the video player. When you click play, the subtitles will be displayed automatically (if the player supports it and the user enables subtitles).

Most modern browsers and video players support subtitle switching. You can select different subtitles (if there are multiple subtitle tracks) via the subtitle button in the video control bar.

VTT Subtitle Notes ​

  • Browser Compatibility: Almost all modern browsers (such as Chrome, Firefox, Edge, etc.) support the <video> element and WebVTT subtitles. As long as the VTT file and MP4 file are correctly associated, the subtitles should be displayed when playing the video in the browser.

  • Cannot Be Directly Embedded in MP4 Files: VTT subtitle files cannot be directly embedded into MP4 files like SRT or other subtitle formats. MP4 files themselves do not contain VTT subtitle tracks. You need to use an external subtitle file and associate it via the HTML5 <track> tag.

  • VTT Subtitle Styles: In browsers, WebVTT subtitles can be styled to some extent through CSS. If you need to customize the appearance of the subtitles, you can further modify the styles through JavaScript and CSS.

ASS Subtitle Format ​

ASS (Advanced SubStation Alpha) is a feature-rich subtitle format widely used in anime, Karaoke subtitles, and other scenarios requiring complex subtitle effects. It supports rich style control, including font, color, position, shadow, and outline.

Below is an example of an ASS subtitle.

[Script Info]
; Script generated by FFmpeg/Lavc60.27.100
ScriptType: v4.00+
PlayResX: 384
PlayResY: 288
ScaledBorderAndShadow: yes
YCbCr Matrix: None

[V4+ Styles]
Format: Name, Fontname, Fontsize, PrimaryColour, SecondaryColour, OutlineColour, BackColour, Bold, Italic, Underline, StrikeOut, ScaleX, ScaleY, Spacing, Angle, BorderStyle, Outline, Shadow, Alignment, MarginL, MarginR, MarginV, Encoding
Style: Default,黑体,16,&hffffff,&HFFFFFF,&h000000,&H0,0,0,0,0,100,100,0,0,1,1,0,2,10,10,10,1
[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:01.95,0:00:04.93,Default,,0,0,0,,This is an ancient galaxy,
Dialogue: 0,0:00:05.42,0:00:08.92,Default,,0,0,0,,We have been observing it for several years,
Dialogue: 0,0:00:09.38,0:00:13.32,Default,,0,0,0,,The Webb Telescope recently sent back many previously undiscovered photos.

ASS Subtitle Structure ​

A standard ASS subtitle file contains several parts:

  1. [Script Info]: Basic information about the script, such as title, original subtitle author, etc.
  2. [V4+ Styles]: Subtitle style definitions, each style can be referenced by different subtitle lines.
  3. [Events]: Actual subtitle events, defining the appearance time, disappearance time, and specific content of the subtitles.

1. [Script Info] Section ​

This section contains the metadata of the subtitle file, defining some basic information about the subtitles.

ini
[Script Info]
Title: Subtitle Title
Original Script: Subtitle Author
ScriptType: v4.00+
PlayDepth: 0
PlayResX: 1920
PlayResY: 1080
ScaledBorderAndShadow: yes
YCbCr Matrix: None
  • Title: The title of the subtitle file.
  • Original Script: The author information of the original subtitles.
  • ScriptType: Defines the script version, usually v4.00+.
  • PlayResX and PlayResY: Define the resolution of the video, indicating the display effect of the subtitles at this resolution.
  • PlayDepth: The color depth of the video, generally 0.
  • ScaledBorderAndShadow: Specifies whether to scale the outline and shadow of the subtitle according to the screen resolution. yes to scale, no not to scale
  • YCbCr Matrix: Specifies the YCbCr matrix used for color conversion. In video processing and subtitle rendering, YCbCr is a color space commonly used for video encoding and decoding. This setting may affect how subtitles are displayed in different color spaces.

2. [V4+ Styles] Section ​

This section defines the style of the subtitles, and each style can control the font, color, shadow, etc. of the subtitles through fields. The format is as follows:

ini
[V4+ Styles]
Format: Name, Fontname, Fontsize, PrimaryColour, SecondaryColour, OutlineColour, BackColour, Bold, Italic, Underline, StrikeOut, ScaleX, ScaleY, Spacing, Angle, BorderStyle, Outline, Shadow, Alignment, MarginL, MarginR, MarginV, Encoding
Style: Default,Arial,20,&H00FFFFFF,&H0000FFFF,&H00000000,&H00000000,-1,0,0,0,100,100,0,0,1,1,0,2,10,10,20,1

Field Explanation: ​

  1. Name: The name of the style, used for reference.

    • Example: Default, indicating this is the default style.

  2. Fontname: The font name.

    • Example: Arial, the subtitles will use the Arial font.

  3. Fontsize: The font size.

    • Example: 20, the font size is 20.

  4. PrimaryColour: The primary subtitle color, indicating the main color of the subtitles (usually the color of the displayed text).

    • Example: &H00FFFFFF, white font. The color value format is &HAABBGGRR, where AA is the transparency.

  5. SecondaryColour: The secondary subtitle color, usually used for the transition color in Karaoke subtitles.

    • Example: &H0000FFFF, blue.

  6. OutlineColour: The outline color.

    • Example: &H00000000, black outline.

  7. BackColour: The background color, usually used when BorderStyle=3 (subtitles with a background box).

    • Example: &H00000000, black background.

  8. Bold: Bold setting.

    • Example: -1 indicates bold, 0 indicates non-bold.

  9. Italic: Italic setting.

    • Example: 0 indicates non-italic, -1 indicates italic.

  10. Underline: Underline setting.

    • Example: 0 indicates no underline.

  11. StrikeOut: Strikethrough setting.

    • Example: 0 indicates no strikethrough.

  12. ScaleX: Horizontal scaling ratio, 100 indicates normal ratio.

    • Example: 100, indicating no scaling.

  13. ScaleY: Vertical scaling ratio.

    • Example: 100, indicating no scaling.

  14. Spacing: Character spacing.

    • Example: 0, indicating no extra spacing.

  15. Angle: Subtitle rotation angle.

    • Example: 0, indicating no rotation.

  16. BorderStyle: Border style, defines whether the subtitle has an outline or background box.

    • Example: 1 indicates an outline but no background box, 3 indicates a background box.

  17. Outline: Outline thickness.

    • Example: 1, indicating the outline thickness is 1.

  18. Shadow: Shadow depth.

    • Example: 0, indicating no shadow.

  19. Alignment: Subtitle alignment, using numbers 1-9 to define different alignment positions.

    • Example: 2, indicating centered alignment.

    Alignment explanation:

    • 1: Bottom left
    • 2: Bottom center
    • 3: Bottom right
    • 4: Middle left
    • 5: Middle center
    • 6: Middle right
    • 7: Top left
    • 8: Top center
    • 9: Top right

  20. MarginL, MarginR, MarginV: Left, right, and vertical margins, in pixels.

    • Example: 10, 10, 20, indicating left and right margins of 10 pixels, and a vertical margin of 20 pixels.

  21. Encoding: Encoding format, 1 indicates ANSI encoding, 0 indicates default encoding.

3. [Events] Section ​

This section defines the actual subtitle events, including timestamps, subtitle content, and styles used.

ini
[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:01.00,0:00:05.00,Default,,0,0,0,,This is the first subtitle
Dialogue: 0,0:00:06.00,0:00:10.00,Default,,0,0,0,,This is the second subtitle

Field Explanation: ​

  1. Layer: Layer, controls the stacking order of subtitles, the larger the number, the higher the layer.

    • Example: 0, indicating the default layer.

  2. Start: Subtitle start time, the format is hours:minutes:seconds.milliseconds.

    • Example: 0:00:01.00, indicating the subtitle starts at 1 second.

  3. End: Subtitle end time.

    • Example: 0:00:05.00, indicating the subtitle ends at 5 seconds.

  4. Style: The name of the subtitle style used, referencing the style defined in [V4+ Styles].

    • Example: Default, using the style named Default.

  5. Name: Optional field, usually used for character name annotations.

  6. MarginL, MarginR, MarginV: The left, right, and vertical margins of the subtitle, overriding the values defined in the style.

  7. Effect: Subtitle effects, usually used for Karaoke subtitles, etc.

  8. Text: The actual content of the subtitle, you can use ASS format control codes to achieve line breaks, special styles, and positioning, etc.

Example Subtitle Event ​

ini
Dialogue: 0,0:00:01.00,0:00:05.00,Default,,0,0,0,,{\pos(960,540)}This is the first subtitle
  • {\pos(960,540)}: Controls the subtitle to be displayed at a specific position on the screen (960 pixels horizontally, 540 pixels vertically).
  • This is the first subtitle: The actual subtitle text displayed.

Color Settings in ASS ​

Taking &HAABBGGRR as an example, &HAABBGGRR is a hexadecimal format for representing colors, which contains the transparency of the color and the value of the color itself. This format is used to define the color properties of subtitles, such as PrimaryColour, OutlineColour, and BackColour.

The meanings are as follows:

  • AA: Transparency (Alpha channel), indicating the transparency of the color.
  • BB: Blue component.
  • GG: Green component.
  • RR: Red component.

The specific byte order is: Alpha (transparency) - Blue - Green - Red.

If you don't want to use transparency, you can directly ignore the value in the AA position, for example, &HBBGGRR.

Transparency and Color Values ​

  • Completely Transparent: The color is completely transparent, i.e., invisible. The representation is &H00BBGGRR, where the AA part is 00 (completely transparent).

    Example:

    plaintext
    &H00FFFFFF
    • Here, &H00FFFFFF represents completely transparent white. The transparency is 00 (completely transparent), and the color is FFFFFF (white).

  • Completely Opaque: The color is completely opaque, i.e., the color display effect is most obvious. The representation is &HFFBBGGRR, where the AA part is FF (completely opaque).

    Example:

    plaintext
    &HFF000000
    • Here, &HFF000000 represents completely opaque black. The transparency is FF (completely opaque), and the color is 000000 (black).

Actual Color Examples ​

  1. Completely Transparent Red:

    plaintext
    &H00FF0000
    • Transparency 00 (completely transparent), color FF0000 (red).

  2. Completely Opaque Green:

    plaintext
    &HFF00FF00
    • Transparency FF (completely opaque), color 00FF00 (green).
  • The AA part in &HAABBGGRR controls the transparency, and the BB, GG, RR parts control the color.
  • Completely Transparent: Transparency 00, for example, &H00FF0000 represents completely transparent red.
  • Completely Opaque: Transparency FF, for example, &HFFFF0000 represents completely opaque red.