WebVTT API: Introduction to The Web Video Text Tracks Format

( ) is a format for displaying timed text tracks (such as subtitles or captions) using the element. The primary purpose of WebVTT files is to add text overlays to a . WebVTT is a text based format, which must be encoded using . Where you can use spaces you can also use tabs. There is also a small API available to represent and manage these tracks and the data needed to perform the playback of the text at the correct times. Web Video Text Tracks Format WebVTT UTF-8 WebVTT files The MIME type of WebVTT is . text/vtt A WebVTT file ( ) contains cues, which can be either a single line or multiple lines, as shown below: .vtt WebVTT body The structure of a WebVTT consists of the following components, some of them optional, in this order: An optional byte order mark (BOM). The string " ". WEBVTT An optional text header to the right of . WEBVTT There must be at least one space after . WEBVTT You could use this to add a description to the file. You may use anything in the text header except newlines or the string " ". A blank line, which is equivalent to two consecutive newlines. --> Zero or more cues or comments. Zero or more blank lines. Example 1 - Simplest possible WEBVTT file WEBVTT Example 2 - Very simple WebVTT file with a text header WEBVTT - This file has no cues. Example 3 - Common WebVTT example with a header and cues WEBVTT - This file has cues. : : --> : : - What? - Where are we now? : : --> : : - This is big bat country. : : --> : : - [ Bats Screeching ] - They won re after the bugs. 14 00 01 14.815 00 01 18.114 15 00 01 18.171 00 01 20.991 16 00 01 21.058 00 01 23.868 't get in your hair. They' Let's re-examine one of our previous examples, and look at the cue structure in a bit more detail. Inner structure of a WebVTT file WEBVTT : --> : - Never drink liquid nitrogen. : --> : - It will perforate your stomach. - You could die. NOTE This is the last line the file 00 01.000 00 04.000 00 05.000 00 09.000 in In the case of each cue: The first line is started with a time, which is the starting time for showing the text that appears underneath. On the same line, we then have a string of . --> We finish the first line with a second time, which is the ending time for showing the associated text. We can then have one or more lines that start with a hyphen (-), each containing part of the text track to be shown. We can also place comments in our file, to help us remember important information about the parts of our file. These should be on separate lines, starting with the string . You'll find more about these in the below section. .vtt NOTE It is important to not use "extra" blank lines within a cue, for example between the timings line and the cue payload. WebVTT is line based; a blank line will close the cue. WebVTT comments Comments are an optional component that can be used to add information to a WebVTT file. Comments are intended for those reading the file and are not seen by users. Comments may contain newlines but cannot contain a blank line, which is equivalent to two consecutive newlines. A blank line signifies the end of a comment. A comment cannot contain the string " ", the ampersand character ( ), or the less-than sign ( ). If you wish to use such characters, you need to escape them using for example ; for ampersand and ; for less-than. It is also recommended that you use the greater-than escape sequence ( ;) instead of the greater-than character ( ) to avoid confusion with tags. --> & A comment consists of three parts: The string . NOTE A space or a newline. Zero or more characters other than those noted above. Example 4 - Common WebVTT example NOTE This is a comment Example 5 - Multi-line comment NOTE Another comment that is spanning more than one line. NOTE You can also make a comment across more than one line way. this Example 6 - Common comment usage WEBVTT - Translation that film I like NOTE This translation was done by Kyle so that some friends can watch it their parents. : : --> : : - Ta en kopp varmt te. - Det är inte varmt. : : --> : : - Har en kopp te. - Det smakar som te. NOTE This last line may not translate well. : : --> : : - Ta en kopp of with 1 00 02 15.000 00 02 20.000 2 00 02 20.000 00 02 25.000 3 00 02 25.000 00 02 30.000 Styling WebTT cues You can style WebVTT cues by looking for elements which match the pseudo-element. ::cue Within site CSS video::cue { background-image: linear-gradient(to bottom, dimgray, lightgray); color: papayawhip; } video::cue(b) { : peachpuff; } color Here, all video elements are styled to use a gray linear gradient as their backgrounds, with a foreground color of " ". In addition, text boldfaced using the element are colored " ". papayawhip peachpuff The HTML snippet below actually handles displaying the media itself. "video.webm" track default src "track.vtt" video Within the WebVTT file itself You can also define the style directly in the WebVTT file. In this case, you insert your CSS rules into the file with each rule preceded by the string " " all by itelf on a line of text, as shown below: STYLE WEBVTT STYLE ::cue { background-image: linear-gradient(to bottom, dimgray, lightgray); color: papayawhip; } NOTE comment blocks can be used between style blocks. STYLE ::cue(b) { : peachpuff; } : : --> : : - Hello world . NOTE style blocks cannot appear after the first cue. We can also use identifiers inside WebVTT file, which can be used for defining a new style for some particular cues in the file. The example where we wanted the transcription text to be red highlighted and the other part to remain normal, we can define it as follows using CSS. Where it must be noted that the CSS uses escape sequences the way they are used in HTML pages: WEBVTT : --> : That’s an, an, that’s an L! crédit de transcription : --> : Transcrit par Célestes™ 1 00 00.000 00 02.000 00 04.000 00 05.000 ::cue(#\ ) { : lime; } ::cue(#crédit\ de\ transcription) { : red; } 31 color color Positioning of text tracks is also supported, by including positioning information after the timings in a cue, as seen below (see for more information): Cue settings WEBVTT : : --> : : position: %,line-left align:left size: % Where did he go? : : --> : : position: % align:right size: % I think he went down lane. : : --> : : position: %,line-right align:center size: % What are you waiting ? 00 00 00.000 00 00 04.000 10 35 00 00 03.000 00 00 06.500 90 35 this 00 00 04.000 00 00 06.500 45 35 for WebVTT cues A cue is a single subtitle block that has a single start time, end time, and textual payload. Example 6 consists of the header, a blank line, and then five cues separated by blank lines. A cue consists of five components: An optional cue identifier followed by a newline. Cue timings. Optional cue settings with at least one space before the first and between each setting. One or more newlines. The cue payload text. Example 7 - Example of a cue - Title Crawl : : --> : : line: position: % size: % align:start Some time ago a place rather distant.... 1 00 00 05.000 00 00 10.000 0 20 60 in The identifier is a name that identifies the cue. It can be used to reference the cue from a script. It must not contain a newline and cannot contain the string " ". It must end with a single newline. They do not have to be unique, although it is common to number them (e.g., 1, 2, 3, ...). Cue identifier --> Example 8 - Cue identifier from Example 7 Title Crawl Example 9 - Common usage of identifiers WEBVTT : : --> : : This is the first subtitle. : : --> : : This is the second. : : --> : : Third 1 00 00 22.230 00 00 24.606 2 00 00 30.739 00 00 34.074 3 00 00 34.159 00 00 35.743 A cue timing indicates when the cue is shown. It has a start and end time which are represented by timestamps. The end time must be greater than the start time, and the start time must be greater than or equal to all previous start times. Cues may have overlapping timings. Cue timings If the WebVTT file is being used for chapters ( is ) then the file cannot have overlapping timings. kind chapters Each cue timing contains five components: Timestamp for start time. At least one space. The string " ". --> At least one space. Timestamp for end time. Which must be greater than the start time. The timestamps must be in one of two formats: mm:ss.ttt hh:mm:ss.ttt Where the components are defined as follows: is hours. hh Must be at least two digits. Hours can be greater than two digits (e.g., 9999:00:00.000). is minutes. mm Must be between 00 and 59 inclusive. is seconds. ss Must be between 00 and 59 inclusive. is miliseconds. ttt Must be between 000 and 999 inclusive. Example 10 - Basic cue timing examples : : --> : : : : --> : : : : --> : : : : --> : : 00 00 22.230 00 00 24.606 00 00 30.739 00 00 34.074 00 00 34.159 00 00 35.743 00 00 35.827 00 00 40.122 Example 11 - Overlapping cue timing examples : : --> : : : : --> : : : : --> : : 00 00 00.000 00 00 10.000 00 00 05.000 00 01 00.000 00 00 30.000 00 00 50.000 Example 12 - Non-overlapping cue timing examples : : --> : : : : --> : : : : --> : : : : --> : : 00 00 00.000 00 00 10.000 00 00 10.000 00 01 00.581 00 01 00.581 00 02 00.100 00 02 01.000 00 02 01.000 Cue settings are optional components used to position where the cue payload text will be displayed over the video. This includes whether the text is displayed horizontally or vertically. There can be zero or more of them, and they can be used in any order so long as each setting is used no more than once. Cue settings The cue settings are added to the right of the cue timings. There must be one or more spaces between the cue timing and the first setting and between each setting. A setting's name and value are separated by a colon. The settings are case sensitive so use lower case as shown. There are five cue settings: vertical Indicates that the text will be displayed vertically rather than horizontally, such as in some Asian languages. line Specifies where text appears vertically. If vertical is set, line specifies where text appears horizontally. Value can be a line number. The line height is the height of the first line of the cue as it appears on the video. Positive numbers indicate top down. Negative numbers indicate bottom up. Or value can be a percentage. Must be an integer (i.e., no decimals) between 0 and 100 inclusive. Must be followed by a percent sign (%). position Specifies where the text will appear horizontally. If vertical is set, position specifies where the text will appear vertically. Value is a percentage. Must be an integer (no decimals) between 0 and 100 inclusive. Must be followed by a percent sign (%). size Specifies the width of the text area. If vertical is set, size specifies the height of the text area. Value is a percentage. Must be an integer (i.e., no decimals) between 0 and 100 inclusive. Must be followed by a percent sign (%). align Specifies the alignment of the text. Text is aligned within the space given by the size cue setting if it is set. The first line demonstrates no settings. The second line might be used to overlay text on a sign or label. The third line might be used for a title. The last line might be used for an Asian language. Example 13 - Cue setting examples : : --> : : : : --> : : line: % position: % align:start : : --> : : line: position: % size: % align:start : : --> : : vertical:rt line: align:end 00 00 05.000 00 00 10.000 00 00 05.000 00 00 10.000 63 72 00 00 05.000 00 00 10.000 0 20 60 00 00 05.000 00 00 10.000 -1 Cue payload The payload is where the main information or content is located. In normal usage the payload contains the subtitles to be displayed. The payload text may contain newlines but it cannot contain a blank line, which is equivalent to two consecutive newlines. A blank line signifies the end of a cue. A cue text payload cannot contain the string " ", the ampersand character ( ), or the less-than sign ( ). Instead use the escape sequence " " for ampersand and " " for less-than. It is also recommended that you use the greater-than escape sequence " " instead of the greater-than character ( ) to avoid confusion with tags. If you are using the WebVTT file for metadata these restrictions do not apply. --> & In addition to the three escape sequences mentioned above, there are fours others. They are listed in the table below. There are a number of tags, such as , that can be used. However, if the WebVTT file is used in a element where the attribute is chapters then you cannot use tags. Cue payload text tags kind Timestamp tag The timestamp must be greater that the cue's start timestamp, greater than any previous timestamp in the cue payload, and less than the cue's end timestamp. The is the text between the timestamp and the next timestamp or to the end of the payload if there is not another timestamp in the payload. Any text before the in the payload is . Any text beyond the is . This enables karaoke style captions. active text active text previous text active text future text Example 12 - Karaoke style text : --> : When the moon hits your eye : : --> : : Like a big-a pizza pie : : --> : : That 1 00 16.500 00 18.500 00 17.500 1 00 00 18.500 00 00 20.500 00 19.000 00 19.500 00 20.000 1 00 00 20.500 00 00 21.500 's amore The following tags are the HTML tags allowed in a cue and require opening and closing tags (e.g., ). text ( ) Class tag Style the contained text using a CSS class. Example 14 - Class tag text ( ) Italics tag Italicize the contained text. Example 15 - Italics tag text ( ) Bold tag Bold the contained text. Example 16 - Bold tag text ( ) Underline tag Underline the contained text. Example 17 - Underline tag text ( ) Ruby tag Used with ruby text tags to display (i.e., small annotative characters above other characters). ruby characters Example 18 - Ruby tag WWW World Wide Web /rt>oui yes ruby ( ) Ruby text tag Used with ruby tags to display (i.e., small annotative characters above other characters). ruby characters Example 19 - Ruby text tag WWW World Wide Web /rt>oui yes ruby ( ) Voice tag Similar to class tag, also used to style the contained text using CSS. Example 20 - Voice tag text Interfaces There are two interfaces or APIs used in WebVTT which are: It is used for providing an interface in Document Object Model API, where different attributes supported by it can be used to prepare and alter the cues in number of ways. VTTCue interface Constructor is the first point for starting the Cue which is defined using the default constructor VTTCue(startTime, endTime, text) where starting time, ending time and text for cue can be adjusted. After that we can set the region for that particular cue to which this cue belongs using cue.region. Vertical, horizontal, line, lineAlign, Position, positionAlign, text, size and Align can be used to alter the cue and its formation, just like we can alter the objects form, shape and visibility in HTML using CSS. But the VTTCue interface is within the WebVTT provides the vast range of adjustment variables which can be used directly to alter the Cue. Following interface can be used to expose WebVTT cues in DOM API: enum AutoKeyword { }; enum DirectionSetting { , , }; enum LineAlignSetting { , , }; enum PositionAlignSetting { , , , }; enum AlignSetting { , , , , }; [Constructor(double startTime, double endTime, DOMString text)] interface VTTCue : TextTrackCue { attribute VTTRegion? region; attribute DirectionSetting vertical; attribute boolean snapToLines; attribute (double or AutoKeyword) line; attribute LineAlignSetting lineAlign; attribute (double or AutoKeyword) position; attribute PositionAlignSetting positionAlign; attribute double size; attribute AlignSetting align; attribute DOMString text; DocumentFragment getCueAsHTML(); }; "auto" "" /* horizontal */ "rl" "lr" "start" "center" "end" "line-left" "center" "line-right" "auto" "start" "center" "end" "left" "right" This is the second interface in WebVTT API. VTT Region interface The new keyword can be used for defining a new VTTRegion object which can then be used for containing the multiple cues. There are several properties of VTTRegion which are width, lines, regionAnchorX, RegionAnchorY, viewportAnchorX, viewportAnchorY and scroll that can be used to specify the look and feel of this VTT region. The interface code is given below which can be used to expose the WebVTT regions in DOM API: enum ScrollSetting { , }; [Constructor] interface VTTRegion { attribute double width; attribute long lines; attribute double regionAnchorX; attribute double regionAnchorY; attribute double viewportAnchorX; attribute double viewportAnchorY; attribute ScrollSetting scroll; }; "" /* none */ "up" Methods and properties The methods used in WebVTT are those which are used to alter the cue or region as the attributes for both interfaces are different. We can categorize them for better understanding relating to each interface in WebVTT: VTTCue The methods which are available in this interface are: GetCueAsHTML to get the HTML of that Cue. VTT Constructor for creating new objects of Cues. Autokeyword. DirectionSetting: to set the direction of caption or text in a file. LineAlignment: to adjust the line alignment. PositionAlignSetting: to adjust the position of text. VTTRegion The methods used for region are listed below along with description of their functionality: ScrollSetting: For adjusting the scrolling setting of all nodes present in given region. VTT Region Constructor: for construction of new VTT Regions. Tutorial on how to write a WebVTT file There are few steps that can be followed to write a simple webVTT file. Before start, it must be noted that you can make use of a notepad and then save the file as ‘.vtt’ file. Steps are given below: 1. Open a notepad. 2. The first line of WebVTT is standardized similar in the way some other languages require you to put headers as the file starts to indicate the file type. One the very first line you have to write. WEBVTT 3. Leave the second line blank, and on the third line the time for first cue is to be specified. For example, for a first cue starting at time 1 second and ending at 5 seconds, it is written as: : --> : 00 01.000 00 05.000 On the next line you can write the caption for this cue which will run from 1st second to the 5th second, inclusive. Following the similar steps, a complete WebVTT file for specific video or audio file can be made. CSS pseudo-classes CSS pseudo classes allow us to classify the type of object which we want to differentiate from other types of objects. It works in similar manner in WebVTT files as it works in HTML file. It is one of the good features supported by WebVTT is the localization and use of class elements which can be used in same way they are used in HTML and CSS to classify the style for particular type of objects, but here these are used for styling and classifying the Cues as shown below: WEBVTT : --> : J’ai commencé le basket à l 04 02.500 04 05.000 'âge de 13, 14 ans 04:05.001 --> 04:07.800 Sur les playground , ici à Montpellier In the above example it can be observed that we can use the identifier and pseudo class name for defining the language of caption, where tag is for italics. The type of pseudo class is determined by the selector it is using and working is similar in nature as it works in HTML. Following CSS pseudo classes can be used: Lang (Lanugage): e.g., p:lang(it). Link: e.g., a:link. Nth-last-child: e.g., p:nth-last-child(2). Nth-child(n): e.g., p:nth-child(2). Where p and a are the tags which are used in HTML for paragraph and link, respectively and they can be replaced by identifiers which are used for Cues in WebVTT file. Specifications Browser compatibility VTTCue interface TextTrack interface Prior to Firefox 50, the enum (representing possible values for ) incorrectly included the value " " instead of " ". This has been corrected. Notes AlignSetting VTTCue.align middle center WebVTT was implemented in Firefox 24 behind the preference , which is disabled by default; you can enable it by setting this preference to . WebVTT is enabled by default starting in Firefox 31 and can be disabled by setting the preference to . media.webvtt.enabled true false Prior to Firefox 58, the keyword was creating objects, but they were not being used. Firefox 58 now fully supports VTTRegion and its use; however, this feature is disabled by default behind the preference ; set it to to enable region support in Firefox 58. Regions are enabled by default starting in Firefox 59 (see bugs and ). REGION VTTRegion media.webvtt.regions.enabled true bug 1338030 bug 1415805 Credits Source: https://developer.mozilla.org/en-US/docs/Web/API/WebVTT_API Published under licence Open CC Attribution ShareAlike 3.0