Captions and Subtitles

Transform has extensive support for captions and subtitles. Captions and subtitles both represent timed text but differ in the following way:

Captions are carried as metadata attached to the samples in a video stream.
Subtitles are carried in a separate stream independent of the video or audio.

Subtitle Formats

Transform supports the following subtitle Format identifiers:

Format	Description
`c890`	Cavena 890 Subtitle Format
`dvbs`	DVB Subtitles
`dvbt`	DVB Teletext
`idcp`	Interop DCP Subtitle Format
`lams`	Lambda CAP Subtitle Format
`mcc`	MacCaption VANC Format
`pacs`	Screen PAC Subtitle Format
`scc`	Scenarist Closed Captions
`sdcp`	SMPTE DCP Subtitle Format
`spru`	Spruce STL Subtitle Format
`stl`	EBU Subtitle Exchange Format (STL)
`srt`	SubRip File Format (SRT)
`ttml`	W3C Timed Text Format (TTML, ISMC, SMPTE-TT, EBU-TT)
`ttxt`	EBU Teletext
`tx3g`	3GPP Timed Text
`vtt`	Web Video Text Track (WebVTT)

Subtitle Conversion

Use the Format property to convert a subtitle stream to a different format. For example the following composition converts the input subtitles to WebVTT format:

---
input:
  container:
  - name: input1
    subtitle:
    - name: s1
    video: []
    audio: []
output:
  container:
  - name: output1
    type: 'vtt '
    subtitle:
    - route:
        name: s1
      properties:
        Format: vtt
    video: []
    audio: []

{
  "input": {
    "container": [
      {
        "name": "input1",
        "subtitle": [
          {
            "name": "s1"
          }
        ],
        "video": [],
        "audio": []
      }
    ]
  },
  "output": {
    "container": [
      {
        "name": "output1",
        "type": "vtt ",
        "subtitle": [
          {
            "route": {
              "name": "s1"
            },
            "properties": {
              "Format": "vtt"
            }
          }
        ],
        "video": [],
        "audio": []
      }
    ]
  }
}

Subtitle Overlay

Use the overlay operator to overlay (or burn-in) subtitles on the video stream. For example the following composition overlays the input subtitles:

---
input:
  container:
  - name: input1
    video:
    - name: v1
    audio:
    - name: a1
    subtitle:
    - name: s1
output:
  container:
  - name: output1
    type: 'mov '
    video:
    - overlay:
      - name: v1
      - name: s1
        properties:
          Kind: subtitle
          FontFamily: Arial
      properties:
        Format: avci_100_720p30
    audio:
    - route:
        name: a1
      properties:
        Format: pcm_24int_little

{
  "input": {
    "container": [
      {
        "name": "input1",
        "video": [
          {
            "name": "v1"
          }
        ],
        "audio": [
          {
            "name": "a1"
          }
        ],
        "subtitle": [
          {
            "name": "s1"
          }
        ]
      }
    ]
  },
  "output": {
    "container": [
      {
        "name": "output1",
        "type": "mov ",
        "video": [
          {
            "overlay": [
              {
                "name": "v1"
              },
              {
                "name": "s1",
                "properties": {
                  "Kind": "subtitle",
                  "FontFamily": "Arial"
                }
              }
            ],
            "properties": {
              "Format": "avci_100_720p30"
            }
          }
        ],
        "audio": [
          {
            "route": {
              "name": "a1"
            },
            "properties": {
              "Format": "pcm_24int_little"
            }
          }
        ]
      }
    ]
  }
}

The following properties can be applied to a subtitle overlay:

Property	Type	Description
`FontFamily`	string	Specifies the font family name for subtitle text.
`StringForegroundColor`	string	Specifies the text foreground color as a TTML color value: `#rrggbb` `#rrggbbaa` `rgb(r,g,b)` `rgba(r,g,b,a)` `<namedcolor>`
`StringBackgroundColor`	string	Specifies the text background color as a TTML color value.
`RegionBackgroundColor`	string	Specifies the region background color as a TTML color value.

Caption Formats

Transform supports the following caption Format identifiers:

Format	Description
`a53`	ATSC A/53
`a72`	ATSC A/72
`scte20`	SCTE-20
`c608`	CEA-608 Closed Captions (Field 1 or 2)
`c618`	CEA-608 Closed Captions (Field 1)
`c628`	CEA-608 Closed Captions (Field 2)
`c708`	CEA-708 Closed Captions

Caption Extraction

Use the route operator to extract closed caption or teletext metadata from a video stream. For example the following composition extracts CEA-608 captions from the input stream and produces a sidecar SCC output:

---
input:
  container:
  - name: input1
    video:
    - name: v1
    audio: []
    subtitle: []
output:
  container:
  - name: output1
    type: 'scc '
    subtitle:
    - route:
        name: v1
        properties:
          Kind: captions
          Format: c608
      properties:
        Format: scc
    video: []
    audio: []

{
  "input": {
    "container": [
      {
        "name": "input1",
        "video": [
          {
            "name": "v1"
          }
        ],
        "audio": [],
        "subtitle": []
      }
    ]
  },
  "output": {
    "container": [
      {
        "name": "output1",
        "type": "scc ",
        "subtitle": [
          {
            "route": {
              "name": "v1",
              "properties": {
                "Kind": "captions",
                "Format": "c608"
              }
            },
            "properties": {
              "Format": "scc"
            }
          }
        ],
        "video": [],
        "audio": []
      }
    ]
  }
}

Caption Overlay

Use the overlay operator to overlay (or burn-in) captions on the video stream. For example the following composition overlays CEA-608 captions. Note that the Kind property configures the overlay operator to display "open" captions:

---
input:
  container:
  - name: input1
    video:
    - name: v1
      detach:
      - properties:
          Format: c608
    audio:
    - name: a1
output:
  container:
  - name: output1
    type: 'mov '
    video:
    - overlay:
      - name: v1
      - name: v1
        properties:
          Kind: captions
          Format: c608
          CaptionChannel: 0
      properties:
        Format: avci_100_720p30
    audio:
    - route:
        name: a1
      properties:
        Format: pcm_24int_little

{
  "input": {
    "container": [
      {
        "name": "input1",
        "video": [
          {
            "name": "v1",
            "detach": [
              {
                "type": "c608"
              }
            ]
          }
        ],
        "audio": [
          {
            "name": "a1"
          }
        ]
      }
    ]
  },
  "output": {
    "container": [
      {
        "name": "output1",
        "type": "mov ",
        "video": [
          {
            "overlay": [
              {
                "name": "v1"
              },
              {
                "name": "v1",
                "properties": {
                  "Kind": "captions",
                  "Format": "c608",
                  "CaptionChannel": 0
                }
              }
            ],
            "properties": {
              "Format": "avci_100_720p30"
            }
          }
        ],
        "audio": [
          {
            "route": {
              "name": "a1"
            },
            "properties": {
              "Format": "pcm_24int_little"
            }
          }
        ]
      }
    ]
  }
}

The following properties can be applied to the caption overlay:

Property	Type	Description
CaptionChannel	integer	Specifies the caption decoder channel: 0-3 CEA-608 CC1-CC4, 10-15 CEA-708 Service 1-6.

Caption Insertion

Use the merge operator to insert captions into a video stream. For example the following composition inserts captions from a sidecar SCC input:

---
input:
  reference: media
  container:
  - name: media
    video:
    - name: v1
    audio:
    - name: a1
  - name: SCC
    subtitle:
    - name: s1
output:
  container:
  - type: 'mov '
    name: output1
    video:
    - route:
        name: v1
      merge:
        name: s1
        properties:
          Kind: captions
          Format: a72
      properties:
        Format: avci_100_720p30
    audio:
    - route:
        name: a1
      properties:
        Format: pcm_24int_little
    subtitle: []

{
   "input":{
      "reference":"media",
      "container":[
         {
            "name":"media",
            "video":[
               {
                  "name":"v1"
               }
            ],
            "audio":[
               {
                  "name":"a1"
               }
            ]
         },
         {
            "name":"SCC",
            "subtitle":[
               {
                  "name":"s1"
               }
            ]
         }
      ]
   },
   "output":{
      "container":[
         {
            "type":"mov ",
            "name":"output1",
            "video":[
               {
                  "route":{
                     "name":"v1"
                  },
                  "merge":{
                     "name":"s1",
                     "properties":{
                        "Kind":"captions",
                        "Format":"c708",
                        "Location":"a72c"
                     }
                  },
                  "properties":{
                     "Format":"avci_100_720p30"
                  }
               }
            ],
            "audio":[
               {
                  "route":{
                     "name":"a1"
                  },
                  "properties":{
                     "Format":"pcm_24int_little"
                  }
               }
            ],
            "subtitle":[]
         }
      ]
   }
}