| Microsoft DirectX 8.1 (C++) |
The IAMLine21Decoder interface is implemented by the Line 21 Decoder filter. This interface provides access to closed-captioned information and settings and is primarily used by applications to turn closed-captioning on and off.
Closed-captioned information is transmitted on line 21 of field 1 in the vertical blanking interval (VBI) of television signals. Video cassette recorders record this information on video tape, and you can use the Line21 Decoder and other Microsoft® DirectShow® filters to capture the line 21 data and save it on disk in a media file format such as Audio-Video Interleaved (AVI). The closed-captioned information appears as a separate stream within the media file.
Closed-captioned text is currently used in live television programming as well as in DVD movies. DVD movies contain line 21 data as part of the user data section of each Group of Pictures (GOP) in the video stream. Television receiver cards with Windows Driver Model (WDM) drivers provide line 21 data.
Applications can call the IAMLine21Decoder::GetDrawBackgroundMode and IAMLine21Decoder::SetDrawBackgroundMode methods so the user can select transparent or opaque captioning.
Requirements
Requires Il21dec.h.
In addition to the methods inherited from IUnknown, the IAMLine21Decoder interface exposes the following methods.
| Method | Description |
| GetDecoderLevel | Retrieves the closed-captioned decoder level. |
| GetCurrentService | Retrieves the current closed captioning service selected by the user. |
| SetCurrentService | Sets the current closed captioning service. |
| GetServiceState | Retrieves the closed captioning service state (on or off). |
| SetServiceState | Sets the closed captioning service state. |
| GetOutputFormat | Retrieves information about output video characteristics such as size and bit depth. |
| SetOutputFormat | Not implemented on the DirectShow Line21 Decoder Filter. Sets information that describes output video characteristics such as size and bit depth. |
| GetBackgroundColor | Retrieves the physical color to use as background for overlays. |
| SetBackgroundColor | Sets the physical color to use as background for overlays. |
| GetRedrawAlways | Retrieves whether the renderer should redraw the whole output bitmap for each sample. |
| SetRedrawAlways | Sets whether the renderer should redraw the whole output bitmap for each sample. |
| GetDrawBackgroundMode | Retrieves whether the caption text background should be opaque or transparent. |
| SetDrawBackgroundMode | Sets whether to make the caption text background opaque or transparent. |