| Microsoft DirectX 9.0 |
The IAMLine21Decoder interface provides access to closed-captioned information and settings. The Line 21 Decoder filter exposes this interface. Applications can use this interface to enable or disable closed captions, select the closed captioning service, and set other closed captioning properties.
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 used in television programming and 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.
Requirements
Requires Il21dec.h.
In addition to the methods inherited from IUnknown, the IAMLine21Decoder interface exposes the following methods.
| Method | Description |
| GetBackgroundColor | Retrieves the background color used by the Line 21 Decoder filter for overlay. |
| GetCurrentService | Retrieves the current closed captioning service. |
| GetDecoderLevel | Retrieves the closed-captioned decoder level. |
| GetDrawBackgroundMode | Indicates whether the filter draws the captions on a transparent background or an opaque background. |
| GetOutputFormat | Retrieves the Line 21 Decoder filter's output format. |
| GetRedrawAlways | Indicates whether the filter redraws the entire output bitmap for each sample. |
| GetServiceState | Indicates whether closed captioning is on or off. |
| SetBackgroundColor | Sets the background color that the filter uses for overlay. |
| SetCurrentService | Sets the closed captioning service. |
| SetDrawBackgroundMode | Specifies whether the filter draws the captions on a transparent background or an opaque background. |
| SetOutputFormat | Sets the filter's output format. (Not implemented) |
| SetRedrawAlways | Specifies whether the filter redraws the entire output bitmap for each sample. |
| SetServiceState | Enables or disables closed captions. |