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IAMLine21Decoder Interface

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Microsoft DirectShow 9.0

IAMLine21Decoder Interface

The IAMLine21Decoder interface sets and retrieves information about closed captions.

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.

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.

Requirements

Header: Include il21dec.h.

Library: Use Strmiids.lib.