APIREF.md

HLS Segmentation and FFmpeg API Workflow

What is HLS?

HTTP Live Streaming (HLS) is a protocol used to stream audio and video content over the HTTP protocol in small, segmented files. HLS supports adaptive bitrate streaming, where the quality of the stream adjusts according to the viewer's available bandwidth.

HLS Segmentation Process

The process of converting an audio file into HLS involves dividing the audio content into smaller segments, each with a fixed duration. The segments are then linked together in a master playlist (.m3u8), which clients use to stream the content.

The main steps involved in the process are:

1. Open the Input File:

   • avformat_open_input() is used to open the input file, which contains the audio stream.
   • avformat_find_stream_info() retrieves stream information, including codec parameters.

2. Create the Output Context:

   • avformat_alloc_output_context2() initializes a new output context for the HLS format.
   • We create a new stream using avformat_new_stream() to hold the audio data for the HLS segments.

3. Set HLS Encoding Options:

   • We configure HLS segment duration and playlist settings using the av_dict_set() function to set dictionary options such as hls_time for segment duration and hls_flags for segment independence.

4. Encode and Write Segments:

   • The program reads packets using av_read_frame(), converts timestamps and durations as necessary, and writes them to the HLS stream using av_interleaved_write_frame().
   • Each packet corresponds to an audio frame, which is appended to the output segment.

5. Finalize the Output:

   • av_write_trailer() is called to finalize the playlist and close the output context.

This process ensures that the input audio file is divided into smaller chunks for HLS streaming, making it more suitable for network transmission and adaptive bitrate adjustments.

API Calls Reference

• avformat_open_input(): Opens the input file.
• avformat_find_stream_info(): Retrieves stream info.
• avformat_alloc_output_context2(): Allocates the output context for HLS.
• avformat_new_stream(): Creates a new stream for the output context.
• avcodec_parameters_copy(): Copies codec parameters from input to output stream.
• av_dict_set(): Sets options for HLS encoding, including segment duration.
• av_read_frame(): Reads packets from the input file.
• av_interleaved_write_frame(): Writes packets to the output context.
• av_write_trailer(): Finalizes the playlist and closes the output file.

Logical flow illustration (mermaid)

flowchart TD
    A[Start] --> B[Initialize FFmpeg Network]
    B --> C[Open Input File - avformat_open_input]
    C --> D{Input File Opened?}
    D -->|No| E[Log Error and Exit]
    D -->|Yes| F[Retrieve Stream Info - avformat_find_stream_info]
    F --> G{Stream Info Found?}
    G -->|No| E
    G -->|Yes| H[Find Audio Stream - Loop Over Streams]
    H --> I{Audio Stream Found?}
    I -->|No| E
    I -->|Yes| J[Allocate Output Context - avformat_alloc_output_context2]
    J --> K{Output Context Allocated?}
    K -->|No| E
    K -->|Yes| L[Create New Stream for Output - avformat_new_stream]
    L --> M{Stream Created?}
    M -->|No| E
    M -->|Yes| N[Copy Codec Parameters - avcodec_parameters_copy]
    N --> O{Codec Parameters Copied?}
    O -->|No| E
    O -->|Yes| P[Open Output File - avio_open]
    P --> Q{Output File Opened?}
    Q -->|No| E
    Q -->|Yes| R[Set HLS Options - av_dict_set]
    R --> S[Write Header - avformat_write_header]
    S --> T{Header Written?}
    T -->|No| E
    T -->|Yes| U[Read and Process Packets - av_read_frame]
    U --> V{Packet Stream Index Matches?}
    V -->|No| W[Skip Packet]
    V -->|Yes| X[Rescale Timestamps and Durations]
    X --> Y[Write Packet to Output - av_interleaved_write_frame]
    Y --> Z{Packet Written?}
    Z -->|No| E
    Z -->|Yes| U
    U --> AA[Finalize and Close - av_write_trailer]
    AA --> AB[Close Input and Output Contexts]
    AB --> AC[End]

    classDef error fill:#f9f,stroke:#333,stroke-width:2px;
    class E error;

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