提供了编程的基础技术教程

zl程序教程

您现在的位置是：首页 >  硬件

当前栏目

ffmpeg 自定义IO

IO 自定义 FFMPEG
2023-06-13 09:13:58 时间

本篇介绍

最近遇到一个问题，需要让ffmpeg支持android的URI路径媒体文件。最后用自定义IO解决了，而且对外部没有任何影响。本篇总结下解决过程。

自定义IO

自定义IO:

在使用ffmpeg打开文件的时候，可以是绝对路径，也可以是http路径，还可以其他路径，ffmpeg是如何支持该能力的呢？ 在ffmpeg内部有很多歌URLProtocol，表示某个来源的文件，ffmpeg会使用这些protocol挨个匹配输入的文件，看哪个protocol可以打开。那什么是URLProtocol呢?

typedef struct URLProtocol {
    const char *name;
    int     (*url_open)( URLContext *h, const char *url, int flags);
    /**
     * This callback is to be used by protocols which open further nested
     * protocols. options are then to be passed to ffurl_open_whitelist()
     * or ffurl_connect() for those nested protocols.
     */
    int     (*url_open2)(URLContext *h, const char *url, int flags, AVDictionary **options);
    int     (*url_accept)(URLContext *s, URLContext **c);
    int     (*url_handshake)(URLContext *c);

    /**
     * Read data from the protocol.
     * If data is immediately available (even less than size), EOF is
     * reached or an error occurs (including EINTR), return immediately.
     * Otherwise:
     * In non-blocking mode, return AVERROR(EAGAIN) immediately.
     * In blocking mode, wait for data/EOF/error with a short timeout (0.1s),
     * and return AVERROR(EAGAIN) on timeout.
     * Checking interrupt_callback, looping on EINTR and EAGAIN and until
     * enough data has been read is left to the calling function; see
     * retry_transfer_wrapper in avio.c.
     */
    int     (*url_read)( URLContext *h, unsigned char *buf, int size);
    int     (*url_write)(URLContext *h, const unsigned char *buf, int size);
    int64_t (*url_seek)( URLContext *h, int64_t pos, int whence);
    int     (*url_close)(URLContext *h);
    int (*url_read_pause)(URLContext *h, int pause);
    int64_t (*url_read_seek)(URLContext *h, int stream_index,
                             int64_t timestamp, int flags);
    int (*url_get_file_handle)(URLContext *h);
    int (*url_get_multi_file_handle)(URLContext *h, int **handles,
                                     int *numhandles);
    int (*url_get_short_seek)(URLContext *h);
    int (*url_shutdown)(URLContext *h, int flags);
    const AVClass *priv_data_class;
    int priv_data_size;
    int flags;
    int (*url_check)(URLContext *h, int mask);
    int (*url_open_dir)(URLContext *h);
    int (*url_read_dir)(URLContext *h, AVIODirEntry **next);
    int (*url_close_dir)(URLContext *h);
    int (*url_delete)(URLContext *h);
    int (*url_move)(URLContext *h_src, URLContext *h_dst);
    const char *default_whitelist;
} URLProtocol;

看起来就是文件的一些基本操作函数指针，不同的Protocol只要实现对应的文件操作方法即可，目前ffmpeg有以下这几种protocol:

extern const URLProtocol ff_async_protocol;
extern const URLProtocol ff_bluray_protocol;
extern const URLProtocol ff_cache_protocol;
extern const URLProtocol ff_concat_protocol;
extern const URLProtocol ff_concatf_protocol;
extern const URLProtocol ff_crypto_protocol;
extern const URLProtocol ff_data_protocol;
extern const URLProtocol ff_ffrtmpcrypt_protocol;
extern const URLProtocol ff_ffrtmphttp_protocol;
extern const URLProtocol ff_file_protocol;
extern const URLProtocol ff_ftp_protocol;
extern const URLProtocol ff_gopher_protocol;
extern const URLProtocol ff_gophers_protocol;
extern const URLProtocol ff_hls_protocol;
extern const URLProtocol ff_http_protocol;
extern const URLProtocol ff_httpproxy_protocol;
extern const URLProtocol ff_https_protocol;
extern const URLProtocol ff_icecast_protocol;
extern const URLProtocol ff_mmsh_protocol;
extern const URLProtocol ff_mmst_protocol;
extern const URLProtocol ff_md5_protocol;
extern const URLProtocol ff_pipe_protocol;
extern const URLProtocol ff_prompeg_protocol;
extern const URLProtocol ff_rtmp_protocol;
extern const URLProtocol ff_rtmpe_protocol;
extern const URLProtocol ff_rtmps_protocol;
extern const URLProtocol ff_rtmpt_protocol;
extern const URLProtocol ff_rtmpte_protocol;
extern const URLProtocol ff_rtmpts_protocol;
extern const URLProtocol ff_rtp_protocol;
extern const URLProtocol ff_sctp_protocol;
extern const URLProtocol ff_srtp_protocol;
extern const URLProtocol ff_subfile_protocol;
extern const URLProtocol ff_tee_protocol;
extern const URLProtocol ff_tcp_protocol;
extern const URLProtocol ff_tls_protocol;
extern const URLProtocol ff_udp_protocol;
extern const URLProtocol ff_udplite_protocol;
extern const URLProtocol ff_unix_protocol;
extern const URLProtocol ff_libamqp_protocol;
extern const URLProtocol ff_librist_protocol;
extern const URLProtocol ff_librtmp_protocol;
extern const URLProtocol ff_librtmpe_protocol;
extern const URLProtocol ff_librtmps_protocol;
extern const URLProtocol ff_librtmpt_protocol;
extern const URLProtocol ff_librtmpte_protocol;
extern const URLProtocol ff_libsrt_protocol;
extern const URLProtocol ff_libssh_protocol;
extern const URLProtocol ff_libsmbclient_protocol;
extern const URLProtocol ff_libzmq_protocol;

如果需要让ffmpeg支持其他来源的媒体文件，只需要再定义一个新的URLProtocol就行。可是这样就需要修改ffmpeg源码，至少需要让自己添加的protocol添加到ffmpeg的protocol_list里面。这样在很多场景下是行不通的，毕竟修改开源代码带来的维护成本是很高的。ffmpeg已经帮我们想好了解决方法，就是用自定义io，让调用方来告诉ffmpeg如何read，write文件。具体使用的结构就是aviocontext.

typedef struct AVIOContext {
    /**
     * A class for private options.
     *
     * If this AVIOContext is created by avio_open2(), av_class is set and
     * passes the options down to protocols.
     *
     * If this AVIOContext is manually allocated, then av_class may be set by
     * the caller.
     *
     * warning -- this field can be NULL, be sure to not pass this AVIOContext
     * to any av_opt_* functions in that case.
     */
    const AVClass *av_class;

    /*
     * The following shows the relationship between buffer, buf_ptr,
     * buf_ptr_max, buf_end, buf_size, and pos, when reading and when writing
     * (since AVIOContext is used for both):
     *
     **********************************************************************************
     *                                   READING
     **********************************************************************************
     *
     *                            |              buffer_size              |
     *                            |---------------------------------------|
     *                            |                                       |
     *
     *                         buffer          buf_ptr       buf_end
     *                            +---------------+-----------------------+
     *                            |/ / / / / / / /|/ / / / / / /|         |
     *  read buffer:              |/ / consumed / | to be read /|         |
     *                            |/ / / / / / / /|/ / / / / / /|         |
     *                            +---------------+-----------------------+
     *
     *                                                         pos
     *              +-------------------------------------------+-----------------+
     *  input file: |                                           |                 |
     *              +-------------------------------------------+-----------------+
     *
     *
     **********************************************************************************
     *                                   WRITING
     **********************************************************************************
     *
     *                             |          buffer_size                 |
     *                             |--------------------------------------|
     *                             |                                      |
     *
     *                                                buf_ptr_max
     *                          buffer                 (buf_ptr)       buf_end
     *                             +-----------------------+--------------+
     *                             |/ / / / / / / / / / / /|              |
     *  write buffer:              | / / to be flushed / / |              |
     *                             |/ / / / / / / / / / / /|              |
     *                             +-----------------------+--------------+
     *                               buf_ptr can be in this
     *                               due to a backward seek
     *
     *                            pos
     *               +-------------+----------------------------------------------+
     *  output file: |             |                                              |
     *               +-------------+----------------------------------------------+
     *
     */
    unsigned char *buffer;  /**< Start of the buffer. */
    int buffer_size;        /**< Maximum buffer size */
    unsigned char *buf_ptr; /**< Current position in the buffer */
    unsigned char *buf_end; /**< End of the data, may be less than
                                 buffer+buffer_size if the read function returned
                                 less data than requested, e.g. for streams where
                                 no more data has been received yet. */
    void *opaque;           /**< A private pointer, passed to the read/write/seek/...
                                 functions. */
    int (*read_packet)(void *opaque, uint8_t *buf, int buf_size);
    int (*write_packet)(void *opaque, uint8_t *buf, int buf_size);
    int64_t (*seek)(void *opaque, int64_t offset, int whence);
    int64_t pos;            /**< position in the file of the current buffer */
    int eof_reached;        /**< true if was unable to read due to error or eof */
    int error;              /**< contains the error code or 0 if no error happened */
    int write_flag;         /**< true if open for writing */
    int max_packet_size;
    int min_packet_size;    /**< Try to buffer at least this amount of data
                                 before flushing it. */
    unsigned long checksum;
    unsigned char *checksum_ptr;
    unsigned long (*update_checksum)(unsigned long checksum, const uint8_t *buf, unsigned int size);
    /**
     * Pause or resume playback for network streaming protocols - e.g. MMS.
     */
    int (*read_pause)(void *opaque, int pause);
    /**
     * Seek to a given timestamp in stream with the specified stream_index.
     * Needed for some network streaming protocols which don't support seeking
     * to byte position.
     */
    int64_t (*read_seek)(void *opaque, int stream_index,
                         int64_t timestamp, int flags);
    /**
     * A combination of AVIO_SEEKABLE_ flags or 0 when the stream is not seekable.
     */
    int seekable;

    /**
     * avio_read and avio_write should if possible be satisfied directly
     * instead of going through a buffer, and avio_seek will always
     * call the underlying seek function directly.
     */
    int direct;

    /**
     * ',' separated list of allowed protocols.
     */
    const char *protocol_whitelist;

    /**
     * ',' separated list of disallowed protocols.
     */
    const char *protocol_blacklist;

    /**
     * A callback that is used instead of write_packet.
     */
    int (*write_data_type)(void *opaque, uint8_t *buf, int buf_size,
                           enum AVIODataMarkerType type, int64_t time);
    /**
     * If set, don't call write_data_type separately for AVIO_DATA_MARKER_BOUNDARY_POINT,
     * but ignore them and treat them as AVIO_DATA_MARKER_UNKNOWN (to avoid needlessly
     * small chunks of data returned from the callback).
     */
    int ignore_boundary_point;

#if FF_API_AVIOCONTEXT_WRITTEN
    /**
     * @deprecated field utilized privately by libavformat. For a public
     *             statistic of how many bytes were written out, see
     *             AVIOContext::bytes_written.
     */
    attribute_deprecated
    int64_t written;
#endif

    /**
     * Maximum reached position before a backward seek in the write buffer,
     * used keeping track of already written data for a later flush.
     */
    unsigned char *buf_ptr_max;

    /**
     * Read-only statistic of bytes read for this AVIOContext.
     */
    int64_t bytes_read;

    /**
     * Read-only statistic of bytes written for this AVIOContext.
     */
    int64_t bytes_written;
} AVIOContext;

可以看到有很多成员，实际上只需要关心以下三个就够了。

int (*read_packet)(void *opaque, uint8_t *buf, int buf_size);
int (*write_packet)(void *opaque, uint8_t *buf, int buf_size);
int64_t (*seek)(void *opaque, int64_t offset, int whence);

读、写、seek，这三个操作基本就覆盖了ffmpeg需要操作一个媒体文件常用的能力。这儿的opaque就是一个私有数据，在实现自定义io的时候，可以将opaque作为一个实现了读写，seek的对象指针。 接下来可以看下是如何让ffmpeg支持uri的。

image.png

关键代码演示：

    avio_ctx = avio_alloc_context(avio_ctx_buffer, avio_ctx_buffer_size,
                                  0, &bd, &read_packet, &write_packet, &seek_packet);
    if (!avio_ctx) {
        ret = AVERROR(ENOMEM);
        goto end;
    }
    fmt_ctx->pb = avio_ctx;

    ret = avformat_open_input(&fmt_ctx, NULL, NULL, NULL);
    if (ret < 0) {
        fprintf(stderr, "Could not open input\n");
        goto end;
    }

通过avio_alloc_context来分配自定义io对象，并且传入read_packet，write_packet，seek_packet。 形式如上，而bd就是自定义的数据，对于我们，就是用FFmpegProtocolFactory拿到的protocol对象。 这时候剩下的操作就简单了，比如：

int (*read_packet)(void *opaque, uint8_t *buf, int buf_size) {
  ProtocolInterface *p = static_cast<ProtocolInterface*>(opaque);
  return p->Read(buf, buf_size);
}

int (*write_packet)(void *opaque, uint8_t *buf, int buf_size) {
  ProtocolInterface *p = static_cast<ProtocolInterface*>(opaque);
  return p->Write(buf, buf_size);
}

int64_t (*seek)(void *opaque, int64_t offset, int whence) {
  ProtocolInterface *p = static_cast<ProtocolInterface*>(opaque);
  return p->Seek(offset, whence);
}

如果要支持URI，那么在URIProtocol里面，就直接反射到Java，用ContentResolver获取InputStream，就可以支持读写了。

相关文章