ffmpeg enum AVChannel枚举解析

typedef struct AVChannelCustom {
    enum AVChannel id;
    char name[16];
    void *opaque;
} AVChannelCustom;
enum AVChannel {
    ///< Invalid channel index
    AV_CHAN_NONE = -1,
    AV_CHAN_FRONT_LEFT,
    AV_CHAN_FRONT_RIGHT,
    AV_CHAN_FRONT_CENTER,
    AV_CHAN_LOW_FREQUENCY,
    AV_CHAN_BACK_LEFT,
    AV_CHAN_BACK_RIGHT,
    AV_CHAN_FRONT_LEFT_OF_CENTER,
    AV_CHAN_FRONT_RIGHT_OF_CENTER,
    AV_CHAN_BACK_CENTER,
    AV_CHAN_SIDE_LEFT,
    AV_CHAN_SIDE_RIGHT,
    AV_CHAN_TOP_CENTER,
    AV_CHAN_TOP_FRONT_LEFT,
    AV_CHAN_TOP_FRONT_CENTER,
    AV_CHAN_TOP_FRONT_RIGHT,
    AV_CHAN_TOP_BACK_LEFT,
    AV_CHAN_TOP_BACK_CENTER,
    AV_CHAN_TOP_BACK_RIGHT,
    /** Stereo downmix. */
    AV_CHAN_STEREO_LEFT = 29,
    /** See above. */
    AV_CHAN_STEREO_RIGHT,
    AV_CHAN_WIDE_LEFT,
    AV_CHAN_WIDE_RIGHT,
    AV_CHAN_SURROUND_DIRECT_LEFT,
    AV_CHAN_SURROUND_DIRECT_RIGHT,
    AV_CHAN_LOW_FREQUENCY_2,
    AV_CHAN_TOP_SIDE_LEFT,
    AV_CHAN_TOP_SIDE_RIGHT,
    AV_CHAN_BOTTOM_FRONT_CENTER,
    AV_CHAN_BOTTOM_FRONT_LEFT,
    AV_CHAN_BOTTOM_FRONT_RIGHT,
    /** Channel is empty can be safely skipped. */
    AV_CHAN_UNUSED = 0x200,
    /** Channel contains data, but its position is unknown. */
    AV_CHAN_UNKNOWN = 0x300,
    /**
     * Range of channels between AV_CHAN_AMBISONIC_BASE and
     * AV_CHAN_AMBISONIC_END represent Ambisonic components using the ACN system.
     *
     * Given a channel id <i> between AV_CHAN_AMBISONIC_BASE and
     * AV_CHAN_AMBISONIC_END (inclusive), the ACN index of the channel <n> is
     * <n> = <i> - AV_CHAN_AMBISONIC_BASE.
     *
     * @note these values are only used for AV_CHANNEL_ORDER_CUSTOM channel
     * orderings, the AV_CHANNEL_ORDER_AMBISONIC ordering orders the channels
     * implicitly by their position in the stream.
     */
    AV_CHAN_AMBISONIC_BASE = 0x400,
    // leave space for 1024 ids, which correspond to maximum order-32 harmonics,
    // which should be enough for the foreseeable use cases
    AV_CHAN_AMBISONIC_END  = 0x7ff,
};

/**
 * Get the channel with the given index in a channel layout.
 *
 * @param channel_layout input channel layout
 * @return channel with the index idx in channel_layout on success or
 *         AV_CHAN_NONE on failure (if idx is not valid or the channel order is
 *         unspecified)
 */
enum AVChannel
av_channel_layout_channel_from_index(const AVChannelLayout *channel_layout, unsigned int idx);
/**
 * Get a channel described by the given string.
 *
 * This function accepts channel names in the same format as
 * @ref av_channel_from_string().
 *
 * @param channel_layout input channel layout
 * @return a channel described by the given string in channel_layout on success
 *         or AV_CHAN_NONE on failure (if the string is not valid or the channel
 *         order is unspecified)
 */
enum AVChannel
av_channel_layout_channel_from_string(const AVChannelLayout *channel_layout,
                                      const char *name);
/**
 * This is the inverse function of @ref av_channel_name().
 *
 * @return the channel with the given name
 *         AV_CHAN_NONE when name does not identify a known channel
 */
enum AVChannel av_channel_from_string(const char *name);

const AVChannelLayout *layout = codec_ctx_->channel_layout;
const char *name = "FL"; // Front Left
AVChannel channel = av_channel_layout_channel_from_string(layout, name);

const AVChannelLayout *layout = codec_ctx_->channel_layout;
unsigned int idx = 0; // Index of the channel
AVChannel channel = av_channel_layout_channel_from_index(layout, idx);

const char *name = "FL"; // Front Left
AVChannel channel = av_channel_from_string(name);

/**
 * Get a human readable string in an abbreviated form describing a given channel.
 * This is the inverse function of @ref av_channel_from_string().
 *
 * @param buf pre-allocated buffer where to put the generated string
 * @param buf_size size in bytes of the buffer.
 * @return amount of bytes needed to hold the output string, or a negative AVERROR
 *         on failure. If the returned value is bigger than buf_size, then the
 *         string was truncated.
 */
int av_channel_name(char *buf, size_t buf_size, enum AVChannel channel);
/**
 * bprint variant of av_channel_name().
 *
 * @note the string will be appended to the bprint buffer.
 */
void av_channel_name_bprint(struct AVBPrint *bp, enum AVChannel channel_id);
/**
 * Get a human readable string describing a given channel.
 *
 * @param buf pre-allocated buffer where to put the generated string
 * @param buf_size size in bytes of the buffer.
 * @return amount of bytes needed to hold the output string, or a negative AVERROR
 *         on failure. If the returned value is bigger than buf_size, then the
 *         string was truncated.
 */
int av_channel_description(char *buf, size_t buf_size, enum AVChannel channel);
/**
 * bprint variant of av_channel_description().
 *
 * @note the string will be appended to the bprint buffer.
 */
void av_channel_description_bprint(struct AVBPrint *bp, enum AVChannel channel_id);
/**
 * Get the index of a given channel in a channel layout. In case multiple
 * channels are found, only the first match will be returned.
 *
 * @param channel_layout input channel layout
 * @return index of channel in channel_layout on success or a negative number if
 *         channel is not present in channel_layout.
 */
int av_channel_layout_index_from_channel(const AVChannelLayout *channel_layout,
                                         enum AVChannel channel);

char buf[256];
AVChannel channel = AV_CHAN_FRONT_LEFT;
int ret = av_channel_name(buf, sizeof(buf), channel);
if (ret < 0) {
    // Handle error
} else {
    printf("Channel name: %s\n", buf);
}

AVBPrint bp;
av_bprint_init(&bp, 0, AV_BPRINT_SIZE_UNLIMITED);
AVChannel channel = AV_CHAN_FRONT_LEFT;
av_channel_name_bprint(&bp, channel);
printf("Channel name: %s\n", bp.str);
av_bprint_finalize(&bp, NULL);

char buf[256];
AVChannel channel = AV_CHAN_FRONT_LEFT;
int ret = av_channel_description(buf, sizeof(buf), channel);
if (ret < 0) {
    // Handle error
} else {
    printf("Channel description: %s\n", buf);
}

AVBPrint bp;
av_bprint_init(&bp, 0, AV_BPRINT_SIZE_UNLIMITED);
AVChannel channel = AV_CHAN_FRONT_LEFT;
av_channel_description_bprint(&bp, channel);
printf("Channel description: %s\n", bp.str);
av_bprint_finalize(&bp, NULL);

const AVChannelLayout *layout = codec_ctx_->channel_layout;
AVChannel channel = AV_CHAN_FRONT_LEFT;
int index = av_channel_layout_index_from_channel(layout, channel);
if (index < 0) {
    printf("Channel is not present in the layout\n");
} else {
    printf("Index of the channel in the layout: %d\n", index);
}