1.使用注意api
2.结构体以下:缓存
typedef struct AVPacket{ /** * A reference to the reference-counted buffer where the packet data is * stored. * May be NULL, then the packet data is not reference-counted. */ AVBufferRef *buf; //用来管理data指针引用的数据缓存,经过av_packet_ref() 和 av_packet_unref() //来使buf->buffer->refcount成员引用计数+-,若是引用计数为0,则释放buffer.
//而buf->buffer存储的则是ffmpeg读取出来的未解码数据
/** * Presentation timestamp in AVStream->time_base units; the time at which * the decompressed packet will be presented to the user. * Can be AV_NOPTS_VALUE if it is not stored in the file. * pts MUST be larger or equal to dts as presentation cannot happen before * decompression, unless one wants to view hex dumps. Some formats misuse * the terms dts and pts/cts to mean something different. Such timestamps * must be converted to true pts/dts before they are stored in AVPacket. */ int64_t pts; //显示时间戳,须要所属媒体流AVStream的time_base时基来换算出当前显示的标准时间(时分秒) //好比dpts = av_q2d(AVStream->time_base) * AVPacket->pts; int64_t dts; //解码时间戳,须要所属媒体流AVStream的time_base时基来换算出当前显示的标准时间(时分秒) uint8_t *data; //指向未解码数据(实际指向buf->buffer所指向的地址)
int size; //data的大小
int stream_index; //标识该AVPacket所属的视频/音频流
int flags; //标识,结合AV_PKT_FLAG使用,好比:
//#define AV_PKT_FLAG_KEY 0x0001 关键帧
//#define AV_PKT_FLAG_CORRUPT 0x0002 损坏的数据
//#define AV_PKT_FLAG_DISCARD 0x0004 丢弃的数据
/**
* Additional packet data that can be provided by the container.
* Packet can contain several types of side information. */
AVPacketSideData *side_data; //容器提供的一些附加数据
int side_data_elems; //边缘数据元数个数
/**
* Duration of this packet in AVStream->time_base units, 0 if unknown.
* Equals next_pts - this_pts in presentation order. */
int64_t duration; //数据的时长,须要所属媒体流AVStream的time_base时基来换算出当前的标准时间,未知则值为默认值0
int64_t pos; //数据在流媒体中的位置,未知则值为默认值-1
}AVPacket;
3.AVPacket经常使用函数以下:app
4.AVPacket解码示例:less
AVPacket *packet = av_packet_alloc(); // 建立一个packet while(av_read_frame(pFormatCtx,packet)) { if(packet->stream_index == audio_index) { ... } else if(packet->stream_index == video_index) { ... } av_packet_unref(packet); // 引用计数-1,若是为0,则释放压缩数据所在的空间 } av_packet_free(packet); //释放packet,若是还想使用,则须要从新alloc
5.AVPacket函数分析ide
av_packet_alloc():初始化函数
AVPacket *av_packet_alloc(void) { AVPacket *pkt = av_mallocz(sizeof(AVPacket)); if (!pkt) return pkt; av_packet_unref(pkt); return pkt; }
建立一个AVPacket的实例,但该函数并不会为数据分配空间,其指向数据域的指针为NULL。ui
av_packet_unref():引用减1.若为0则释放压缩数据this
void av_packet_unref(AVPacket *pkt)
{
av_packet_free_side_data(pkt);
av_buffer_unref(&pkt->buf); av_init_packet(pkt); pkt->data = NULL; pkt->size = 0; }
将AVPacket->buf->buffer->refcount成员减1(数据域的引用技术减为0时会自动释放),替代了旧api(av_free_packet)spa
av_packet_free():释放AVPacket自己指针
void av_packet_free(AVPacket **pkt) { if (!pkt || !*pkt) return; av_packet_unref(*pkt); av_freep(pkt); }
首先将AVPacket->buf->buffer->refcount成员减1(数据域的引用技术减为0时会自动释放),而后再释放为AVPacket分配的空间。
av_packet_ref():从src复制到一个初始化好的dst中,并引用+1
int av_packet_ref(AVPacket *dst, const AVPacket *src) { int ret; ret = av_packet_copy_props(dst, src); //复制部分红员(好比:pts,dts,pos,duration,side_data)到dst if (ret < 0) return ret; if (!src->buf) { //若是src->buf为空,则为dst新分配一个数据域,并将src->data复制到dst->buf->data ret = packet_alloc(&dst->buf, src->size); if (ret < 0) goto fail; av_assert1(!src->size || src->data); if (src->size) memcpy(dst->buf->data, src->data, src->size); dst->data = dst->buf->data; } else { //不为空,则调用av_buffer_ref()来使引用+1,并将dst->buf指向src->buf, dst->buf = av_buffer_ref(src->buf); if (!dst->buf) { ret = AVERROR(ENOMEM); goto fail; } dst->data = src->data; //而后将src->data复制到dst->data中 } dst->size = src->size; return 0; fail: av_packet_free_side_data(dst); return ret; }
av_packet_ref将src内容复制到一个建立好的dst中.须要注意: dst必须提早已经注册好
av_packet_clone():建立并返回一个复制好的AVPacket(在音视频同步处理中用到该函数)
AVPacket *av_packet_clone(const AVPacket *src) { AVPacket *ret = av_packet_alloc(); if (!ret) return ret; if (av_packet_ref(ret, src)) av_packet_free(&ret); return ret; }
av_packet_from_data(AVPacket *pkt, uint8_t *data, int size): 经过压缩数据来初始化一个AVPacket(pkt必须是建立好的),通常在读取流媒体时使用,由于解码函数的参数必须是AVPacket.
int av_packet_from_data(AVPacket *pkt, uint8_t *data, int size) { if (size >= INT_MAX - AV_INPUT_BUFFER_PADDING_SIZE) return AVERROR(EINVAL); pkt->buf = av_buffer_create(data, size + AV_INPUT_BUFFER_PADDING_SIZE, av_buffer_default_free, NULL, 0); //建立新的AVBufferRef,并初始化 if (!pkt->buf) return AVERROR(ENOMEM); pkt->data = data; pkt->size = size; return 0; }