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FFmpeg的 libavcodec 模块完成音视频多媒体的编解码模块。老版本的 FFmpeg 将avcodec_decode_video2()作为视频的解码函数 API#xff0c;将avcodec_decode_audio4()作为音频的解码函数 API#xff1b;从 3.4版本开始已经将二者标记为废弃过时 API#xff08;attribut…介绍
FFmpeg的 libavcodec 模块完成音视频多媒体的编解码模块。老版本的 FFmpeg 将avcodec_decode_video2()作为视频的解码函数 API将avcodec_decode_audio4()作为音频的解码函数 API从 3.4版本开始已经将二者标记为废弃过时 APIattribute_deprecated。新版本 FFmpeg 将 avcodec_send_packet() 与 avcodec_receive_frame() 作为音视频的解码函数 API但同时仍然保留了对老接口的兼容通过avcodec_decode_video2()、avcodec_decode_audio4()调用 compat_decode()完成对新 API 的封装。
//具体可以参考 FFmpeg 中 doc/APIchanges 中的记录.2016-04-21 - 7fc329e - lavc 57.37.100 - avcodec.hAdd a new audio/video encoding and decoding API with decoupled inputand output -- avcodec_send_packet(), avcodec_receive_frame(),avcodec_send_frame() and avcodec_receive_packet().2017-09-26 - b1cf151c4d - lavc 57.106.102 - avcodec.hDeprecate AVCodecContext.refcounted_frames. This was useful for deprecatedAPI only (avcodec_decode_video2/avcodec_decode_audio4). The new decode APIs(avcodec_send_packet/avcodec_receive_frame) always work with referencecounted frames.视频解码详细流程
以解码 H264标准为例从 main 函数到最后的 MB 宏块解码。
解码的核心 API 介绍
avcodec_send_packet()
API 申明介绍 * param avctx codec context* param[in] avpkt The input AVPacket. Usually, this will be a single video* frame, or several complete audio frames.* Ownership of the packet remains with the caller, and the* decoder will not write to the packet. The decoder may create* a reference to the packet data (or copy it if the packet is* not reference-counted).* Unlike with older APIs, the packet is always fully consumed,* and if it contains multiple frames (e.g. some audio codecs),* will require you to call avcodec_receive_frame() multiple* times afterwards before you can send a new packet.* It can be NULL (or an AVPacket with data set to NULL and* size set to 0); in this case, it is considered a flush* packet, which signals the end of the stream. Sending the* first flush packet will return success. Subsequent ones are* unnecessary and will return AVERROR_EOF. If the decoder* still has frames buffered, it will return them after sending* a flush packet.** return 0 on success, otherwise negative error code:* AVERROR(EAGAIN): input is not accepted in the current state - user* must read output with avcodec_receive_frame() (once* all output is read, the packet should be resent, and* the call will not fail with EAGAIN).* AVERROR_EOF: the decoder has been flushed, and no new packets can* be sent to it (also returned if more than 1 flush* packet is sent)* AVERROR(EINVAL): codec not opened, it is an encoder, or requires flush* AVERROR(ENOMEM): failed to add packet to internal queue, or similar* other errors: legitimate decoding errors*/
int avcodec_send_packet(AVCodecContext *avctx, const AVPacket *avpkt);avcodec_send_packet()定义解析 该 API 可以理解为 FFmpeg 完成音视频解码的核心函数从函数的实现中可以看到首先会调用av_bsf_send_packet()函数将输入的 AVPacket 拷贝到 buffer 中。最后当 buffer_frame为空的时候就调用decode_receive_frame_internal()函数完成真正的解码。 decode_receive_frame_internal()内部会调用decode_simple_receive_frame()完成解码。 decode_simple_receive_frame()内部会调用decode_simple_internal()完成解码。 而在decode_simple_internal()通过函数指针(*decode())指向不同的解码器完成解码不同标准的解码过程如果涉及到多线程解码还会涉及到ff_thread_decode_frame()多线程解码不详细介绍。 (*decode())函数指针 在 decode_simple_internal()通过ret avctx-codec-decode(avctx, frame, got_frame, pkt)实现对不同解码器的调用完成具体的解码过程。 以h264解码为例h264_decode_frame()完成了具体的解码过程其中核心函数是 decode_nal_units()进行 NAUL 解码其中ff_H2645_packet_split()进行码流解析 ff_h264_execute_decode_slices()进行 Slice 级解码。 在ff_h264_execute_decode_slices()核心函数是 decode_slice()对每个 Slice 进行解码。 decode_slice()中是视频数据的核心解码过程主要分成四个模块熵解码、宏块解码、环路滤波、错误隐藏。其中 ff_h264_decode_mb_cabac()完成熵解码如果熵解码是 cavlc则会ff_h264_decode_mb_cavlac 完成熵解码ff_h264_decode_mb()完成宏块 MB 解码loop_filter()完成环路滤波er_add_slice()完成错误隐藏处理。
avcodec_receive_frame()
API 申明介绍
/*** Return decoded output data from a decoder.//从解码器返回解码后的输出数据** param avctx codec context* param frame This will be set to a reference-counted video or audio* frame (depending on the decoder type) allocated by the* decoder. Note that the function will always call* av_frame_unref(frame) before doing anything else.** return* 0: success, a frame was returned* AVERROR(EAGAIN): output is not available in this state - user must try* to send new input* AVERROR_EOF: the decoder has been fully flushed, and there will be* no more output frames* AVERROR(EINVAL): codec not opened, or it is an encoder* AVERROR_INPUT_CHANGED: current decoded frame has changed parameters* with respect to first decoded frame. Applicable* when flag AV_CODEC_FLAG_DROPCHANGED is set.* other negative values: legitimate decoding errors*/
int avcodec_receive_frame(AVCodecContext *avctx, AVFrame *frame);avcodec_receive_frame()定义解析 从avcodec_receive_frame()函数内部实现可以看到核心逻辑相对简单先判断buffer_frame里是否有数据如果有直接调用av_frame_move_ref()完成frame 拷贝过程解码后数据从buffer_frame拷贝到 frame 中即把数据从 AVCodecContext 中拷贝到 AVFrame 中如果 buffer_frame中没有数据则需要调用decode_receive_frame_internal()完成具体的解码该步骤和 send packet 模块相同。
官方解码实例
/** Copyright (c) 2001 Fabrice Bellard** Permission is hereby granted, free of charge, to any person obtaining a copy* of this software and associated documentation files (the Software), to deal* in the Software without restriction, including without limitation the rights* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell* copies of the Software, and to permit persons to whom the Software is* furnished to do so, subject to the following conditions:** The above copyright notice and this permission notice shall be included in* all copies or substantial portions of the Software.** THE SOFTWARE IS PROVIDED AS IS, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL* THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN* THE SOFTWARE.*//*** file* video decoding with libavcodec API example** example decode_video.c*/#include stdio.h
#include stdlib.h
#include string.h#include libavcodec/avcodec.h#define INBUF_SIZE 4096static void pgm_save(unsigned char *buf, int wrap, int xsize, int ysize,char *filename)
{FILE *f;int i;f fopen(filename,w);fprintf(f, P5\n%d %d\n%d\n, xsize, ysize, 255);for (i 0; i ysize; i)fwrite(buf i * wrap, 1, xsize, f);fclose(f);
}static void decode(AVCodecContext *dec_ctx, AVFrame *frame, AVPacket *pkt,const char *filename)
{char buf[1024];int ret;ret avcodec_send_packet(dec_ctx, pkt);if (ret 0) {fprintf(stderr, Error sending a packet for decoding\n);exit(1);}while (ret 0) {ret avcodec_receive_frame(dec_ctx, frame);if (ret AVERROR(EAGAIN) || ret AVERROR_EOF)return;else if (ret 0) {fprintf(stderr, Error during decoding\n);exit(1);}printf(saving frame %3d\n, dec_ctx-frame_number);fflush(stdout);/* the picture is allocated by the decoder. no need tofree it */snprintf(buf, sizeof(buf), %s-%d, filename, dec_ctx-frame_number);pgm_save(frame-data[0], frame-linesize[0],frame-width, frame-height, buf);}
}int main(int argc, char **argv)
{const char *filename, *outfilename;const AVCodec *codec;AVCodecParserContext *parser;AVCodecContext *c NULL;FILE *f;AVFrame *frame;uint8_t inbuf[INBUF_SIZE AV_INPUT_BUFFER_PADDING_SIZE];uint8_t *data;size_t data_size;int ret;AVPacket *pkt;if (argc 2) {fprintf(stderr, Usage: %s input file output file\nAnd check your input file is encoded by mpeg1video please.\n, argv[0]);exit(0);}filename argv[1];outfilename argv[2];pkt av_packet_alloc();if (!pkt)exit(1);/* set end of buffer to 0 (this ensures that no overreading happens for damaged MPEG streams) */memset(inbuf INBUF_SIZE, 0, AV_INPUT_BUFFER_PADDING_SIZE);/* find the MPEG-1 video decoder */codec avcodec_find_decoder(AV_CODEC_ID_MPEG1VIDEO);if (!codec) {fprintf(stderr, Codec not found\n);exit(1);}parser av_parser_init(codec-id);if (!parser) {fprintf(stderr, parser not found\n);exit(1);}c avcodec_alloc_context3(codec);if (!c) {fprintf(stderr, Could not allocate video codec context\n);exit(1);}/* For some codecs, such as msmpeg4 and mpeg4, width and heightMUST be initialized there because this information is notavailable in the bitstream. *//* open it */if (avcodec_open2(c, codec, NULL) 0) {fprintf(stderr, Could not open codec\n);exit(1);}f fopen(filename, rb);if (!f) {fprintf(stderr, Could not open %s\n, filename);exit(1);}frame av_frame_alloc();if (!frame) {fprintf(stderr, Could not allocate video frame\n);exit(1);}while (!feof(f)) {/* read raw data from the input file */data_size fread(inbuf, 1, INBUF_SIZE, f);if (!data_size)break;/* use the parser to split the data into frames */data inbuf;while (data_size 0) {ret av_parser_parse2(parser, c, pkt-data, pkt-size,data, data_size, AV_NOPTS_VALUE, AV_NOPTS_VALUE, 0);if (ret 0) {fprintf(stderr, Error while parsing\n);exit(1);}data ret;data_size - ret;if (pkt-size)decode(c, frame, pkt, outfilename);}}/* flush the decoder */decode(c, frame, NULL, outfilename);fclose(f);av_parser_close(parser);avcodec_free_context(c);av_frame_free(frame);av_packet_free(pkt);return 0;
}
参考
http://ffmpeg.org/
