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 API（attribute_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.h
Add a new audio/video encoding and decoding API with decoupled input
and output -- avcodec_send_packet(), avcodec_receive_frame(),
avcodec_send_frame() and avcodec_receive_packet().
2017-09-26 - b1cf151c4d - lavc 57.106.102 - avcodec.h
Deprecate AVCodecContext.refcounted_frames. This was useful for deprecated
API only (avcodec_decode_video2/avcodec_decode_audio4). The new decode APIs
(avcodec_send_packet/avcodec_receive_frame) always work with reference
counted 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 4096
static 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 to
free 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>\n"
"And 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 height
MUST be initialized there because this information is not
available 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;
}