大家都知道，ffmpeg是一个强大的转码程序，但是ffmpeg功能实在太多了，以至于我们读ffmpeg.c都觉得特别头疼！其实你我都只是想看看mp4转ts，或avi转mkv怎么做的~
好吧，其实ffmpeg的源码里有sample code，是简化版的转码程序，读起来就容易多了！
具体位置：ffmpeg/doc/examples/transcoding.c。
我把这个代码拉出来，修改了一下，单独编译成一个程序。也很好用。
为了让更多的朋友了解，我决定博此一文，说说这个转码程序！
1 源码+详细注释
#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libavfilter/avfiltergraph.h>
#include <libavfilter/buffersink.h>
#include <libavfilter/buffersrc.h>
#include <libavutil/opt.h>
#include <libavutil/pixdesc.h>
static AVFormatContext *ifmt_ctx;//DEMUX,比如把输入的input.mp4拆分为audio和video的ES数据
static AVFormatContext *ofmt_ctx;//MUX，比如把编码好的audio/video ES数据打包成output.ts
typedef struct FilteringContext {
AVFilterContext *buffersink_ctx;
AVFilterContext *buffersrc_ctx;
AVFilterGraph *filter_graph;
} FilteringContext;
static FilteringContext *filter_ctx;
//打开并解析要转码的源文件(如input.mp4)，并分配相应的decoder
//比如，源文件是input.mp4(H264+AAC),那么就有2个stream，分配2个decoder，H264 decoder & AAC decoder
static int open_input_file(const char *filename)
{
int ret;
unsigned int i;
ifmt_ctx = NULL;
if ((ret = avformat_open_input(&ifmt_ctx, filename, NULL, NULL)) < 0) {
printf("Cannot open input file\n");
return ret;
}
if ((ret = avformat_find_stream_info(ifmt_ctx, NULL)) < 0) {
printf("Cannot find stream information\n");
return ret;
}
//一般有2个streams,一个video, 一个audio
for (i = 0; i < ifmt_ctx->nb_streams; i++) {
AVStream *stream;
AVCodecContext *codec_ctx;
stream = ifmt_ctx->streams[i];
codec_ctx = stream->codec;
/* Reencode video & audio and remux subtitles etc. */
if (codec_ctx->codec_type == AVMEDIA_TYPE_VIDEO
|| codec_ctx->codec_type == AVMEDIA_TYPE_AUDIO) {
/* Open decoder */
//为stream寻找一个合适的decoder，for example, video stream use H264 decoder, audio stream use AAC decoder
ret = avcodec_open2(codec_ctx,
avcodec_find_decoder(codec_ctx->codec_id), NULL);
if (ret < 0) {
printf("Failed to open decoder for stream #%u\n", i);
return ret;
}
}
}
av_dump_format(ifmt_ctx, 0, filename, 0);
return 0;
}
//打开输出文件，如我们的目标是转码成TS文件，output.ts
//分配相应的encoder, 本例，encoder的codec与decoder的一样。也就是, output.ts的codec还是和input.mp4一样(H264+AAC)
static int open_output_file(const char *filename)
{
AVStream *out_stream;
AVStream *in_stream;
AVCodecContext *dec_ctx, *enc_ctx;
AVCodec *encoder;
int ret;
unsigned int i;
ofmt_ctx = NULL;
avformat_alloc_output_context2(&ofmt_ctx, NULL, NULL, filename);
if (!ofmt_ctx) {
printf("Could not create output context\n");
return AVERROR_UNKNOWN;
}
for (i = 0; i < ifmt_ctx->nb_streams; i++) {
out_stream = avformat_new_stream(ofmt_ctx, NULL);
if (!out_stream) {
printf("Failed allocating output stream\n");
return AVERROR_UNKNOWN;
}
in_stream = ifmt_ctx->streams[i];
dec_ctx = in_stream->codec;
enc_ctx = out_stream->codec;
if (dec_ctx->codec_type == AVMEDIA_TYPE_VIDEO
|| dec_ctx->codec_type == AVMEDIA_TYPE_AUDIO) {
// in this example, we choose transcoding to same codec
//假设输入input.mp4(H264+AAC), 那输出还是H264+AAC, 只不过容器变成ts, 输出名字output.ts
encoder = avcodec_find_encoder(dec_ctx->codec_id);
if (!encoder) {
av_log(NULL, AV_LOG_FATAL, "Necessary encoder not found\n");
return AVERROR_INVALIDDATA;
}
/* In this example, we transcode to same properties (picture size,
* sample rate etc.). These properties can be changed for output
* streams easily using filters */
if (dec_ctx->codec_type == AVMEDIA_TYPE_VIDEO) {
enc_ctx->height = dec_ctx->height;
enc_ctx->width = dec_ctx->width;
enc_ctx->sample_aspect_ratio = dec_ctx->sample_aspect_ratio;
/* take first format from list of supported formats */
enc_ctx->pix_fmt = encoder->pix_fmts[0];
/* video time_base can be set to whatever is handy and supported by encoder */
enc_ctx->time_base = dec_ctx->time_base;
} else {
enc_ctx->sample_rate = dec_ctx->sample_rate;
enc_ctx->channel_layout = dec_ctx->channel_layout;
enc_ctx->channels = av_get_channel_layout_nb_channels(enc_ctx->channel_layout);
/* take first format from list of supported formats */
enc_ctx->sample_fmt = encoder->sample_fmts[0];
enc_ctx->time_base = (AVRational){1, enc_ctx->sample_rate};
}
/* Third parameter can be used to pass settings to encoder */
ret = avcodec_open2(enc_ctx, encoder, NULL);
if (ret < 0) {
printf("Cannot open video encoder for stream #%u\n", i);
return ret;
}
} else if (dec_ctx->codec_type == AVMEDIA_TYPE_UNKNOWN) {
av_log(NULL, AV_LOG_FATAL, "Elementary stream #%d is of unknown type, cannot proceed\n", i);
return AVERROR_INVALIDDATA;
} else {
/* if this stream must be remuxed */
ret = avcodec_copy_context(ofmt_ctx->streams[i]->codec,
ifmt_ctx->streams[i]->codec);
if (ret < 0) {
printf("Copying stream context failed\n");
return ret;
}
}
if (ofmt_ctx->oformat->flags & AVFMT_GLOBALHEADER)
enc_ctx->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;
}
av_dump_format(ofmt_ctx, 0, filename, 1);
if (!(ofmt_ctx->oformat->flags & AVFMT_NOFILE)) {
ret = avio_open(&ofmt_ctx->pb, filename, AVIO_FLAG_WRITE);
if (ret < 0) {
printf("Could not open output file '%s'", filename);
return ret;
}
}
/* init muxer, write output file header */
ret = avformat_write_header(ofmt_ctx, NULL);
if (ret < 0) {
printf("Error occurred when opening output file\n");
return ret;
}
return 0;
}
//filter不要怕，请看http://blog.youkuaiyun.com/newchenxf/article/details/51364105
//其实对本例，你可以忽略其细节，就把filter当作FIFO，输入就是buffersrc，输出就是buffersink，buffer无非在这走了个过场而已。
//不过不用filter，直接把buffer从decoder送给encoder，也是可以的。只不过缺点是，这buffer就不能改了，不像用filter，你还可以处理buffer
static int init_filter(FilteringContext* fctx, AVCodecContext *dec_ctx,
AVCodecContext *enc_ctx, const char *filter_spec)
{
char args[512];
int ret = 0;
AVFilter *buffersrc = NULL;
AVFilter *buffersink = NULL;
AVFilterContext *buffersrc_ctx = NULL;
AVFilterContext *buffersink_ctx = NULL;
AVFilterInOut *outputs = avfilter_inout_alloc();
AVFilterInOut *inputs = avfilter_inout_alloc();
AVFilterGraph *filter_graph = avfilter_graph_alloc();
if (!outputs || !inputs || !filter_graph) {
ret = AVERROR(ENOMEM);
goto end;
}
if (dec_ctx->codec_type == AVMEDIA_TYPE_VIDEO) {
buffersrc = avfilter_get_by_name("buffer");
buffersink = avfilter_get_by_name("buffersink");
if (!buffersrc || !buffersink) {
printf("filtering source or sink element not found\n");
ret = AVERROR_UNKNOWN;
goto end;
}
snprintf(args, sizeof(args),
"video_size=%dx%d:pix_fmt=%d:time_base=%d/%d:pixel_aspect=%d/%d",
dec_ctx->width, dec_ctx->height, dec_ctx->pix_fmt,
dec_ctx->time_base.num, dec_ctx->time_base.den,
dec_ctx->sample_aspect_ratio.num,
dec_ctx->sample_aspect_ratio.den);
ret = avfilter_graph_create_filter(&buffersrc_ctx, buffersrc, "in",
args, NULL, filter_graph);
if (ret < 0) {
printf("Cannot create buffer source\n");
goto end;
}
ret = avfilter_graph_create_filter(&buffersink_ctx, buffersink, "out",
NULL, NULL, filter_graph);
if (ret < 0) {
printf("Cannot create buffer sink\n");
goto end;
}
ret = av_opt_set_bin(buffersink_ctx, "pix_fmts",
(uint8_t*)&enc_ctx->pix_fmt, sizeof(enc_ctx->pix_fmt),
AV_OPT_SEARCH_CHILDREN);
if (ret < 0) {
printf("Cannot set output pixel format\n");
goto end;
}
} else if (dec_ctx->codec_type == AVMEDIA_TYPE_AUDIO) {
buffersrc = avfilter_get_by_name("abuffer");
buffersink = avfilter_get_by_name("abuffersink");
if (!buffersrc || !buffersink) {
printf("filtering source or sink element not found\n");
ret = AVERROR_UNKNOWN;
goto end;
}
if (!dec_ctx->channel_layout)
dec_ctx->channel_layout =
av_get_default_channel_layout(dec_ctx->channels);
snprintf(args, sizeof(args),
"time_base=%d/%d:sample_rate=%d:sample_fmt=%s:channel_layout=0x%"PRIx64,
dec_ctx->time_base.num, dec_ctx->time_base.den, dec_ctx->sample_rate,
av_get_sample_fmt_name(dec_ctx->sample_fmt),
dec_ctx->channel_layout);
ret = avfilter_graph_create_filter(&buffersrc_ctx, buffersrc, "in",
args, NULL, filter_graph);
if (ret < 0) {
printf("Cannot create audio buffer source\n");
goto end;
}
ret = avfilter_graph_create_filter(&buffersink_ctx, buffersink, "out",
NULL, NULL, filter_graph);
if (ret < 0) {
printf("Cannot create audio buffer sink\n");
goto end;
}
ret = av_opt_set_bin(buffersink_ctx, "sample_fmts",
(uint8_t*)&enc_ctx->sample_fmt, sizeof(enc_ctx->sample_fmt),
AV_OPT_SEARCH_CHILDREN);
if (ret < 0) {
printf("Cannot set output sample format\n");
goto end;
}
ret = av_opt_set_bin(buffersink_ctx, "channel_layouts",
(uint8_t*)&enc_ctx->channel_layout,
sizeof(enc_ctx->channel_layout), AV_OPT_SEARCH_CHILDREN);
if (ret < 0) {
printf("Cannot set output channel layout\n");
goto end;
}
ret = av_opt_set_bin(buffersink_ctx, "sample_rates",
(uint8_t*)&enc_ctx->sample_rate, sizeof(enc_ctx->sample_rate),
AV_OPT_SEARCH_CHILDREN);
if (ret < 0) {
printf("Cannot set output sample rate\n");
goto end;
}
} else {
ret = AVERROR_UNKNOWN;
goto end;
}
/* Endpoints for the filter graph. */
outputs->name = av_strdup("in");
outputs->filter_ctx = buffersrc_ctx;
outputs->pad_idx = 0;
outputs->next = NULL;
inputs->name = av_strdup("out");
inputs->filter_ctx = buffersink_ctx;
inputs->pad_idx = 0;
inputs->next = NULL;
if (!outputs->name || !inputs->name) {
ret = AVERROR(ENOMEM);
goto end;
}
if ((ret = avfilter_graph_parse_ptr(filter_graph, filter_spec,
&inputs, &outputs, NULL)) < 0)
goto end;
if ((ret = avfilter_graph_config(filter_graph, NULL)) < 0)
goto end;
/* Fill FilteringContext */
fctx->buffersrc_ctx = buffersrc_ctx;
fctx->buffersink_ctx = buffersink_ctx;
fctx->filter_graph = filter_graph;
end:
avfilter_inout_free(&inputs);
avfilter_inout_free(&outputs);
return ret;
}
static int init_filters(void)
{
const char *filter_spec;
unsigned int i;
int ret;
filter_ctx = av_malloc_array(ifmt_ctx->nb_streams, sizeof(*filter_ctx));
if (!filter_ctx)
return AVERROR(ENOMEM);
for (i = 0; i < ifmt_ctx->nb_streams; i++) {
filter_ctx[i].buffersrc_ctx = NULL;
filter_ctx[i].buffersink_ctx = NULL;
filter_ctx[i].filter_graph = NULL;
if (!(ifmt_ctx->streams[i]->codec->codec_type == AVMEDIA_TYPE_AUDIO
|| ifmt_ctx->streams[i]->codec->codec_type == AVMEDIA_TYPE_VIDEO))
continue;
if (ifmt_ctx->streams[i]->codec->codec_type == AVMEDIA_TYPE_VIDEO)
filter_spec = "null"; /* passthrough (dummy) filter for video */
else
filter_spec = "anull"; /* passthrough (dummy) filter for audio */
ret = init_filter(&filter_ctx[i], ifmt_ctx->streams[i]->codec,
ofmt_ctx->streams[i]->codec, filter_spec);
if (ret)
return ret;
}
return 0;
}
static void dumptimebase(const char* flag, int stream_index, AVRational timebase) {
printf("%s[%d]: timebase: %d/%d \n", flag, stream_index, timebase.num, timebase.den);
}
static int encode_write_frame(AVFrame *filt_frame, unsigned int stream_index, int *got_frame) {
int ret;
int got_frame_local;
AVPacket enc_pkt;
//函数指针，看要encode video or audio
int (*enc_func)(AVCodecContext *, AVPacket *, const AVFrame *, int *) =
(ifmt_ctx->streams[stream_index]->codec->codec_type ==
AVMEDIA_TYPE_VIDEO) ? avcodec_encode_video2 : avcodec_encode_audio2;
if (!got_frame)
got_frame = &got_frame_local;
printf("Encoding frame\n");
/* encode filtered frame */
enc_pkt.data = NULL;
enc_pkt.size = 0;
av_init_packet(&enc_pkt);
ret = enc_func(ofmt_ctx->streams[stream_index]->codec, &enc_pkt,
filt_frame, got_frame);
av_frame_free(&filt_frame);
if (ret < 0)
return ret;
if (!(*got_frame))
return 0;
// 编码完成，准备做muxing，做之前，又得把time_base，从encoder，转到输出的time_base
enc_pkt.stream_index = stream_index;
printf("stream[%d]: finish encoding, enc_pkt.pts = %lld\n", stream_index, enc_pkt.pts);
dumptimebase("output streams", stream_index, ofmt_ctx->streams[stream_index]->time_base);
dumptimebase("output streams codec ", stream_index, ofmt_ctx->streams[stream_index]->codec->time_base);
av_packet_rescale_ts(&enc_pkt,
ofmt_ctx->streams[stream_index]->codec->time_base,
ofmt_ctx->streams[stream_index]->time_base);
printf("stream[%d], after scale, now enc_pkt.pts = %lld\n", stream_index, enc_pkt.pts);
printf("Muxing frame\n");
/* mux encoded frame */
ret = av_interleaved_write_frame(ofmt_ctx, &enc_pkt);
return ret;
}
static int filter_encode_write_frame(AVFrame *frame, unsigned int stream_index)
{
int ret;
AVFrame *filt_frame;
printf("Pushing decoded frame to filters\n");
// push the decoded frame into the filtergraph
//其实你如果不知道filter，也不怕，就把它当作一个FIFO，buffersrc是FIFO的输入，buffersinkFIFO的输出
//这里做的，无非就是先把buffer送到FIFO，待会要encode时，再从FIFO拿数据，其实很简单的
ret = av_buffersrc_add_frame_flags(filter_ctx[stream_index].buffersrc_ctx,
frame, 0);
if (ret < 0) {
printf("Error while feeding the filtergraph\n");
return ret;
}
/* pull filtered frames from the filtergraph */
while (1) {
filt_frame = av_frame_alloc();
if (!filt_frame) {
ret = AVERROR(ENOMEM);
break;
}
printf("Pulling filtered frame from filters\n");
//如上文所说，就当作从FIFO拿出数据
ret = av_buffersink_get_frame(filter_ctx[stream_index].buffersink_ctx,
filt_frame);
if (ret < 0) {
/* if no more frames for output - returns AVERROR(EAGAIN)
* if flushed and no more frames for output - returns AVERROR_EOF
* rewrite retcode to 0 to show it as normal procedure completion
*/
if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)
ret = 0;
av_frame_free(&filt_frame);
break;
}
filt_frame->pict_type = AV_PICTURE_TYPE_NONE;
//好了，FIFO走了个过场，真的要开始编码了
ret = encode_write_frame(filt_frame, stream_index, NULL);
if (ret < 0)
break;
}
return ret;
}
static int flush_encoder(unsigned int stream_index)
{
int ret;
int got_frame;
if (!(ofmt_ctx->streams[stream_index]->codec->codec->capabilities &
AV_CODEC_CAP_DELAY))
return 0;
while (1) {
av_log(NULL, AV_LOG_INFO, "Flushing stream #%u encoder\n", stream_index);
ret = encode_write_frame(NULL, stream_index, &got_frame);
if (ret < 0)
break;
if (!got_frame)
return 0;
}
return ret;
}
int main(int argc, char **argv)
{
int ret;
AVPacket packet = { .data = NULL, .size = 0 };
AVFrame *frame = NULL;
enum AVMediaType type;
unsigned int stream_index;
unsigned int i;
int got_frame;
int (*dec_func)(AVCodecContext *, AVFrame *, int *, const AVPacket *);
if (argc != 3) {
printf("Usage: %s <input file> <output file>\n", argv[0]);
return 1;
}
//任何ffmpeg程序，都得av_register_all()，哈哈!
av_register_all();
avfilter_register_all();
if ((ret = open_input_file(argv[1])) < 0)
goto end;
if ((ret = open_output_file(argv[2])) < 0)
goto end;
if ((ret = init_filters()) < 0)
goto end;
/* read all packets */
while (1) {
//从输入文件读取一帧ES数据，要么是音频，要么是视频
if ((ret = av_read_frame(ifmt_ctx, &packet)) < 0)
break;
//stream_index表示这一帧是属于哪个流(要么是视频流，要么是音频流, 一般视频流，stream_index=0,音频流stream_index=1
stream_index = packet.stream_index;
type = ifmt_ctx->streams[packet.stream_index]->codec->codec_type;
//printf("Demuxer gave frame of stream_index %u\n", stream_index);
if (filter_ctx[stream_index].filter_graph) {
printf("Going to reencode&filter the frame\n");
//先分配一个空帧，准备存储解码的数据
frame = av_frame_alloc();
if (!frame) {
ret = AVERROR(ENOMEM);
break;
}
//packet的pts的time_base是基于编码格式的，比如TS码流，就是"1/90000"，要解码了，所以要scale到decoder，decoder的time_base一般="1/FPS"
printf("stream[%d]: before scale, packet pts %lld\n", stream_index, packet.pts);
av_packet_rescale_ts(&packet,
ifmt_ctx->streams[stream_index]->time_base,
ifmt_ctx->streams[stream_index]->codec->time_base);
dumptimebase("streams", stream_index, ifmt_ctx->streams[stream_index]->time_base);
dumptimebase("streams codec", stream_index, ifmt_ctx->streams[stream_index]->codec->time_base);
printf("stream[%d]: after scale, packet pts %lld\n", stream_index, packet.pts);
//函数指针，根据情况，解码音频或者视频
dec_func = (type == AVMEDIA_TYPE_VIDEO) ? avcodec_decode_video2 :
avcodec_decode_audio4;
ret = dec_func(ifmt_ctx->streams[stream_index]->codec, frame,
&got_frame, &packet);
if (ret < 0) {
av_frame_free(&frame);
printf("Decoding failed\n");
break;
}
//解码成功!准备开始编码咯
if (got_frame) {
//其实，这时候解码出来的frame，pts一般=AV_NOPTS_VALUE，即64位的最小值-9223372036854775808(0x8000000000000000)
printf("stream_index %d: decode a frame: frame->pts %lld, %lld \n", stream_index, frame->pts, AV_NOPTS_VALUE);
//所以，就调用av_frame_get_best_effort_timestamp，这个函数获得的，就是上面packet.pts
//av_frame_get_best_effort_timestamp是由宏定义的，搜索ffmpeg源码是搜索不到的。
//宏定义在ffmpeg-2.7.1\libavutil\internal.h
//type av_##name##_get_##field(const str *s) { return s->field; } \ void av_##name##_set_##field(str *s, type v) { s->field = v; }
//set函数,就在上面做decode的时候调用的，所以get才能获得packet.pts
frame->pts = av_frame_get_best_effort_timestamp(frame);
printf("stream index %d: av_frame_get_best_effort_timestamp, frame->pts %lld\n", stream_index, frame->pts);
//准备编码并写入输出文件
ret = filter_encode_write_frame(frame, stream_index);
av_frame_free(&frame);
if (ret < 0)
goto end;
} else {
printf("didn't got frame? \n");
av_frame_free(&frame);
}
} else {
/* remux this frame without reencoding */
av_packet_rescale_ts(&packet,
ifmt_ctx->streams[stream_index]->time_base,
ofmt_ctx->streams[stream_index]->time_base);
ret = av_interleaved_write_frame(ofmt_ctx, &packet);
if (ret < 0)
goto end;
}
av_packet_unref(&packet);
}
/* flush filters and encoders */
for (i = 0; i < ifmt_ctx->nb_streams; i++) {
/* flush filter */
if (!filter_ctx[i].filter_graph)
continue;
ret = filter_encode_write_frame(NULL, i);
if (ret < 0) {
printf("Flushing filter failed\n");
goto end;
}
/* flush encoder */
ret = flush_encoder(i);
if (ret < 0) {
printf("Flushing encoder failed\n");
goto end;
}
}
//写输出文件的尾巴
av_write_trailer(ofmt_ctx);
end:
av_packet_unref(&packet);
av_frame_free(&frame);
for (i = 0; i < ifmt_ctx->nb_streams; i++) {
avcodec_close(ifmt_ctx->streams[i]->codec);
if (ofmt_ctx && ofmt_ctx->nb_streams > i && ofmt_ctx->streams[i] && ofmt_ctx->streams[i]->codec)
avcodec_close(ofmt_ctx->streams[i]->codec);
if (filter_ctx && filter_ctx[i].filter_graph)
avfilter_graph_free(&filter_ctx[i].filter_graph);
}
av_free(filter_ctx);
avformat_close_input(&ifmt_ctx);
if (ofmt_ctx && !(ofmt_ctx->oformat->flags & AVFMT_NOFILE))
avio_closep(&ofmt_ctx->pb);
avformat_free_context(ofmt_ctx);
if (ret < 0)
printf("Error occurred: %s\n", av_err2str(ret));
return ret ? 1 : 0;
}
2 详细解析
看了代码，可能你还意犹未尽。
那么，我先来说说代码的流程。
既然是转码，那整理流程就是，先把一个源文件，比如input.mp4，拆包（DEMUX），获得编码过的数据packet，然后把packet送给decoder解码，获得audio/video的原始数据frame，再把frame送给encoder编码，又变成编码过的数据packet，然后把packet打包(MUX)成另一种格式，比如output.ts。
程序流程图就不说了，代码量那么少，一看就知道。
3 关于PTS
本例最难懂的就是PTS的计算。
time_base
time_base称为时间基准，在不同的阶段(结构体)，每个time_base具体的值不一样，ffmpeg提供函数av_packet_rescale_ts()在各个time_base中进行切换。
time_base主要在2个结构体最常用，一个是AVStream。
//libavformat/avformat.h
typedef struct AVStream {
AVCodecContext *codec;
/**
* This is the fundamental unit of time (in seconds) in terms
* of which frame timestamps are represented.
*
* decoding: set by libavformat
* encoding: May be set by the caller before avformat_write_header() to
* provide a hint to the muxer about the desired timebase. In
* avformat_write_header(), the muxer will overwrite this field
* with the timebase that will actually be used for the timestamps
* written into the file (which may or may not be related to the
* user-provided one, depending on the format).
*/
AVRational time_base;
//......
}一个是AVCodecContext
//libavcodec/avcodec.h
typedef struct AVCodecContext {
const AVClass *av_class;
enum AVMediaType codec_type; /* see AVMEDIA_TYPE_xxx */
const struct AVCodec *codec;
/**
* This is the fundamental unit of time (in seconds) in terms
* of which frame timestamps are represented. For fixed-fps content,
* timebase should be 1/framerate and timestamp increments should be
* identically 1.
* This often, but not always is the inverse of the frame rate or field rate
* for video. 1/time_base is not the average frame rate if the frame rate is not
* constant.
*
* Like containers, elementary streams also can store timestamps, 1/time_base
* is the unit in which these timestamps are specified.
* As example of such codec time base see ISO/IEC 14496-2:2001(E)
* vop_time_increment_resolution and fixed_vop_rate
* (fixed_vop_rate == 0 implies that it is different from the framerate)
*
* - encoding: MUST be set by user.
* - decoding: the use of this field for decoding is deprecated.
* Use framerate instead.
*/
AVRational time_base;AVRational 本身很简单，定义是
typedef struct AVRational{
int num; ///< numerator
int den; ///< denominator
} AVRational;什么意思呢？
以video为例子。
在AVStream的阶段，也就是还没解码之前的原始数据，假设是mp4文件，FPS = 15，那么
time_base={1， 15}，那么
懂第一帧到开始，每一ES packet的PTS就是1， 2， 3，。。。。
为何呢？
简单啊，比如第一帧，packet.pts=1，实际的packet.pts = 1/15 = 0.0666s
同理第二帧，packet.pts=2，实际的packet.pts = 2/15 = 0.1333s
到第15帧，packet.pts=15，实际的packet.pts = 15/15 = 1s
也符合我们的预想嘛！
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