FFMPEG多线程编码器一般以在Slice内分功能模块进行多线程编码，如h263，h263P，msmpeg(v1, v2, v3)，wmv1。包含以下几个线程：(1)Pre_estimation_motion_thread运动估计前的准备;(2)Estimation_motion_thread运动估计;(3)Mb_var_thread宏块其他变量;(4)Encode_thread编码主线程。当然也有例外，如FFV1编码器按Slice为线程单位进行多线程编码。
FFMPEG多线程解码器分为Frame级和Slice级两种，Slice级多线程同时解码一帧中不同的部分。Frame级多线程同时接受多帧码流，实现并行解码，当前帧处于显示状态时，未来的几帧已经在其他线程中被解码。
1. Slice Threading
FFmpeg中，dvvideo_decoder, ffv1_decoder, h264_decoder, mpeg2_video_decoder和mpeg_video_decoder均支持了Slice Threading。
实现方法是：首先为codecContext注册注册多线程处理函数excute()，Codec解码过程中处理Slice时调用avctx->excute()。excute()启动Slice解码工作线程开始多线程解码，同时快速返回开始下一Slice的解析和解码。
Frame Threading主线程和解码线程的同步如图1所示。
图1 Frame Threading主线程和解码线程的同步
2. Frame Threading
目前为止支持Frame Threading的解码器有h264_decoder, huffyuv_decoder, ffvhuff_decoder, mdec_decoder, mimic_decoder, mpeg4_decoder, theora_decoder, vp3_decoder和vp8_decoder。
Frame Threading有如下限制：用户函数draw_horiz_band()必须是线程安全的；为了提升性能，用户应该为codec提供线程安全的get_buffer()回调函数；用户必须能处理多线程带来的延时。另外，支持Frame Threading的codec要求每个包包含一个完整帧。Buffer内容在ff_thread_await_progress()调用之前不能读，同样，包括加边draw_edges()在内的处理，在ff_thread_report_progress()调用之后，Buffer内容不能写。
每个线程都有以下四个状态。如图2所示，为了保证线程安全，若Codec未实现update_thread_context()和线程安全的get_buffer()，则必须在解码完成后才能将状态转换为STATUS_SETUP_FINISHED，意味着下一个线程只能在当前线程解码完成后才能开始解码。
而如图3所示，如果Codec实现update_thread_context()和线程安全的get_buffer()，线程状态可以在解码开始之前转换为STATUS_SETUP_FINISHED，这样，下一个线程就可能与当前线程并行。
图2 Codec未实现update_thread_context()和线程安全的get_buffer()，线程状态转换
图3 Codec实现update_thread_context()和线程安全的get_buffer()，线程状态转换
解码主线程通过调用submit_packet()将码流交给对应的解码线程。主线程和解码线程的同步如图4所示。
图4 Frame Threading主线程和解码线程的同步
/**
* Set the threading algorithms used.//设置线程的使用算法
* Threading requires more than one thread.//需要一个以上的线程
* Frame threading requires entire frames to be passed to the codec,//帧线程需要整个帧被传递到编码解码器
* and introduces extra decoding delay, so is incompatible with low_delay.// 并引入了额外的解码延迟，所以是不符合low_delay
* @param avctx The context.
*/
//有效的线程参数
static void validate_thread_parameters(AVCodecContext *avctx)
{
//实现帧线程支持，需要在配置codec的时候设置codec的capabilities，flags，flags2
int frame_threading_supported = (avctx->codec->capabilities & CODEC_CAP_FRAME_THREADS)
&& !(avctx->flags & CODEC_FLAG_TRUNCATED)
&& !(avctx->flags & CODEC_FLAG_LOW_DELAY)
&& !(avctx->flags2 & CODEC_FLAG2_CHUNKS);
if (avctx->thread_count == 1) { //多线程要在双核以上的机器上才行
avctx->active_thread_type = 0;
} else if (frame_threading_supported && (avctx->thread_type & FF_THREAD_FRAME)) {// 在codec初始化的时候设置avctx->thread_type |=FF_THREAD_FRAME
avctx->active_thread_type = FF_THREAD_FRAME; //线程的类型设置为帧并行
} else if (avctx->codec->capabilities & CODEC_CAP_SLICE_THREADS &&
avctx->thread_type & FF_THREAD_SLICE) { //同样的要实现片级并行则需要在codec初始化的时候设置条件
avctx->active_thread_type = FF_THREAD_SLICE;
} else if (!(avctx->codec->capabilities & CODEC_CAP_AUTO_THREADS)) {
avctx->thread_count = 1;
avctx->active_thread_type = 0;
}
if (avctx->thread_count > MAX_AUTO_THREADS)
av_log(avctx, AV_LOG_WARNING,
"Application has requested %d threads. Using a thread count greater than %d is not recommended.\n",
avctx->thread_count, MAX_AUTO_THREADS);
}
注释：
如果不知到帧与片 可以查看之前的转载文章 http://blog.csdn.net/jwzhangjie/article/details/8739139
//解码函数
int attribute_align_arg avcodec_decode_video2(AVCodecContext *avctx, AVFrame *picture,
int *got_picture_ptr,
const AVPacket *avpkt)
{
AVCodecInternal *avci = avctx->internal;
int ret;
// copy to ensure we do not change avpkt
AVPacket tmp = *avpkt;
if (avctx->codec->type != AVMEDIA_TYPE_VIDEO) { //如果不是视频类型，则退出
av_log(avctx, AV_LOG_ERROR, "Invalid media type for video\n");
return AVERROR(EINVAL);
}
*got_picture_ptr = 0; /
if ((avctx->coded_width || avctx->coded_height) && av_image_check_size(avctx->coded_width, avctx->coded_height, 0, avctx))
return AVERROR(EINVAL);
avcodec_get_frame_defaults(picture);
if (!avctx->refcounted_frames)
av_frame_unref(&avci->to_free);
//如果是帧并行解码的话
//需要在codec初始化的时候设置codec->capabilities |= CODEC_CAP_DELAY，查看注释，设置avctx->active_thread_type |= FF_THREAD_FRAME
if ((avctx->codec->capabilities & CODEC_CAP_DELAY) || avpkt->size || (avctx->active_thread_type & FF_THREAD_FRAME)) {
int did_split = av_packet_split_side_data(&tmp);
apply_param_change(avctx, &tmp);
avctx->pkt = &tmp;
//需要配置configure --enable-pthreads(我现在最新的默认生成的config.h就是#define HAVE_THREADS 1) ,设置avctx->active_thread_type |= FF_THREAD_FRAME
if (HAVE_THREADS && avctx->active_thread_type & FF_THREAD_FRAME)
ret = ff_thread_decode_frame(avctx, picture, got_picture_ptr,&tmp);//进入帧级并行解码
else {//进入单线程解码
ret = avctx->codec->decode(avctx, picture, got_picture_ptr, &tmp);
picture->pkt_dts = avpkt->dts;
if(!avctx->has_b_frames){
av_frame_set_pkt_pos(picture, avpkt->pos);
}
//FIXME these should be under if(!avctx->has_b_frames)
/* get_buffer is supposed to set frame parameters */
if (!(avctx->codec->capabilities & CODEC_CAP_DR1)) {
if (!picture->sample_aspect_ratio.num) picture->sample_aspect_ratio = avctx->sample_aspect_ratio;
if (!picture->width) picture->width = avctx->width;
if (!picture->height) picture->height = avctx->height;
if (picture->format == AV_PIX_FMT_NONE) picture->format = avctx->pix_fmt;
}
}
add_metadata_from_side_data(avctx, picture);
emms_c(); //needed to avoid an emms_c() call before every return;
avctx->pkt = NULL;
if (did_split) {
ff_packet_free_side_data(&tmp);
if(ret == tmp.size)
ret = avpkt->size;
}
if (ret < 0 && picture->data[0])
av_frame_unref(picture);
if (*got_picture_ptr) {
if (!avctx->refcounted_frames) {
avci->to_free = *picture;
avci->to_free.extended_data = avci->to_free.data;
}
avctx->frame_number++;
av_frame_set_best_effort_timestamp(picture,
guess_correct_pts(avctx,
picture->pkt_pts,
picture->pkt_dts));
}
} else
ret = 0;
/* many decoders assign whole AVFrames, thus overwriting extended_data;
* make sure it's set correctly */
//许多解码器分配整个的AVFrames ，从而覆盖extended_data相关
picture->extended_data = picture->data;
return ret;
}
注释：
/**
* Encoder or decoder requires flushing with NULL input at the end in order to
* give the complete and correct output.
*
* NOTE: If this flag is not set, the codec is guaranteed to never be fed with
* with NULL data. The user can still send NULL data to the public encode
* or decode function, but libavcodec will not pass it along to the codec
* unless this flag is set.
*
* Decoders:
* The decoder has a non-zero delay and needs to be fed with avpkt->data=NULL,
* avpkt->size=0 at the end to get the delayed data until the decoder no longer
* returns frames.
*
* Encoders:
* The encoder needs to be fed with NULL data at the end of encoding until the
* encoder no longer returns data.
*
* NOTE: For encoders implementing the AVCodec.encode2() function, setting this
* flag also means that the encoder must set the pts and duration for
* each output packet. If this flag is not set, the pts and duration will
* be determined by libavcodec from the input frame.
*/
#define CODEC_CAP_DELAY 0x0020
//帧级解码函数，在avcodec_open2的时候，就会判断片还是帧解码，分析见下一篇
int ff_thread_decode_frame(AVCodecContext *avctx,
AVFrame *picture, int *got_picture_ptr,
AVPacket *avpkt)
{
FrameThreadContext *fctx = avctx->thread_opaque; //thread_opaque见注释1
int finished = fctx->next_finished;//从返回结果输出的下一个内容
PerThreadContext *p;//查看注释2
int err;
/*
* Submit a packet to the next decoding thread.
*提交数据包发送到下一个解码线程
*/
p = &fctx->threads[fctx->next_decoding];
err = update_context_from_user(p->avctx, avctx);//将用户提交的avctx复制到p->avctx里面
if (err) return err;
err = submit_packet(p, avpkt);
if (err) return err;
/*
* If we're still receiving the initial packets, don't return a frame.
*如果我们仍然接收的初步数据包，不返回帧
*/
if (fctx->delaying) {
if (fctx->next_decoding >= (avctx->thread_count-1)) fctx->delaying = 0;
*got_picture_ptr=0;
if (avpkt->size)
return avpkt->size;
}
/*
* Return the next available frame from the oldest thread.//返回下一个可用帧从最早的线程
* If we're at the end of the stream, then we have to skip threads that
* didn't output a frame, because we don't want to accidentally signal
* EOF (avpkt->size == 0 && *got_picture_ptr == 0).//因为我们不希望发生意外信号EOF，所以在流的最后，我们必须跳过线程，而不输出帧
*/
do {
p = &fctx->threads[finished++];
if (p->state != STATE_INPUT_READY) {
pthread_mutex_lock(&p->progress_mutex);
while (p->state != STATE_INPUT_READY)//如果数据的状态不是STATE_INPUT_READY，线程进行等待，直到线程pthread_cond_signal(&p->progress_cond);
pthread_cond_wait(&p->output_cond, &p->progress_mutex);
pthread_mutex_unlock(&p->progress_mutex);
}
av_frame_move_ref(picture, &p->frame);
*got_picture_ptr = p->got_frame;
picture->pkt_dts = p->avpkt.dts;
/*
* A later call with avkpt->size == 0 may loop over all threads,
* including this one, searching for a frame to return before being
* stopped by the "finished != fctx->next_finished" condition.
* Make sure we don't mistakenly return the same frame again.
*/
p->got_frame = 0;
if (finished >= avctx->thread_count) finished = 0;
} while (!avpkt->size && !*got_picture_ptr && finished != fctx->next_finished);
update_context_from_thread(avctx, p->avctx, 1);
if (fctx->next_decoding >= avctx->thread_count) fctx->next_decoding = 0;
fctx->next_finished = finished;
/* return the size of the consumed packet if no error occurred */
return (p->result >= 0) ? avpkt->size : p->result;
}
注释：
1.
/**
* thread opaque
* Can be used by execute() to store some per AVCodecContext stuff.
//能够使用execute()函数来存储每个AVCodecContext数据
* - encoding: set by execute()
* - decoding: set by execute()
*/
void *thread_opaque;
2.
/**
* Context used by codec threads and stored in their AVCodecContext thread_opaque.
*上下文所使用的编解码器线程和存储在他们的AVCodecContext thread_opaque
*/
typedef struct PerThreadContext {
struct FrameThreadContext *parent;
pthread_t thread;
int thread_init;
pthread_cond_t input_cond; ///< Used to wait for a new packet from the main thread.从主线程等待一个新的数据包。
pthread_cond_t progress_cond; ///< Used by child threads to wait for progress to change.
pthread_cond_t output_cond; ///< Used by the main thread to wait for frames to finish.用于由主线程等待帧来完成
pthread_mutex_t mutex; ///< Mutex used to protect the contents of the PerThreadContext.
pthread_mutex_t progress_mutex; ///< Mutex used to protect frame progress values and progress_cond.
AVCodecContext *avctx; ///< Context used to decode packets passed to this thread.
AVPacket avpkt; ///< Input packet (for decoding) or output (for encoding).
uint8_t *buf; ///< backup storage for packet data when the input packet is not refcounted
int allocated_buf_size; ///< Size allocated for buf
AVFrame frame; ///< Output frame (for decoding) or input (for encoding).
int got_frame; ///< The output of got_picture_ptr from the last avcodec_decode_video() call.
int result; ///< The result of the last codec decode/encode() call.
enum { //这个可以看之前文章中提到的帧级解码状态图
STATE_INPUT_READY, ///< Set when the thread is awaiting a packet.
STATE_SETTING_UP, ///< Set before the codec has called ff_thread_finish_setup().
STATE_GET_BUFFER, /**<
* Set when the codec calls get_buffer().
* State is returned to STATE_SETTING_UP afterwards.
*/
STATE_SETUP_FINISHED ///< Set after the codec has called ff_thread_finish_setup().
} state;
/**
* Array of frames passed to ff_thread_release_buffer().
* Frames are released after all threads referencing them are finished.
*/
AVFrame *released_buffers;
int num_released_buffers;
int released_buffers_allocated;
AVFrame *requested_frame; ///< AVFrame the codec passed to get_buffer()
int requested_flags; ///< flags passed to get_buffer() for requested_frame
} PerThreadContext;
//在初始化codec后，接下来就是打开解码器
int attribute_align_arg avcodec_open2(AVCodecContext *avctx, const AVCodec *codec, AVDictionary **options)
{
int ret = 0;
AVDictionary *tmp = NULL;
if (avcodec_is_open(avctx)) //返回avctx-》internal，见注释1
return 0;
if ((!codec && !avctx->codec)) {
av_log(avctx, AV_LOG_ERROR, "No codec provided to avcodec_open2()\n");
return AVERROR(EINVAL);
}
if ((codec && avctx->codec && codec != avctx->codec)) {
av_log(avctx, AV_LOG_ERROR, "This AVCodecContext was allocated for %s, "
"but %s passed to avcodec_open2()\n", avctx->codec->name, codec->name);
return AVERROR(EINVAL);
}
if (!codec)
codec = avctx->codec;
//判断额外数据，在硬解吗h264的时候必须人为设置sps pps给extradata
if (avctx->extradata_size < 0 || avctx->extradata_size >= FF_MAX_EXTRADATA_SIZE)
return AVERROR(EINVAL);
if (options) //一般options设置NULL
av_dict_copy(&tmp, *options, 0);
ret = ff_lock_avcodec(avctx);
if (ret < 0)
return ret;
avctx->internal = av_mallocz(sizeof(AVCodecInternal));
if (!avctx->internal) {
ret = AVERROR(ENOMEM);
goto end;
}
avctx->internal->pool = av_mallocz(sizeof(*avctx->internal->pool));
if (!avctx->internal->pool) {
ret = AVERROR(ENOMEM);
goto free_and_end;
}
if (codec->priv_data_size > 0) {
if (!avctx->priv_data) {
avctx->priv_data = av_mallocz(codec->priv_data_size);
if (!avctx->priv_data) {
ret = AVERROR(ENOMEM);
goto end;
}
if (codec->priv_class) {
*(const AVClass **)avctx->priv_data = codec->priv_class;
av_opt_set_defaults(avctx->priv_data);
}
}
if (codec->priv_class && (ret = av_opt_set_dict(avctx->priv_data, &tmp)) < 0)
goto free_and_end;
} else {
avctx->priv_data = NULL;
}//上面主要是分配内存
if ((ret = av_opt_set_dict(avctx, &tmp)) < 0)
goto free_and_end;
// only call avcodec_set_dimensions() for non H.264/VP6F codecs so as not to overwrite previously setup dimensions
if (!(avctx->coded_width && avctx->coded_height && avctx->width && avctx->height &&
(avctx->codec_id == AV_CODEC_ID_H264 || avctx->codec_id == AV_CODEC_ID_VP6F))) {
if (avctx->coded_width && avctx->coded_height)
avcodec_set_dimensions(avctx, avctx->coded_width, avctx->coded_height);
else if (avctx->width && avctx->height)
avcodec_set_dimensions(avctx, avctx->width, avctx->height);
}
if ((avctx->coded_width || avctx->coded_height || avctx->width || avctx->height)
&& ( av_image_check_size(avctx->coded_width, avctx->coded_height, 0, avctx) < 0
|| av_image_check_size(avctx->width, avctx->height, 0, avctx) < 0)) {
av_log(avctx, AV_LOG_WARNING, "Ignoring invalid width/height values\n");
avcodec_set_dimensions(avctx, 0, 0);
}
/* if the decoder init function was already called previously,如果解码器的init函数已经调用过
* free the already allocated subtitle_header before overwriting it 释放已经分配subtitle_header ，在它覆盖前
*/
if (av_codec_is_decoder(codec))
av_freep(&avctx->subtitle_header);
if (avctx->channels > FF_SANE_NB_CHANNELS) {
ret = AVERROR(EINVAL);
goto free_and_end;
}
avctx->codec = codec;
if ((avctx->codec_type == AVMEDIA_TYPE_UNKNOWN || avctx->codec_type == codec->type) &&
avctx->codec_id == AV_CODEC_ID_NONE) {
avctx->codec_type = codec->type;
avctx->codec_id = codec->id;
}
if (avctx->codec_id != codec->id || (avctx->codec_type != codec->type
&& avctx->codec_type != AVMEDIA_TYPE_ATTACHMENT)) {
av_log(avctx, AV_LOG_ERROR, "Codec type or id mismatches\n");
ret = AVERROR(EINVAL);
goto free_and_end;
}
avctx->frame_number = 0;
avctx->codec_descriptor = avcodec_descriptor_get(avctx->codec_id);
if (avctx->codec->capabilities & CODEC_CAP_EXPERIMENTAL &&
avctx->strict_std_compliance > FF_COMPLIANCE_EXPERIMENTAL) {
const char *codec_string = av_codec_is_encoder(codec) ? "encoder" : "decoder";
AVCodec *codec2;
av_log(NULL, AV_LOG_ERROR,
"The %s '%s' is experimental but experimental codecs are not enabled, "
"add '-strict %d' if you want to use it.\n",
codec_string, codec->name, FF_COMPLIANCE_EXPERIMENTAL);
codec2 = av_codec_is_encoder(codec) ? avcodec_find_encoder(codec->id) : avcodec_find_decoder(codec->id);
if (!(codec2->capabilities & CODEC_CAP_EXPERIMENTAL))
av_log(NULL, AV_LOG_ERROR, "Alternatively use the non experimental %s '%s'.\n",
codec_string, codec2->name);
ret = AVERROR_EXPERIMENTAL;
goto free_and_end;
}
if (avctx->codec_type == AVMEDIA_TYPE_AUDIO &&
(!avctx->time_base.num || !avctx->time_base.den)) {
avctx->time_base.num = 1;
avctx->time_base.den = avctx->sample_rate;
}
if (!HAVE_THREADS)
av_log(avctx, AV_LOG_WARNING, "Warning: not compiled with thread support, using thread emulation\n");
if (CONFIG_FRAME_THREAD_ENCODER) {
ff_unlock_avcodec(); //we will instanciate a few encoders thus kick the counter to prevent false detection of a problem
ret = ff_frame_thread_encoder_init(avctx, options ? *options : NULL);
ff_lock_avcodec(avctx);
if (ret < 0)
goto free_and_end;
}
//帧线程编码
if (HAVE_THREADS && !avctx->thread_opaque
&& !(avctx->internal->frame_thread_encoder && (avctx->active_thread_type&FF_THREAD_FRAME))) {
ret = ff_thread_init(avctx);//会根据用户设置不同的变量来判断是片，帧，来初始化线程
if (ret < 0) {
goto free_and_end;
}
}
if (!HAVE_THREADS && !(codec->capabilities & CODEC_CAP_AUTO_THREADS))
avctx->thread_count = 1;
if (avctx->codec->max_lowres < avctx->lowres || avctx->lowres < 0) {
av_log(avctx, AV_LOG_ERROR, "The maximum value for lowres supported by the decoder is %d\n",
avctx->codec->max_lowres);
ret = AVERROR(EINVAL);
goto free_and_end;
}
//下面就会根据是编码器还是解码器来初始化
if (av_codec_is_encoder(avctx->codec)) {
int i;
if (avctx->codec->sample_fmts) {
for (i = 0; avctx->codec->sample_fmts[i] != AV_SAMPLE_FMT_NONE; i++) {
if (avctx->sample_fmt == avctx->codec->sample_fmts[i])
break;
if (avctx->channels == 1 &&
av_get_planar_sample_fmt(avctx->sample_fmt) ==
av_get_planar_sample_fmt(avctx->codec->sample_fmts[i])) {
avctx->sample_fmt = avctx->codec->sample_fmts[i];
break;
}
}
if (avctx->codec->sample_fmts[i] == AV_SAMPLE_FMT_NONE) {
char buf[128];
snprintf(buf, sizeof(buf), "%d", avctx->sample_fmt);
av_log(avctx, AV_LOG_ERROR, "Specified sample format %s is invalid or not supported\n",
(char *)av_x_if_null(av_get_sample_fmt_name(avctx->sample_fmt), buf));
ret = AVERROR(EINVAL);
goto free_and_end;
}
}
if (avctx->codec->pix_fmts) {
for (i = 0; avctx->codec->pix_fmts[i] != AV_PIX_FMT_NONE; i++)
if (avctx->pix_fmt == avctx->codec->pix_fmts[i])
break;
if (avctx->codec->pix_fmts[i] == AV_PIX_FMT_NONE
&& !((avctx->codec_id == AV_CODEC_ID_MJPEG || avctx->codec_id == AV_CODEC_ID_LJPEG)
&& avctx->strict_std_compliance <= FF_COMPLIANCE_UNOFFICIAL)) {
char buf[128];
snprintf(buf, sizeof(buf), "%d", avctx->pix_fmt);
av_log(avctx, AV_LOG_ERROR, "Specified pixel format %s is invalid or not supported\n",
(char *)av_x_if_null(av_get_pix_fmt_name(avctx->pix_fmt), buf));
ret = AVERROR(EINVAL);
goto free_and_end;
}
}
if (avctx->codec->supported_samplerates) {
for (i = 0; avctx->codec->supported_samplerates[i] != 0; i++)
if (avctx->sample_rate == avctx->codec->supported_samplerates[i])
break;
if (avctx->codec->supported_samplerates[i] == 0) {
av_log(avctx, AV_LOG_ERROR, "Specified sample rate %d is not supported\n",
avctx->sample_rate);
ret = AVERROR(EINVAL);
goto free_and_end;
}
}
if (avctx->codec->channel_layouts) {
if (!avctx->channel_layout) {
av_log(avctx, AV_LOG_WARNING, "Channel layout not specified\n");
} else {
for (i = 0; avctx->codec->channel_layouts[i] != 0; i++)
if (avctx->channel_layout == avctx->codec->channel_layouts[i])
break;
if (avctx->codec->channel_layouts[i] == 0) {
char buf[512];
av_get_channel_layout_string(buf, sizeof(buf), -1, avctx->channel_layout);
av_log(avctx, AV_LOG_ERROR, "Specified channel layout '%s' is not supported\n", buf);
ret = AVERROR(EINVAL);
goto free_and_end;
}
}
}
if (avctx->channel_layout && avctx->channels) {
int channels = av_get_channel_layout_nb_channels(avctx->channel_layout);
if (channels != avctx->channels) {
char buf[512];
av_get_channel_layout_string(buf, sizeof(buf), -1, avctx->channel_layout);
av_log(avctx, AV_LOG_ERROR,
"Channel layout '%s' with %d channels does not match number of specified channels %d\n",
buf, channels, avctx->channels);
ret = AVERROR(EINVAL);
goto free_and_end;
}
} else if (avctx->channel_layout) {
avctx->channels = av_get_channel_layout_nb_channels(avctx->channel_layout);
}
if(avctx->codec_type == AVMEDIA_TYPE_VIDEO &&
avctx->codec_id != AV_CODEC_ID_PNG // For mplayer
) {
if (avctx->width <= 0 || avctx->height <= 0) {
av_log(avctx, AV_LOG_ERROR, "dimensions not set\n");
ret = AVERROR(EINVAL);
goto free_and_end;
}
}
if ( (avctx->codec_type == AVMEDIA_TYPE_VIDEO || avctx->codec_type == AVMEDIA_TYPE_AUDIO)
&& avctx->bit_rate>0 && avctx->bit_rate<1000) {
av_log(avctx, AV_LOG_WARNING, "Bitrate %d is extremely low, maybe you mean %dk\n", avctx->bit_rate, avctx->bit_rate);
}
if (!avctx->rc_initial_buffer_occupancy)
avctx->rc_initial_buffer_occupancy = avctx->rc_buffer_size * 3 / 4;
}
avctx->pts_correction_num_faulty_pts =
avctx->pts_correction_num_faulty_dts = 0;
avctx->pts_correction_last_pts =
avctx->pts_correction_last_dts = INT64_MIN;
if ( avctx->codec->init && (!(avctx->active_thread_type&FF_THREAD_FRAME)
|| avctx->internal->frame_thread_encoder)) {
ret = avctx->codec->init(avctx);
if (ret < 0) {
goto free_and_end;
}
}
ret=0;
//上面部分是判断codec是否是编码器，然后如果是编码器则进行选择编码的方式，这里我们只讨论解码，下面的解码器也是一样，根据不同的条件，进行编码初始化
if (av_codec_is_decoder(avctx->codec)) {
if (!avctx->bit_rate)
avctx->bit_rate = get_bit_rate(avctx);//如果没有设置bit_rate则会更加avctx类型将avctx->bit_rate复制给
/* validate channel layout from the decoder */
if (avctx->channel_layout) {
int channels = av_get_channel_layout_nb_channels(avctx->channel_layout);
if (!avctx->channels)
avctx->channels = channels;
else if (channels != avctx->channels) {
char buf[512];
av_get_channel_layout_string(buf, sizeof(buf), -1, avctx->channel_layout);
av_log(avctx, AV_LOG_WARNING,
"Channel layout '%s' with %d channels does not match specified number of channels %d: "
"ignoring specified channel layout\n",
buf, channels, avctx->channels);
avctx->channel_layout = 0;
}
}
if (avctx->channels && avctx->channels < 0 ||
avctx->channels > FF_SANE_NB_CHANNELS) {
ret = AVERROR(EINVAL);
goto free_and_end;
}
if (avctx->sub_charenc) {
if (avctx->codec_type != AVMEDIA_TYPE_SUBTITLE) {
av_log(avctx, AV_LOG_ERROR, "Character encoding is only "
"supported with subtitles codecs\n");
ret = AVERROR(EINVAL);
goto free_and_end;
} else if (avctx->codec_descriptor->props & AV_CODEC_PROP_BITMAP_SUB) {
av_log(avctx, AV_LOG_WARNING, "Codec '%s' is bitmap-based, "
"subtitles character encoding will be ignored\n",
avctx->codec_descriptor->name);
avctx->sub_charenc_mode = FF_SUB_CHARENC_MODE_DO_NOTHING;
} else {
/* input character encoding is set for a text based subtitle
* codec at this point */
if (avctx->sub_charenc_mode == FF_SUB_CHARENC_MODE_AUTOMATIC)
avctx->sub_charenc_mode = FF_SUB_CHARENC_MODE_PRE_DECODER;
if (avctx->sub_charenc_mode == FF_SUB_CHARENC_MODE_PRE_DECODER) {
#if CONFIG_ICONV
iconv_t cd = iconv_open("UTF-8", avctx->sub_charenc);
if (cd == (iconv_t)-1) {
av_log(avctx, AV_LOG_ERROR, "Unable to open iconv context "
"with input character encoding \"%s\"\n", avctx->sub_charenc);
ret = AVERROR(errno);
goto free_and_end;
}
iconv_close(cd);
#else
av_log(avctx, AV_LOG_ERROR, "Character encoding subtitles "
"conversion needs a libavcodec built with iconv support "
"for this codec\n");
ret = AVERROR(ENOSYS);
goto free_and_end;
#endif
}
}
}
}
end:
ff_unlock_avcodec();
if (options) {
av_dict_free(options);
*options = tmp;
}
return ret;
free_and_end:
av_dict_free(&tmp);
av_freep(&avctx->priv_data);
if (avctx->internal)
av_freep(&avctx->internal->pool);
av_freep(&avctx->internal);
avctx->codec = NULL;
goto end;
}
注释：
1. /**
* Private context used for internal data.
*私有的上下文内容用于内部数据
* Unlike priv_data, this is not codec-specific. It is used in general
* libavcodec functions.
*不像priv_data ，这是不特定的编解码器。这是用在一般libavcodec的功能
*/
struct AVCodecInternal *internal;
//用来判断是帧还是片线程初始化
int ff_thread_init(AVCodecContext *avctx)
{
if (avctx->thread_opaque) {
av_log(avctx, AV_LOG_ERROR, "avcodec_thread_init is ignored after avcodec_open\n");
return -1;
}
#if HAVE_W32THREADS
w32thread_init();
#endif
if (avctx->codec) {
//有效的线程参数，将会设置avctx->active_thread_type，进而判断进入下面if的哪个条件
validate_thread_parameters(avctx);//查看FFmepg 多线程解码历程 - 1:validate_thread_parameters 讲解
if (avctx->active_thread_type&FF_THREAD_SLICE)
return thread_init(avctx);
else if (avctx->active_thread_type&FF_THREAD_FRAME)
return frame_thread_init(avctx); //下一篇讲解frame_thread_init的具体内容
}
return 0;
}
//ff_thread_init选择帧线程初始化，就会进入frame_thread_init
static int frame_thread_init(AVCodecContext *avctx)
{
int thread_count = avctx->thread_count;
const AVCodec *codec = avctx->codec;
AVCodecContext *src = avctx;
FrameThreadContext *fctx;
int i, err = 0;
//如果我们在初始化codec的时候没有设置thread_count或者设置为0，则就会自动根据手机设备来获取
if (!thread_count) {
int nb_cpus = ff_get_logical_cpus(avctx);
if ((avctx->debug & (FF_DEBUG_VIS_QP | FF_DEBUG_VIS_MB_TYPE)) || avctx->debug_mv)
nb_cpus = 1;
// use number of cores + 1 as thread count if there is more than one
if (nb_cpus > 1)
thread_count = avctx->thread_count = FFMIN(nb_cpus + 1, MAX_AUTO_THREADS); //#define FFMIN(a,b) ((a) > (b) ? (b) : (a))
else
thread_count = avctx->thread_count = 1;
}
if (thread_count <= 1) { //如果thread_count <= 1那么即使进入帧级线程初始化，在解码的时候也不会调用帧级解码函数的
avctx->active_thread_type = 0;
return 0;
}
avctx->thread_opaque = fctx = av_mallocz(sizeof(FrameThreadContext));
fctx->threads = av_mallocz(sizeof(PerThreadContext) * thread_count); //相当与初始化线程池，个数就是thread_count
pthread_mutex_init(&fctx->buffer_mutex, NULL);
fctx->delaying = 1;
for (i = 0; i < thread_count; i++) {
AVCodecContext *copy = av_malloc(sizeof(AVCodecContext));
PerThreadContext *p = &fctx->threads[i];
pthread_mutex_init(&p->mutex, NULL);
pthread_mutex_init(&p->progress_mutex, NULL);
pthread_cond_init(&p->input_cond, NULL);
pthread_cond_init(&p->progress_cond, NULL);
pthread_cond_init(&p->output_cond, NULL);
p->parent = fctx;
p->avctx = copy;
if (!copy) {
err = AVERROR(ENOMEM);
goto error;
}
*copy = *src;
copy->thread_opaque = p;
copy->pkt = &p->avpkt;
if (!i) {
src = copy;
if (codec->init)
err = codec->init(copy);
//更新下一个线程的AVCodecContext参考线程的上下文中的值
update_context_from_thread(avctx, copy, 1);
} else {
copy->priv_data = av_malloc(codec->priv_data_size);
if (!copy->priv_data) {
err = AVERROR(ENOMEM);
goto error;
}
memcpy(copy->priv_data, src->priv_data, codec->priv_data_size);
copy->internal = av_malloc(sizeof(AVCodecInternal));
if (!copy->internal) {
err = AVERROR(ENOMEM);
goto error;
}
*copy->internal = *src->internal;
copy->internal->is_copy = 1;
if (codec->init_thread_copy)
err = codec->init_thread_copy(copy);
}
if (err) goto error;
///下面出创建线程，调用frame_worker_thread
err = AVERROR(pthread_create(&p->thread, NULL, frame_worker_thread, p));
p->thread_init= !err;
if(!p->thread_init)
goto error;
}
return 0;
error:
frame_thread_free(avctx, i+1);
return err;
}
/在ff_thread_decode_frame中会调用submit_packet将码流交给对应的解码线程，来实现线程状态的改变，具体的流程图见下面图
static int submit_packet(PerThreadContext *p, AVPacket *avpkt)
{
FrameThreadContext *fctx = p->parent;
PerThreadContext *prev_thread = fctx->prev_thread;
const AVCodec *codec = p->avctx->codec;
if (!avpkt->size && !(codec->capabilities & CODEC_CAP_DELAY)) return 0;
pthread_mutex_lock(&p->mutex);
release_delayed_buffers(p);
if (prev_thread) {
int err;
if (prev_thread->state == STATE_SETTING_UP) {
pthread_mutex_lock(&prev_thread->progress_mutex);
while (prev_thread->state == STATE_SETTING_UP) //判断之前的线程状态为STATE_SETTING_UP的时候，进行线程的等待，
pthread_cond_wait(&prev_thread->progress_cond, &prev_thread->progress_mutex);
pthread_mutex_unlock(&prev_thread->progress_mutex);
}
err = update_context_from_thread(p->avctx, prev_thread->avctx, 0);//如果之前的线程状态变为STATE_SETUP_FINISHED，然后从线程中哦功能更新内容
if (err) {
pthread_mutex_unlock(&p->mutex);
return err;
}
}
av_buffer_unref(&p->avpkt.buf);
p->avpkt = *avpkt;
if (avpkt->buf)
p->avpkt.buf = av_buffer_ref(avpkt->buf);
else {
av_fast_malloc(&p->buf, &p->allocated_buf_size, avpkt->size + FF_INPUT_BUFFER_PADDING_SIZE);
p->avpkt.data = p->buf;
memcpy(p->buf, avpkt->data, avpkt->size);
memset(p->buf + avpkt->size, 0, FF_INPUT_BUFFER_PADDING_SIZE);
}
p->state = STATE_SETTING_UP;
pthread_cond_signal(&p->input_cond);//发送码流输入准备好的信号给解码线程pthread_cond_waite(&p->input_cond),让解码线程
pthread_mutex_unlock(&p->mutex);
/*
* If the client doesn't have a thread-safe get_buffer(),
* then decoding threads call back to the main thread,
* and it calls back to the client here.
*/
if (!p->avctx->thread_safe_callbacks && (
#if FF_API_GET_BUFFER
p->avctx->get_buffer ||
#endif
p->avctx->get_buffer2 != avcodec_default_get_buffer2)) {
while (p->state != STATE_SETUP_FINISHED && p->state != STATE_INPUT_READY) {
pthread_mutex_lock(&p->progress_mutex);
while (p->state == STATE_SETTING_UP)
pthread_cond_wait(&p->progress_cond, &p->progress_mutex);
if (p->state == STATE_GET_BUFFER) {
p->result = ff_get_buffer(p->avctx, p->requested_frame, p->requested_flags);
p->state = STATE_SETTING_UP;
pthread_cond_signal(&p->progress_cond);
}
pthread_mutex_unlock(&p->progress_mutex);
}
}
fctx->prev_thread = p;
fctx->next_decoding++;
return 0;
}
主线程接收数据，准备完毕后发出pthread_cond_signal(&p->input_cond)信号，然后解码线程接收到以后，设置线程状态为finish_setup，发送广播p->process_cond，进行解码，
解码线程解完以后，发送p->progress_cond信号，主线程回调，从解码线程中获取get_buffer，解码之后的数据，然后发送p->progress_cond信号给解码线程，然后解码线程发出p->output_cond信号给主线程，主线程输出解码之后的frame给用户
注释1：