ffmpeg源码分析：transcode_init()函数

transcode_init()函数是在转换前做准备工作的.此处看一下它的真面目,不废话,看注释吧:

//为转换过程做准备
static int transcode_init(OutputFile *output_files,
		int nb_output_files,
		InputFile *input_files,
		int nb_input_files)
{
	int ret = 0, i, j, k;
	AVFormatContext *oc;
	AVCodecContext *codec, *icodec;
	OutputStream *ost;
	InputStream *ist;
	char error[1024];
	int want_sdp = 1;

	/* init framerate emulation */
	//初始化帧率仿真(转换时是不按帧率来的,但如果要求帧率仿真,就可以做到)
	for (i = 0; i < nb_input_files; i++)
	{
		InputFile *ifile = &input_files[i];
		//如果一个输入文件被要求帧率仿真(指的是即使是转换也像播放那样按照帧率来进行),
		//则为这个文件中所有流记录下开始时间
		if (ifile->rate_emu)
			for (j = 0; j < ifile->nb_streams; j++)
				input_streams[j + ifile->ist_index].start = av_gettime();
	}

	/* output stream init */
	for (i = 0; i < nb_output_files; i++)
	{
		//什么也没做,只是做了个判断而已
		oc = output_files[i].ctx;
		if (!oc->nb_streams && !(oc->oformat->flags & AVFMT_NOSTREAMS))
		{
			av_dump_format(oc, i, oc->filename, 1);
			av_log(NULL, AV_LOG_ERROR,
					"Output file #%d does not contain any stream\n", i);
			return AVERROR(EINVAL);
		}
	}

	//轮循所有的输出流,跟据对应的输入流,设置其编解码器的参数
	for (i = 0; i < nb_output_streams; i++)
	{
		//轮循所有的输出流
		ost = &output_streams[i];
		//输出流对应的FormatContext
		oc = output_files[ost->file_index].ctx;
		//取得输出流对应的输入流
		ist = &input_streams[ost->source_index];

		//attachment_filename是不是这样的东西:一个文件,它单独容纳一个输出流?此处不懂
		if (ost->attachment_filename)
			continue;

		codec = ost->st->codec;//输出流的编解码器结构
		icodec = ist->st->codec;//输入流的编解码器结构

		//先把能复制的复制一下
		ost->st->disposition = ist->st->disposition;
		codec->bits_per_raw_sample = icodec->bits_per_raw_sample;
		codec->chroma_sample_location = icodec->chroma_sample_location;

		//如果只是复制一个流(不用解码后再编码),则把输入流的编码参数直接复制给输出流
		//此时是不需要解码也不需要编码的，所以不需打开解码器和编码器
		if (ost->stream_copy)
		{
			//计算输出流的编解码器的extradata的大小,然后分配容纳extradata的缓冲
			//然后把输入流的编解码器的extradata复制到输出流的编解码器中
			uint64_t extra_size = (uint64_t) icodec->extradata_size
					+ FF_INPUT_BUFFER_PADDING_SIZE;

			if (extra_size > INT_MAX)	{
				return AVERROR(EINVAL);
			}

			/* if stream_copy is selected, no need to decode or encode */
			codec->codec_id = icodec->codec_id;
			codec->codec_type = icodec->codec_type;

			if (!codec->codec_tag){
				if (!oc->oformat->codec_tag
					||av_codec_get_id(oc->oformat->codec_tag,icodec->codec_tag) == codec->codec_id
					||av_codec_get_tag(oc->oformat->codec_tag,icodec->codec_id) <= 0)
					codec->codec_tag = icodec->codec_tag;
			}

			codec->bit_rate = icodec->bit_rate;
			codec->rc_max_rate = icodec->rc_max_rate;
			codec->rc_buffer_size = icodec->rc_buffer_size;
			codec->extradata = av_mallocz(extra_size);
			if (!codec->extradata){
				return AVERROR(ENOMEM);
			}
			memcpy(codec->extradata, icodec->extradata, icodec->extradata_size);
			codec->extradata_size = icodec->extradata_size;

			//重新鼓捣一下time base(这家伙就是帧率)
			codec->time_base = ist->st->time_base;
			//如果输出文件是avi,做一点特殊处理
			if (!strcmp(oc->oformat->name, "avi"))	{
				if (copy_tb < 0
						&& av_q2d(icodec->time_base) * icodec->ticks_per_frame	>
							2 * av_q2d(ist->st->time_base)
						&& av_q2d(ist->st->time_base) < 1.0 / 500
						|| copy_tb == 0)
				{
					codec->time_base = icodec->time_base;
					codec->time_base.num *= icodec->ticks_per_frame;
					codec->time_base.den *= 2;
				}
			}
			else if (!(oc->oformat->flags & AVFMT_VARIABLE_FPS))
			{
				if (copy_tb < 0
						&& av_q2d(icodec->time_base) * icodec->ticks_per_frame
								> av_q2d(ist->st->time_base)
						&& av_q2d(ist->st->time_base) < 1.0 / 500
						|| copy_tb == 0)
				{
					codec->time_base = icodec->time_base;
					codec->time_base.num *= icodec->ticks_per_frame;
				}
			}

			//再修正一下帧率
			av_reduce(&codec->time_base.num, &codec->time_base.den,
					codec->time_base.num, codec->time_base.den, INT_MAX);

			//单独复制各不同媒体自己的编码参数
			switch (codec->codec_type)
			{
			case AVMEDIA_TYPE_AUDIO:
				//音频的
				if (audio_volume != 256){
					av_log(	NULL,AV_LOG_FATAL,
							"-acodec copy and -vol are incompatible (frames are not decoded)\n");
					exit_program(1);
				}
				codec->channel_layout = icodec->channel_layout;
				codec->sample_rate = icodec->sample_rate;
				codec->channels = icodec->channels;
				codec->frame_size = icodec->frame_size;
				codec->audio_service_type = icodec->audio_service_type;
				codec->block_align = icodec->block_align;
				break;
			case AVMEDIA_TYPE_VIDEO:
				//视频的
				codec->pix_fmt = icodec->pix_fmt;
				codec->width = icodec->width;
				codec->height = icodec->height;
				codec->has_b_frames = icodec->has_b_frames;
				if (!codec->sample_aspect_ratio.num){
					codec->sample_aspect_ratio = ost->st->sample_aspect_ratio =
							ist->st->sample_aspect_ratio.num ?ist->st->sample_aspect_ratio :
									ist->st->codec->sample_aspect_ratio.num ?ist->st->codec->sample_aspect_ratio :(AVRational){0, 1};
				}
				ost->st->avg_frame_rate = ist->st->avg_frame_rate;
                break;
            case AVMEDIA_TYPE_SUBTITLE:
            	//字幕的
                codec->width  = icodec->width;
                codec->height = icodec->height;
                break;
            case AVMEDIA_TYPE_DATA:
            case AVMEDIA_TYPE_ATTACHMENT:
            	//??的
                break;
            default:
                abort();
            }
	}
	else
	{
		//如果不是复制,就麻烦多了

		//获取编码器
		if (!ost->enc)
			ost->enc = avcodec_find_encoder(ost->st->codec->codec_id);

		//因为需要转换,所以既需解码又需编码
		ist->decoding_needed = 1;
		ost->encoding_needed = 1;

		switch(codec->codec_type)
		{
		case AVMEDIA_TYPE_AUDIO:
			//鼓捣音频编码器的参数,基本上是把一些不合适的参数替换掉
			ost->fifo = av_fifo_alloc(1024);//音频数据所在的缓冲
			if (!ost->fifo)	{
				return AVERROR(ENOMEM);
			}

			//采样率
			if (!codec->sample_rate)
				codec->sample_rate = icodec->sample_rate;
			choose_sample_rate(ost->st, ost->enc);
			codec->time_base = (AVRational){1, codec->sample_rate};

			//样点格式
			if (codec->sample_fmt == AV_SAMPLE_FMT_NONE)
				codec->sample_fmt = icodec->sample_fmt;
			choose_sample_fmt(ost->st, ost->enc);

			//声道
			if (ost->audio_channels_mapped)	{
				/* the requested output channel is set to the number of
				 * -map_channel only if no -ac are specified */
				if (!codec->channels)	{
					codec->channels = ost->audio_channels_mapped;
					codec->channel_layout = av_get_default_channel_layout(codec->channels);
					if (!codec->channel_layout)	{
						av_log(NULL, AV_LOG_FATAL, "Unable to find an appropriate channel layout for requested number of channel\n);
						exit_program(1);
					}
				}
				/* fill unused channel mapping with -1 (which means a muted
				 * channel in case the number of output channels is bigger
				 * than the number of mapped channel) */
				for (j = ost->audio_channels_mapped; j < FF_ARRAY_ELEMS(ost->audio_channels_map); j++)
				<span>	</span>ost->audio_channels_map[j] = -1;
			}else if (!codec->channels){
				codec->channels = icodec->channels;
				codec->channel_layout = icodec->channel_layout;
			}
			if (av_get_channel_layout_nb_channels(codec->channel_layout) != codec->channels)
				codec->channel_layout = 0;

			//是否需要重采样
			ost->audio_resample = codec->sample_rate != icodec->sample_rate || audio_sync_method > 1;
			ost->audio_resample |= codec->sample_fmt != icodec->sample_fmt	||
					codec->channel_layout != icodec->channel_layout;
			icodec->request_channels = codec->channels;
			ost->resample_sample_fmt = icodec->sample_fmt;
			ost->resample_sample_rate = icodec->sample_rate;
			ost->resample_channels = icodec->channels;
			break;
		case AVMEDIA_TYPE_VIDEO:
			//鼓捣视频编码器的参数,基本上是把一些不合适的参数替换掉
			if (codec->pix_fmt == PIX_FMT_NONE)
				codec->pix_fmt = icodec->pix_fmt;
			choose_pixel_fmt(ost->st, ost->enc);
			if (ost->st->codec->pix_fmt == PIX_FMT_NONE){
				av_log(NULL, AV_LOG_FATAL, "Video pixel format is unknown, stream cannot be encoded\n");
				exit_program(1);
			}

			//宽高
			if (!codec->width || !codec->height){
				codec->width = icodec->width;
				codec->height = icodec->height;
			}

			//视频是否需要重采样
			ost->video_resample = codec->width != icodec->width ||
					codec->height != icodec->height ||
					codec->pix_fmt != icodec->pix_fmt;
			if (ost->video_resample){
				codec->bits_per_raw_sample= frame_bits_per_raw_sample;
			}

			ost->resample_height = icodec->height;
			ost->resample_width = icodec->width;
			ost->resample_pix_fmt = icodec->pix_fmt;

			//计算帧率
			if (!ost->frame_rate.num)
				ost->frame_rate = ist->st->r_frame_rate.num ?
						ist->st->r_frame_rate : (AVRational){25,1};
			if (ost->enc && ost->enc->supported_framerates && !ost->force_fps)	{
				int idx = av_find_nearest_q_idx(ost->frame_rate,ost->enc->supported_framerates);
				ost->frame_rate = ost->enc->supported_framerates[idx];
			}
			codec->time_base = (AVRational)	{ost->frame_rate.den, ost->frame_rate.num};
			if( av_q2d(codec->time_base) < 0.001 &&
					video_sync_method &&
					(video_sync_method==1 ||
						(video_sync_method<0 &&	!
							(oc->oformat->flags & AVFMT_VARIABLE_FPS))))
			{
				av_log(oc, AV_LOG_WARNING, "Frame rate very high for a muxer not effciciently supporting it.\n"
						"Please consider specifiying a lower framerate, a different muxer or -vsync 2\n");
			}
		<span>	</span>for (j = 0; j < ost->forced_kf_count; j++)
				ost->forced_kf_pts[j] = av_rescale_q(ost->forced_kf_pts[j],
						AV_TIME_BASE_Q,	codec->time_base);
				break;
			case AVMEDIA_TYPE_SUBTITLE:
				break;
			default:
				abort();
				break;
			}
			/* two pass mode */
			if (codec->codec_id != CODEC_ID_H264 &&
					(codec->flags & (CODEC_FLAG_PASS1 | CODEC_FLAG_PASS2)))
			{
				char logfilename[1024];
				FILE *f;

				snprintf(logfilename, sizeof(logfilename), "%s-%d.log",
						pass_logfilename_prefix ? pass_logfilename_prefix : DEFAULT_PASS_LOGFILENAME_PREFIX,
						i);
				if (codec->flags & CODEC_FLAG_PASS2){
					char *logbuffer;
					size_t logbuffer_size;
					if (cmdutils_read_file(logfilename, &logbuffer, &logbuffer_size) < 0){
						av_log(NULL, AV_LOG_FATAL,
								"Error reading log file '%s' for pass-2 encoding\n",
								logfilename);
						exit_program(1);
					}
					codec->stats_in = logbuffer;
				}
				if (codec->flags & CODEC_FLAG_PASS1){
					f = fopen(logfilename, "wb");
					if (!f)	{
						av_log(NULL, AV_LOG_FATAL, "Cannot write log file '%s' for pass-1 encoding: %s\n",
								logfilename, strerror(errno));
						exit_program(1);
					}
					ost->logfile = f;
				}
			}
		}
		if (codec->codec_type == AVMEDIA_TYPE_VIDEO){
			/* maximum video buffer size is 6-bytes per pixel, plus DPX header size (1664)*/
			//计算编码输出缓冲的大小,计算一个最大值
			int size = codec->width * codec->height;
			bit_buffer_size = FFMAX(bit_buffer_size, 7 * size + 10000);
		}
	}

	//分配编码后数据所在的缓冲
	if (!bit_buffer)
		bit_buffer = av_malloc(bit_buffer_size);
	if (!bit_buffer){
		av_log(NULL, AV_LOG_ERROR,
				"Cannot allocate %d bytes output buffer\n",
				bit_buffer_size);
		return AVERROR(ENOMEM);
	}

	//轮循所有输出流,打开每个输出流的编码器
	for (i = 0; i < nb_output_streams; i++)
	{
		ost = &output_streams[i];
		if (ost->encoding_needed){
			//当然,只有在需要编码时才打开编码器
			AVCodec *codec = ost->enc;
			AVCodecContext *dec = input_streams[ost->source_index].st->codec;
			if (!codec)	{
				snprintf(error, sizeof(error),
						"Encoder (codec %s) not found for output stream #%d:%d",
						avcodec_get_name(ost->st->codec->codec_id),
						ost->file_index, ost->index);
				ret = AVERROR(EINVAL);
				goto dump_format;
			}
			if (dec->subtitle_header){
				ost->st->codec->subtitle_header = av_malloc(dec->subtitle_header_size);
				if (!ost->st->codec->subtitle_header){
					ret = AVERROR(ENOMEM);
					goto dump_format;
				}
				memcpy(ost->st->codec->subtitle_header,
						dec->subtitle_header,dec->subtitle_header_size);
				ost->st->codec->subtitle_header_size =	dec->subtitle_header_size;
			}
			//打开啦
			if (avcodec_open2(ost->st->codec, codec, &ost->opts) < 0)	{
				snprintf(error,	sizeof(error),
						"Error while opening encoder for output stream #%d:%d - maybe incorrect parameters such as bit_rate, rate, width or height",
						ost->file_index, ost->index);
				ret = AVERROR(EINVAL);
				goto dump_format;
			}
			assert_codec_experimental(ost->st->codec, 1);
			assert_avoptions(ost->opts);
			if (ost->st->codec->bit_rate && ost->st->codec->bit_rate < 1000)
				av_log(NULL, AV_LOG_WARNING,
						"The bitrate parameter is set too low."
						" It takes bits/s as argument, not kbits/s\n");
			extra_size += ost->st->codec->extradata_size;

			if (ost->st->codec->me_threshold)
				input_streams[ost->source_index].st->codec->debug |= FF_DEBUG_MV;
		}
	}

	//初始化所有的输入流(主要做的就是在需要时打开解码器)
	for (i = 0; i < nb_input_streams; i++)
		if ((ret = init_input_stream(i, output_streams, nb_output_streams,
				error, sizeof(error))) < 0)
			goto dump_format;

	/* discard unused programs */
	for (i = 0; i < nb_input_files; i++){
		InputFile *ifile = &input_files[i];
		for (j = 0; j < ifile->ctx->nb_programs; j++){
			AVProgram *p = ifile->ctx->programs[j];
			int discard = AVDISCARD_ALL;

			for (k = 0; k < p->nb_stream_indexes; k++){
				if (!input_streams[ifile->ist_index + p->stream_index[k]].discard){
					discard = AVDISCARD_DEFAULT;
					break;
				}
			}
			p->discard = discard;
		}
	}

	//打开所有输出文件，写入媒体文件头
	for (i = 0; i < nb_output_files; i++){
		oc = output_files[i].ctx;
		oc->interrupt_callback = int_cb;
		if (avformat_write_header(oc, &output_files[i].opts) < 0){
			snprintf(error,	sizeof(error),
					"Could not write header for output file #%d (incorrect codec parameters ?)",
					i);
			ret = AVERROR(EINVAL);
			goto dump_format;
		}
//        assert_avoptions(output_files[i].opts);
		if (strcmp(oc->oformat->name, "rtp")){
			want_sdp = 0;
		}
	}

	return 0;
}

 

原文地址：http://blog.csdn.net/niu_gao/article/details/7175421