13.官方示例解复用和解编码.c

extern "C" {
#include <libavutil/imgutils.h>
#include <libavutil/samplefmt.h>
#include <libavutil/timestamp.h>
#include <libavformat/avformat.h>
}

static AVFormatContext *fmt_ctx = NULL;
static AVCodecContext *video_dec_ctx = NULL, *audio_dec_ctx;
static int width, height;
static enum AVPixelFormat pix_fmt;
static AVStream *video_stream = NULL, *audio_stream = NULL;
static const char *src_filename = NULL;
static const char *video_dst_filename = NULL;
static const char *audio_dst_filename = NULL;
static FILE *video_dst_file = NULL;
static FILE *audio_dst_file = NULL;

static uint8_t *video_dst_data[4] = { NULL };
static int      video_dst_linesize[4];
static int video_dst_bufsize;

static int video_stream_idx = -1, audio_stream_idx = -1;
static AVFrame *frame = NULL;
static AVPacket pkt;
static int video_frame_count = 0;
static int audio_frame_count = 0;

/* Enable or disable frame reference counting. You are not supposed to support
* both paths in your application but pick the one most appropriate to your
* needs. Look for the use of refcount in this example to see what are the
* differences of API usage between them. */
static int refcount = 0;

static int decode_packet(int *got_frame, int cached)
{
	int ret = 0;
	int decoded = pkt.size;

	*got_frame = 0;

	if (pkt.stream_index == video_stream_idx) {
		/* decode video frame */
		ret = avcodec_decode_video2(video_dec_ctx, frame, got_frame, &pkt);//解码帧 已经弃用不再维护
		if (ret < 0) {
			fprintf(stderr, "Error decoding video frame (%s)\n", av_err2str(ret));
			return ret;
		}

		if (*got_frame) {

			if (frame->width != width || frame->height != height ||
				frame->format != pix_fmt) {//帧大小不对
				/* To handle this change, one could call av_image_alloc again and
				* decode the following frames into another rawvideo file. */
				fprintf(stderr, "Error: Width, height and pixel format have to be "
					"constant in a rawvideo file, but the width, height or "
					"pixel format of the input video changed:\n"
					"old: width = %d, height = %d, format = %s\n"
					"new: width = %d, height = %d, format = %s\n",
					width, height, av_get_pix_fmt_name(pix_fmt),
					frame->width, frame->height,
					av_get_pix_fmt_name(frame->format));
				return -1;
			}

			printf("video_frame%s n:%d coded_n:%d\n",
				cached ? "(cached)" : "",
				video_frame_count++, frame->coded_picture_number);

			/* copy decoded frame to destination buffer:
			* this is required since rawvideo expects non aligned data */
			av_image_copy(video_dst_data, video_dst_linesize,
				(const uint8_t **)(frame->data), frame->linesize,
				pix_fmt, width, height);//复制帧数据到video_dst_data和video_dst_linesize 因为是rawvideo

			/* write to rawvideo file */
			fwrite(video_dst_data[0], 1, video_dst_bufsize, video_dst_file);
		}
	}
	else if (pkt.stream_index == audio_stream_idx) {
		/* decode audio frame */
		ret = avcodec_decode_audio4(audio_dec_ctx, frame, got_frame, &pkt);//解码音频 已经放弃维护
		if (ret < 0) {
			fprintf(stderr, "Error decoding audio frame (%s)\n", av_err2str(ret));
			return ret;
		}
		/* Some audio decoders decode only part of the packet, and have to be
		* called again with the remainder of the packet data.
		* Sample: fate-suite/lossless-audio/luckynight-partial.shn
		* Also, some decoders might over-read the packet. */
		decoded = FFMIN(ret, pkt.size);//该帧的大小

		if (*got_frame) {
			size_t unpadded_linesize = frame->nb_samples * av_get_bytes_per_sample(frame->format);//采样数*采样样本大小
			printf("audio_frame%s n:%d nb_samples:%d pts:%s\n",
				cached ? "(cached)" : "",
				audio_frame_count++, frame->nb_samples,
				av_ts2timestr(frame->pts, &audio_dec_ctx->time_base));

			/* Write the raw audio data samples of the first plane. This works
			* fine for packed formats (e.g. AV_SAMPLE_FMT_S16). However,
			* most audio decoders output planar audio, which uses a separate
			* plane of audio samples for each channel (e.g. AV_SAMPLE_FMT_S16P).
			* In other words, this code will write only the first audio channel
			* in these cases.
			* You should use libswresample or libavfilter to convert the frame
			* to packed data. */
			fwrite(frame->extended_data[0], 1, unpadded_linesize, audio_dst_file);//对于plane是可以的 planar需要使用libswresample or libavfilter进行优化
		}
	}

	/* If we use frame reference counting, we own the data and need
	* to de-reference it when we don't use it anymore */
	if (*got_frame && refcount)
		av_frame_unref(frame);

	return decoded;
}

static int open_codec_context(int *stream_idx,
	AVCodecContext **dec_ctx, AVFormatContext *fmt_ctx, enum AVMediaType type)//根据type从fmt_ctx中返回stream_idx和dec_ctx
{
	int ret, stream_index;
	AVStream *st;
	AVCodec *dec = NULL;
	AVDictionary *opts = NULL;

	ret = av_find_best_stream(fmt_ctx, type, -1, -1, NULL, 0);//根据type 查找流的下标
	if (ret < 0) {
		fprintf(stderr, "Could not find %s stream in input file '%s'\n",
			av_get_media_type_string(type), src_filename);
		return ret;
	}
	else {
		stream_index = ret;
		st = fmt_ctx->streams[stream_index];
		
		/* find decoder for the stream */
		dec = avcodec_find_decoder(st->codecpar->codec_id);//根据流保存的编码id 找到解码器
		if (!dec) {
			fprintf(stderr, "Failed to find %s codec\n",
				av_get_media_type_string(type));
			return AVERROR(EINVAL);
		}

		/* Allocate a codec context for the decoder */
		*dec_ctx = avcodec_alloc_context3(dec);//根据编解码器 为ctx分配空间和初始化
		if (!*dec_ctx) {
			fprintf(stderr, "Failed to allocate the %s codec context\n",
				av_get_media_type_string(type));
			return AVERROR(ENOMEM);
		}

		/* Copy codec parameters from input stream to output codec context */
		if ((ret = avcodec_parameters_to_context(*dec_ctx, st->codecpar)) < 0) {//将codecpar的字段替换dec_ctx内相应的字段
			fprintf(stderr, "Failed to copy %s codec parameters to decoder context\n",
				av_get_media_type_string(type));
			return ret;
		}

		/* Init the decoders, with or without reference counting */
		av_dict_set(&opts, "refcounted_frames", refcount ? "1" : "0", 0);//设置AVDictionary属性 
		if ((ret = avcodec_open2(*dec_ctx, dec, &opts)) < 0) {//根据AVcode初始化一个编解码上下文
			fprintf(stderr, "Failed to open %s codec\n",
				av_get_media_type_string(type));
			return ret;
		}
		*stream_idx = stream_index;
	}

	return 0;
}

static int get_format_from_sample_fmt(const char **fmt,
	enum AVSampleFormat sample_fmt)
{
	int i;
	struct sample_fmt_entry {
		enum AVSampleFormat sample_fmt; const char *fmt_be, *fmt_le;
	} sample_fmt_entries[] = {
		{ AV_SAMPLE_FMT_U8,  "u8",    "u8" },
		{ AV_SAMPLE_FMT_S16, "s16be", "s16le" },
		{ AV_SAMPLE_FMT_S32, "s32be", "s32le" },
		{ AV_SAMPLE_FMT_FLT, "f32be", "f32le" },
		{ AV_SAMPLE_FMT_DBL, "f64be", "f64le" },
	};
	*fmt = NULL;

	for (i = 0; i < FF_ARRAY_ELEMS(sample_fmt_entries); i++) {
		struct sample_fmt_entry *entry = &sample_fmt_entries[i];
		if (sample_fmt == entry->sample_fmt) {
			*fmt = AV_NE(entry->fmt_be, entry->fmt_le);//大小端
			return 0;
		}
	}

	fprintf(stderr,
		"sample format %s is not supported as output format\n",
		av_get_sample_fmt_name(sample_fmt));
	return -1;
}

int main(int argc, char **argv)
{
	int ret = 0, got_frame;

	if (argc != 4 && argc != 5) {
		fprintf(stderr, "usage: %s [-refcount] input_file video_output_file audio_output_file\n"
			"API example program to show how to read frames from an input file.\n"
			"This program reads frames from a file, decodes them, and writes decoded\n"
			"video frames to a rawvideo file named video_output_file, and decoded\n"
			"audio frames to a rawaudio file named audio_output_file.\n\n"
			"If the -refcount option is specified, the program use the\n"
			"reference counting frame system which allows keeping a copy of\n"
			"the data for longer than one decode call.\n"
			"\n", argv[0]);//refcount 引用计数帧系统，但是保存副本的时间比编码长
		exit(1);
	}
	if (argc == 5 && !strcmp(argv[1], "-refcount")) {
		refcount = 1;
		argv++;
	}
	src_filename = argv[1];
	video_dst_filename = argv[2];
	audio_dst_filename = argv[3];

	/* open input file, and allocate format context */
	if (avformat_open_input(&fmt_ctx, src_filename, NULL, NULL) < 0) {//打开输入流，并且读取文件头 fmt_ctx为NULL时 不必再调用avformat_alloc_context
		fprintf(stderr, "Could not open source file %s\n", src_filename);
		exit(1);
	}

	/* retrieve stream information */
	if (avformat_find_stream_info(fmt_ctx, NULL) < 0) {//查找流信息
		fprintf(stderr, "Could not find stream information\n");
		exit(1);
	}

	if (open_codec_context(&video_stream_idx, &video_dec_ctx, fmt_ctx, AVMEDIA_TYPE_VIDEO) >= 0) {//查找视频流 并得到编码器
		video_stream = fmt_ctx->streams[video_stream_idx];

		video_dst_file = fopen(video_dst_filename, "wb");
		if (!video_dst_file) {
			fprintf(stderr, "Could not open destination file %s\n", video_dst_filename);
			ret = 1;
			goto end;
		}

		/* allocate image where the decoded image will be put */
		width = video_dec_ctx->width;
		height = video_dec_ctx->height;
		pix_fmt = video_dec_ctx->pix_fmt;
		ret = av_image_alloc(video_dst_data, video_dst_linesize,
			width, height, pix_fmt, 1);//根据width height pix_fmt分配video_dst_data和video_dst_linesize的一张图片
		if (ret < 0) {
			fprintf(stderr, "Could not allocate raw video buffer\n");
			goto end;
		}
		video_dst_bufsize = ret;
	}

	if (open_codec_context(&audio_stream_idx, &audio_dec_ctx, fmt_ctx, AVMEDIA_TYPE_AUDIO) >= 0) {//查找音频流 并得到编码器
		audio_stream = fmt_ctx->streams[audio_stream_idx];
		audio_dst_file = fopen(audio_dst_filename, "wb");
		if (!audio_dst_file) {
			fprintf(stderr, "Could not open destination file %s\n", audio_dst_filename);
			ret = 1;
			goto end;
		}
	}

	/* dump input information to stderr */
	av_dump_format(fmt_ctx, 0, src_filename, 0);//打印输入输出封装上下文

	if (!audio_stream && !video_stream) {
		fprintf(stderr, "Could not find audio or video stream in the input, aborting\n");
		ret = 1;
		goto end;
	}

	frame = av_frame_alloc();//初始化帧
	if (!frame) {
		fprintf(stderr, "Could not allocate frame\n");
		ret = AVERROR(ENOMEM);
		goto end;
	}

	/* initialize packet, set data to NULL, let the demuxer fill it */
	av_init_packet(&pkt);//初始化压缩包
	pkt.data = NULL;
	pkt.size = 0;

	if (video_stream)
		printf("Demuxing video from file '%s' into '%s'\n", src_filename, video_dst_filename);
	if (audio_stream)
		printf("Demuxing audio from file '%s' into '%s'\n", src_filename, audio_dst_filename);

	/* read frames from the file */
	while (av_read_frame(fmt_ctx, &pkt) >= 0) {
		AVPacket orig_pkt = pkt;
		do {
			ret = decode_packet(&got_frame, 0);//解码一帧
			if (ret < 0)
				break;
			pkt.data += ret;//数据指针移动
			pkt.size -= ret;//数据长度减少
		} while (pkt.size > 0);
		av_packet_unref(&orig_pkt);//删掉缓存
	}

	/* flush cached frames */
	pkt.data = NULL;
	pkt.size = 0;
	do {
		decode_packet(&got_frame, 1);
	} while (got_frame);//清除缓存中的帧

	printf("Demuxing succeeded.\n");

	if (video_stream) {
		printf("Play the output video file with the command:\n"
			"ffplay -f rawvideo -pix_fmt %s -video_size %dx%d %s\n",
			av_get_pix_fmt_name(pix_fmt), width, height,
			video_dst_filename);
	}

	if (audio_stream) {
		enum AVSampleFormat sfmt = audio_dec_ctx->sample_fmt;
		int n_channels = audio_dec_ctx->channels;
		const char *fmt;

		if (av_sample_fmt_is_planar(sfmt)) {
			const char *packed = av_get_sample_fmt_name(sfmt);
			printf("Warning: the sample format the decoder produced is planar "
				"(%s). This example will output the first channel only.\n",
				packed ? packed : "?");
			sfmt = av_get_packed_sample_fmt(sfmt);
			n_channels = 1;
		}

		if ((ret = get_format_from_sample_fmt(&fmt, sfmt)) < 0)
			goto end;

		printf("Play the output audio file with the command:\n"
			"ffplay -f %s -ac %d -ar %d %s\n",
			fmt, n_channels, audio_dec_ctx->sample_rate,
			audio_dst_filename);
	}

end:
	avcodec_free_context(&video_dec_ctx);//释放视频解码器
	avcodec_free_context(&audio_dec_ctx);//释放音频解码器
	avformat_close_input(&fmt_ctx);//释放封装上下文
	if (video_dst_file)
		fclose(video_dst_file);
	if (audio_dst_file)
		fclose(audio_dst_file);
	av_frame_free(&frame);//释放帧
	av_free(video_dst_data[0]);//释放图片

	return ret < 0;
}