1.打開輸入流

avformat_open_input()

int?avformat_open_input	(	AVFormatContext?**?	ps,
		const char *?	url,
		const?AVInputFormat?*?	fmt,
		AVDictionary?**?	options?
	)

Open an input stream and read the header.

打開輸入流, 讀取header, 并分配format 上下文 （allocate format context），具體參見AVFormatContext 結構體

The codecs are not opened. The stream must be closed with?avformat_close_input().

Parameters

ps	Pointer to user-supplied?AVFormatContext?(allocated by avformat_alloc_context). May be a pointer to NULL, in which case an?AVFormatContext?is allocated by this function and written into ps. Note that a user-supplied?AVFormatContext?will be freed on failure.
url	URL of the stream to open.
fmt	If non-NULL, this parameter forces a specific input format. Otherwise the format is autodetected.
options	A dictionary filled with?AVFormatContext?and demuxer-private options. On return this parameter will be destroyed and replaced with a dict containing options that were not found. May be NULL.

Returns

0 on success, a negative AVERROR on failure.

Note

If you want to use custom IO, preallocate the format context and set its pb field.

2.? 檢索流信息

avformat_find_stream_info()

int?avformat_find_stream_info	(	AVFormatContext?*?	ic,
		AVDictionary?**?	options?
	)

Read packets of a media file to get stream information.

This is useful for file formats with no headers such as MPEG. This function also computes the real framerate in case of MPEG-2 repeat frame mode. The logical file position is not changed by this function; examined packets may be buffered for later processing.

Parameters

ic	media file handle
options	If non-NULL, an ic.nb_streams long array of pointers to dictionaries, where i-th member contains options for codec corresponding to i-th stream. On return each dictionary will be filled with options that were not found.

Returns

>=0 if OK, AVERROR_xxx on error

Note

this function isn't guaranteed to open all the codecs, so options being non-empty at return is a perfectly normal behavior.

Todo:

Let the user decide somehow what information is needed so that we do not waste time getting stuff the user does not need.

3.?

open_codec_context()

static?int?open_codec_context ( int?*? stream_idx, AVCodecContext?**? dec_ctx, AVFormatContext?*? fmt_ctx, enum?AVMediaType? type? )

4.?創建用于存放解碼視頻幀的image

av_image_alloc()

int?av_image_alloc	(	uint8_t *?	pointers[4],
		int	linesizes[4],
		int	w,
		int	h,
		enum?AVPixelFormat?	pix_fmt,
		int	align?
	)

Allocate an image with size w and h and pixel format pix_fmt, and fill pointers and linesizes accordingly.

The allocated image buffer has to be freed by using?av_freep(&pointers[0]).

Parameters

align

the value to use for buffer size alignment

Returns

the size in bytes required for the image buffer, a negative error code in case of failure

5.?

av_image_copy av_image_copy()void av_image_copy ( uint8_t * dst_data[4],int dst_linesizes[4],const uint8_t * src_data[4],const int src_linesizes[4],enum AVPixelFormat pix_fmt,int width,int height ) Copy image in src_data to dst_data.Parameters dst_linesizes linesizes for the image in dst_data src_linesizes linesizes for the image in src_data

6.?

av_sample_fmt_is_planar()

int?av_sample_fmt_is_planar

(

enum?AVSampleFormat?

sample_fmt

)

Check if the sample format is planar.

Parameters

sample_fmt

the sample format to inspect

Returns

1 if the sample format is planar, 0 if it is interleaved

7.?傳入planar類型的采樣格式，返回其可轉換的packed類型的采樣格式。例如傳入?AV_SAMPLE_FMT_S32P，其返回值為?AV_SAMPLE_FMT_S32。

av_get_packed_sample_fmt()

enum?AVSampleFormat?av_get_packed_sample_fmt

(

enum?AVSampleFormat?

sample_fmt

)

Get the packed alternative form of the given sample format.

If the passed sample_fmt is already in packed format, the format returned is the same as the input.

Returns

the packed alternative form of the given sample format or AV_SAMPLE_FMT_NONE on error.

ffmpeg 音頻存儲格式：

packed：多個聲道數據交錯存放，所有聲道的數據交錯排放在frame->data[0](即frame->extended_data[0])地址處，數據長度為linesize[0](單位:字節)

planar：每個聲道數據單獨存放，聲道0的起始地址為 frame->data[0](或frame->extended_data[0])，聲道1的起始地址為 frame->data[1](或frame->extended_data[1]) . . . 每個聲道的數據長度都為linesize[0](單位:字節)

判斷是否是 planar類型：av_sample_fmt_is_planar(sample_fmt)

FFmpeg音頻解碼后的數據是存放在AVFrame結構中的。

1）Packed格式，frame.data[0]或frame.extended_data[0]包含所有的音頻數據中。

2）Planar格式，frame.data[i]或者frame.extended_data[i]表示第i個聲道的數據（假設聲道0是第一個）, AVFrame.data數組大小固定為8，如果聲道數超過8，需要從frame.extended_data獲取聲道數據。

參考：FFmpeg: Image related

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