AVFrame关于视音频数据存储

1、概述

这几天用ffmpeg实现简单音频转码，在写程序的过程中发现音频在AVFrame中存储与视频很相似，音频要复杂一些，本人记性不好，所以在这里记录下来，以作备忘。

2、2个数据成员

在AVFrame中有2个很重要的数据成员，一个是data，一个是linesize。data中存储的是未编码的源始数据（不论视音频），linesize中存储的是每行data中数据大小。

data的定义如下：

/**     * pointer to the picture/channel planes.     * This might be different from the first allocated byte     *     * Some decoders access areas outside 0,0 - width,height, please     * see avcodec_align_dimensions2(). Some filters and swscale can read     * up to 16 bytes beyond the planes, if these filters are to be used,     * then 16 extra bytes must be allocated.     */    uint8_t *data[AV_NUM_DATA_POINTERS];

linesize定义如下：

  /**     * For video, size in bytes of each picture line.     * For audio, size in bytes of each plane.     *     * For audio, only linesize[0] may be set. For planar audio, each channel     * plane must be the same size.     *     * For video the linesizes should be multiplies of the CPUs alignment     * preference, this is 16 or 32 for modern desktop CPUs.     * Some code requires such alignment other code can be slower without     * correct alignment, for yet other it makes no difference.     *     * @note The linesize may be larger than the size of usable data -- there     * may be extra padding present for performance reasons.     */    int linesize[AV_NUM_DATA_POINTERS];

注意：当为音频的时候linesize，只有linesize[0]才是有效值，因为左右一样大。

3、存储方式

1、视频

视频相对简单的多，以yuv420为例，图像数据在AVFrame中存储是这样的：

data[0]存储Y

data[1]存储U

data[2]存储V

而他们相对应的大小为：

linesize[0]为Y的大小

linesize[1]为U的大小

linesize[2]为V的大小

2、音频

音频数据则要复杂一些，在音频中分为平面和非平面数据类型，下面是音频数据类型的定义：

/** * Audio Sample Formats * * @par * The data described by the sample format is always in native-endian order. * Sample values can be expressed by native C types, hence the lack of a signed * 24-bit sample format even though it is a common raw audio data format. * * @par * The floating-point formats are based on full volume being in the range * [-1.0, 1.0]. Any values outside this range are beyond full volume level. * * @par * The data layout as used in av_samples_fill_arrays() and elsewhere in FFmpeg * (such as AVFrame in libavcodec) is as follows: * * For planar sample formats, each audio channel is in a separate data plane, * and linesize is the buffer size, in bytes, for a single plane. All data * planes must be the same size. For packed sample formats, only the first data * plane is used, and samples for each channel are interleaved. In this case, * linesize is the buffer size, in bytes, for the 1 plane. */enum AVSampleFormat {    AV_SAMPLE_FMT_NONE = -1,    AV_SAMPLE_FMT_U8,          ///< unsigned 8 bits    AV_SAMPLE_FMT_S16,         ///< signed 16 bits    AV_SAMPLE_FMT_S32,         ///< signed 32 bits    AV_SAMPLE_FMT_FLT,         ///< float    AV_SAMPLE_FMT_DBL,         ///< double    AV_SAMPLE_FMT_U8P,         ///< unsigned 8 bits, planar    AV_SAMPLE_FMT_S16P,        ///< signed 16 bits, planar    AV_SAMPLE_FMT_S32P,        ///< signed 32 bits, planar    AV_SAMPLE_FMT_FLTP,        ///< float, planar    AV_SAMPLE_FMT_DBLP,        ///< double, planar    AV_SAMPLE_FMT_NB           ///< Number of sample formats. DO NOT USE if linking dynamically};

定义中最后带p的为平面数据类型，可以用av_sample_fmt_is_planar来判断此数据类型是否是平面数据类型。

先说非平面数据：

以一个双声道（左右）音频来说，存储格式可能就为LRLRLR.........（左声道在前还是右声道在前没有认真研究过），数据都装在data[0]中，而大小则为linesize[0]。

平面数据：

就有点像视频部分的YUV数据，同样对双声道音频PCM数据，以S16P为例，存储就可能是
plane 0: LLLLLLLLLLLLLLLLLLLLLLLLLL...
plane 1: RRRRRRRRRRRRRRRRRRRR....

对应的存储则为：

data[0]存储plane 0

data[1]存储plane 1

对应的大小则都为linesize[0]，可以用av_get_bytes_per_sample(out_stream->codec->sample_fmt) * out_frame->nb_samples来算出plane的大小。