FFmpeg 4.3 音视频-多路H265监控录放C++开发八，使用SDLVSQT显示yuv文件 ，使用ffmpeg的AVFrame

一. AVFrame 核心回顾，uint8_t *data[AV_NUM_DATA_POINTERS] 和  int linesize[AV_NUM_DATA_POINTERS]

AVFrame 存储的是解码后的数据，（包括音频和视频）例如：yuv数据，或者pcm数据，参考AVFrame结构体的第一句话。

其核心数据为：

AV_NUM_DATA_POINTERS = 8；

uint8_t *data[AV_NUM_DATA_POINTERS];

 int linesize[AV_NUM_DATA_POINTERS];

uint8_t *data[AV_NUM_DATA_POINTERS];

data -->xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
            ^                          ^                        ^
            |                           |                        |
        data[0]                data[1]              data[2]

比如说，当pix_fmt=AV_PIX_FMT_YUV420P时，data中的数据是按照YUV的格式存储的，也就是：

data -->YYYYYYYYYYYYYYYYYYYYYYYYUUUUUUUUUUUVVVVVVVVVVVV
            ^                                                     ^                        ^
            |                                                     |                         |
       data[0]                                           data[1]               data[2]

 int linesize[AV_NUM_DATA_POINTERS];

 linesize是指对应于每一行的大小，为什么需要这个变量，是因为在YUV格式和RGB格式时，每行的大小不一定等于图像的宽度。

linesize = width + padding size(16+16) for YUV
linesize = width*pixel_size  for RGB
padding is needed during Motion Estimation and Motion Compensation for Optimizing MV serach and  P/B frame reconstruction

for RGB only one channel is available
so RGB24 : data[0] = packet rgbrgbrgbrgb......
           linesize[0] = width*3
data[1],data[2],data[3],linesize[1],linesize[2],linesize[2] have no any means for RGB

在二核心函数中 关于linesize[x]字节数的验证 代码，可以参考

/*** This structure describes decoded (raw) audio or video data.** AVFrame must be allocated using av_frame_alloc(). Note that this only* allocates the AVFrame itself, the buffers for the data must be managed* through other means (see below).* AVFrame must be freed with av_frame_free().** AVFrame is typically allocated once and then reused multiple times to hold* different data (e.g. a single AVFrame to hold frames received from a* decoder). In such a case, av_frame_unref() will free any references held by* the frame and reset it to its original clean state before it* is reused again.** The data described by an AVFrame is usually reference counted through the* AVBuffer API. The underlying buffer references are stored in AVFrame.buf /* AVFrame.extended_buf. An AVFrame is considered to be reference counted if at* least one reference is set, i.e. if AVFrame.buf[0] != NULL. In such a case,* every single data plane must be contained in one of the buffers in* AVFrame.buf or AVFrame.extended_buf.* There may be a single buffer for all the data, or one separate buffer for* each plane, or anything in between.** sizeof(AVFrame) is not a part of the public ABI, so new fields may be added* to the end with a minor bump.** Fields can be accessed through AVOptions, the name string used, matches the* C structure field name for fields accessible through AVOptions. The AVClass* for AVFrame can be obtained from avcodec_get_frame_class()*/typedef struct AVFrame {
#define AV_NUM_DATA_POINTERS 8/*** pointer to the picture/channel planes.* This might be different from the first allocated byte. For video,* it could even point to the end of the image data.** All pointers in data and extended_data must point into one of the* AVBufferRef in buf or extended_buf.** 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.** NOTE: Pointers not needed by the format MUST be set to NULL.** @attention In case of video, the data[] pointers can point to the* end of image data in order to reverse line order, when used in* combination with negative values in the linesize[] array.*/uint8_t *data[AV_NUM_DATA_POINTERS];/*** For video, a positive or negative value, which is typically indicating* the size in bytes of each picture line, but it can also be:* - the negative byte size of lines for vertical flipping*   (with data[n] pointing to the end of the data* - a positive or negative multiple of the byte size as for accessing*   even and odd fields of a frame (possibly flipped)** 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 multiples 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.** @attention In case of video, line size values can be negative to achieve* a vertically inverted iteration over image lines.*/int linesize[AV_NUM_DATA_POINTERS];/*** pointers to the data planes/channels.** For video, this should simply point to data[].** For planar audio, each channel has a separate data pointer, and* linesize[0] contains the size of each channel buffer.* For packed audio, there is just one data pointer, and linesize[0]* contains the total size of the buffer for all channels.** Note: Both data and extended_data should always be set in a valid frame,* but for planar audio with more channels that can fit in data,* extended_data must be used in order to access all channels.*/uint8_t **extended_data;/*** @name Video dimensions* Video frames only. The coded dimensions (in pixels) of the video frame,* i.e. the size of the rectangle that contains some well-defined values.** @note The part of the frame intended for display/presentation is further* restricted by the @ref cropping "Cropping rectangle".* @{*/int width, height;/*** @}*//*** number of audio samples (per channel) described by this frame*/int nb_samples;/*** format of the frame, -1 if unknown or unset* Values correspond to enum AVPixelFormat for video frames,* enum AVSampleFormat for audio)*/int format;/*** 1 -> keyframe, 0-> not*/int key_frame;/*** Picture type of the frame.*/enum AVPictureType pict_type;/*** Sample aspect ratio for the video frame, 0/1 if unknown/unspecified.*/AVRational sample_aspect_ratio;/*** Presentation timestamp in time_base units (time when frame should be shown to user).*/int64_t pts;/*** DTS copied from the AVPacket that triggered returning this frame. (if frame threading isn't used)* This is also the Presentation time of this AVFrame calculated from* only AVPacket.dts values without pts values.*/int64_t pkt_dts;/*** Time base for the timestamps in this frame.* In the future, this field may be set on frames output by decoders or* filters, but its value will be by default ignored on input to encoders* or filters.*/AVRational time_base;#if FF_API_FRAME_PICTURE_NUMBER/*** picture number in bitstream order*/attribute_deprecatedint coded_picture_number;/*** picture number in display order*/attribute_deprecatedint display_picture_number;
#endif/*** quality (between 1 (good) and FF_LAMBDA_MAX (bad))*/int quality;/*** for some private data of the user*/void *opaque;/*** When decoding, this signals how much the picture must be delayed.* extra_delay = repeat_pict / (2*fps)*/int repeat_pict;/*** The content of the picture is interlaced.*/int interlaced_frame;/*** If the content is interlaced, is top field displayed first.*/int top_field_first;/*** Tell user application that palette has changed from previous frame.*/int palette_has_changed;#if FF_API_REORDERED_OPAQUE/*** reordered opaque 64 bits (generally an integer or a double precision float* PTS but can be anything).* The user sets AVCodecContext.reordered_opaque to represent the input at* that time,* the decoder reorders values as needed and sets AVFrame.reordered_opaque* to exactly one of the values provided by the user through AVCodecContext.reordered_opaque** @deprecated Use AV_CODEC_FLAG_COPY_OPAQUE instead*/attribute_deprecatedint64_t reordered_opaque;
#endif/*** Sample rate of the audio data.*/int sample_rate;#if FF_API_OLD_CHANNEL_LAYOUT/*** Channel layout of the audio data.* @deprecated use ch_layout instead*/attribute_deprecateduint64_t channel_layout;
#endif/*** AVBuffer references backing the data for this frame. All the pointers in* data and extended_data must point inside one of the buffers in buf or* extended_buf. This array must be filled contiguously -- if buf[i] is* non-NULL then buf[j] must also be non-NULL for all j < i.** There may be at most one AVBuffer per data plane, so for video this array* always contains all the references. For planar audio with more than* AV_NUM_DATA_POINTERS channels, there may be more buffers than can fit in* this array. Then the extra AVBufferRef pointers are stored in the* extended_buf array.*/AVBufferRef *buf[AV_NUM_DATA_POINTERS];/*** For planar audio which requires more than AV_NUM_DATA_POINTERS* AVBufferRef pointers, this array will hold all the references which* cannot fit into AVFrame.buf.** Note that this is different from AVFrame.extended_data, which always* contains all the pointers. This array only contains the extra pointers,* which cannot fit into AVFrame.buf.** This array is always allocated using av_malloc() by whoever constructs* the frame. It is freed in av_frame_unref().*/AVBufferRef **extended_buf;/*** Number of elements in extended_buf.*/int        nb_extended_buf;AVFrameSideData **side_data;int            nb_side_data;/*** @defgroup lavu_frame_flags AV_FRAME_FLAGS* @ingroup lavu_frame* Flags describing additional frame properties.** @{*//*** The frame data may be corrupted, e.g. due to decoding errors.*/
#define AV_FRAME_FLAG_CORRUPT       (1 << 0)
/*** A flag to mark the frames which need to be decoded, but shouldn't be output.*/
#define AV_FRAME_FLAG_DISCARD   (1 << 2)
/*** @}*//*** Frame flags, a combination of @ref lavu_frame_flags*/int flags;/*** MPEG vs JPEG YUV range.* - encoding: Set by user* - decoding: Set by libavcodec*/enum AVColorRange color_range;enum AVColorPrimaries color_primaries;enum AVColorTransferCharacteristic color_trc;/*** YUV colorspace type.* - encoding: Set by user* - decoding: Set by libavcodec*/enum AVColorSpace colorspace;enum AVChromaLocation chroma_location;/*** frame timestamp estimated using various heuristics, in stream time base* - encoding: unused* - decoding: set by libavcodec, read by user.*/int64_t best_effort_timestamp;/*** reordered pos from the last AVPacket that has been input into the decoder* - encoding: unused* - decoding: Read by user.*/int64_t pkt_pos;#if FF_API_PKT_DURATION/*** duration of the corresponding packet, expressed in* AVStream->time_base units, 0 if unknown.* - encoding: unused* - decoding: Read by user.** @deprecated use duration instead*/attribute_deprecatedint64_t pkt_duration;
#endif/*** metadata.* - encoding: Set by user.* - decoding: Set by libavcodec.*/AVDictionary *metadata;/*** decode error flags of the frame, set to a combination of* FF_DECODE_ERROR_xxx flags if the decoder produced a frame, but there* were errors during the decoding.* - encoding: unused* - decoding: set by libavcodec, read by user.*/int decode_error_flags;
#define FF_DECODE_ERROR_INVALID_BITSTREAM   1
#define FF_DECODE_ERROR_MISSING_REFERENCE   2
#define FF_DECODE_ERROR_CONCEALMENT_ACTIVE  4
#define FF_DECODE_ERROR_DECODE_SLICES       8#if FF_API_OLD_CHANNEL_LAYOUT/*** number of audio channels, only used for audio.* - encoding: unused* - decoding: Read by user.* @deprecated use ch_layout instead*/attribute_deprecatedint channels;
#endif/*** size of the corresponding packet containing the compressed* frame.* It is set to a negative value if unknown.* - encoding: unused* - decoding: set by libavcodec, read by user.*/int pkt_size;/*** For hwaccel-format frames, this should be a reference to the* AVHWFramesContext describing the frame.*/AVBufferRef *hw_frames_ctx;/*** AVBufferRef for free use by the API user. FFmpeg will never check the* contents of the buffer ref. FFmpeg calls av_buffer_unref() on it when* the frame is unreferenced. av_frame_copy_props() calls create a new* reference with av_buffer_ref() for the target frame's opaque_ref field.** This is unrelated to the opaque field, although it serves a similar* purpose.*/AVBufferRef *opaque_ref;/*** @anchor cropping* @name Cropping* Video frames only. The number of pixels to discard from the the* top/bottom/left/right border of the frame to obtain the sub-rectangle of* the frame intended for presentation.* @{*/size_t crop_top;size_t crop_bottom;size_t crop_left;size_t crop_right;/*** @}*//*** AVBufferRef for internal use by a single libav* library.* Must not be used to transfer data between libraries.* Has to be NULL when ownership of the frame leaves the respective library.** Code outside the FFmpeg libs should never check or change the contents of the buffer ref.** FFmpeg calls av_buffer_unref() on it when the frame is unreferenced.* av_frame_copy_props() calls create a new reference with av_buffer_ref()* for the target frame's private_ref field.*/AVBufferRef *private_ref;/*** Channel layout of the audio data.*/AVChannelLayout ch_layout;/*** Duration of the frame, in the same units as pts. 0 if unknown.*/int64_t duration;
} AVFrame;

二 核心函数  av_frame_alloc()，av_frame_get_buffer

AVFrame*  avframe1 = av_frame_alloc();

从实现来看，av_frame_alloc 函数只是 给 avframe1分配了空间，但是内部的值都没有，也就是说avframe内部需要空间的都没有分配。

int av_frame_get_buffer(AVFrame *frame, int align);

给传递进来的 frame 的内部元素分配空间，

第一个参数：给那个frame分配空间

第二个参数：分配空间的对齐是按照 align 进行，如果填充的是0，会根据当前CPU给一个默认值，测试在32位 windows上，这个值就是32. 一般都会填写0，使用默认值

/*** Allocate new buffer(s) for audio or video data.** The following fields must be set on frame before calling this function:* - format (pixel format for video, sample format for audio)* - width and height for video* - nb_samples and ch_layout for audio** This function will fill AVFrame.data and AVFrame.buf arrays and, if* necessary, allocate and fill AVFrame.extended_data and AVFrame.extended_buf.* For planar formats, one buffer will be allocated for each plane.** @warning: if frame already has been allocated, calling this function will*           leak memory. In addition, undefined behavior can occur in certain*           cases.** @param frame frame in which to store the new buffers.* @param align Required buffer size alignment. If equal to 0, alignment will be*              chosen automatically for the current CPU. It is highly*              recommended to pass 0 here unless you know what you are doing.** @return 0 on success, a negative AVERROR on error.*/
int av_frame_get_buffer(AVFrame *frame, int align);

内部实现：

可以看到如果是video，则会先判断 width 和 height 是否  > 0

 也就是说，我们在调用这个函数之前，如果是for video，需要保证avframe 的 width 和height 的属性有被设置过。

int av_frame_get_buffer(AVFrame *frame, int align)
{if (frame->format < 0)return AVERROR(EINVAL);FF_DISABLE_DEPRECATION_WARNINGSif (frame->width > 0 && frame->height > 0)return get_video_buffer(frame, align);else if (frame->nb_samples > 0 &&(av_channel_layout_check(&frame->ch_layout)
#if FF_API_OLD_CHANNEL_LAYOUT|| frame->channel_layout || frame->channels > 0
#endif))return get_audio_buffer(frame, align);
FF_ENABLE_DEPRECATION_WARNINGSreturn AVERROR(EINVAL);
}

那么如果我们不设置会有什么问题呢？

试一试

设置一下 width 和height 再来看一下

还是有问题：Invalid argument

void testAVframe() {cout << avcodec_configuration() << endl;AVFrame*  avframe1 = av_frame_alloc();cout << "debug1...." << endl;avframe1->width = 300;avframe1->height = 600;int ret = 0;ret = av_frame_get_buffer(avframe1, 0);if (ret < 0 ) {//如果方法失败，会返回一个 负数，可以通过 av_strerror函数打印这个具体的信息char buf[1024] = { 0 };av_strerror(ret, buf, sizeof(buf));cout << buf << endl;}cout << "debug2......" << endl;
}

那么应该再来看源码中的具体方法：get_video_buffer(frame, align);

源码在 frame.c中，我们看到 在 av_pix_fmt_desc_get(frame->format)中 返回了一个 desc，如果这个desc 为null，也会返回error。那么也就是说，这个frame->format 应该是有必要设置的，如下：

static int get_video_buffer(AVFrame *frame, int align)
{const AVPixFmtDescriptor *desc = av_pix_fmt_desc_get(frame->format);int ret, i, padded_height, total_size;int plane_padding = FFMAX(16 + 16/*STRIDE_ALIGN*/, align);ptrdiff_t linesizes[4];size_t sizes[4];if (!desc)return AVERROR(EINVAL);

为了验证这个问题，我们可以设置一下frame 中的 format 测试一下。发现是可以的。我们这时候再将  avframe1中的 关键数据 打印 看一下。

void testAVframe() {cout << avcodec_configuration() << endl;AVFrame*  avframe1 = av_frame_alloc();cout << "debug1...." << endl;avframe1->width = 300;avframe1->height = 600;//设置 foramt为 AV_PIX_FMT_YUV420P，再次测试avframe1->format = AV_PIX_FMT_YUV420P;int ret = 0;ret = av_frame_get_buffer(avframe1, 0);if (ret < 0 ) {//如果方法失败，会返回一个 负数，可以通过 av_strerror函数打印这个具体的信息char buf[1024] = { 0 };av_strerror(ret, buf, sizeof(buf));cout << buf << endl;}cout << "debug2......" << endl;
}

关于linesize[x]字节数的验证

void testAVframe() {cout << avcodec_configuration() << endl;AVFrame*  avframe1 = av_frame_alloc();cout << "debug1...." << endl;//只设置 宽和高 ，av_frame_get_buffer 函数还是会报错误。avframe1->width = 641 ;avframe1->height = 111;//设置 foramt为 AV_PIX_FMT_YUV420P，再次测试 就成功了avframe1->format = AV_PIX_FMT_RGB24;int ret = 0;ret = av_frame_get_buffer(avframe1, 0);if (ret < 0 ) {//如果方法失败，会返回一个 负数，可以通过 av_strerror函数打印这个具体的信息char buf[1024] = { 0 };av_strerror(ret, buf, sizeof(buf));cout << buf << endl;}cout << "debug2......" << endl;// avframe1 通过 av_frame_get_buffer 函数后，打印相关数据cout<< " 640 *111 yuv420p case , avframe1->linesize[0]  =  " << avframe1->linesize[0] << endl; ///640cout << " 640 *111 yuv420p case , avframe1->linesize[1]  =  " << avframe1->linesize[1] << endl; ///320cout << " 640 *111 yuv420p case , avframe1->linesize[2]  =  " << avframe1->linesize[2] << endl; ///320cout << " 641 *111 yuv420p case , avframe1->linesize[0]  =  " << avframe1->linesize[0] << endl;///672 由于字节对齐，多了一个32字节出来cout << " 641 *111 yuv420p case , avframe1->linesize[1]  =  " << avframe1->linesize[1] << endl;///352 由于字节对齐，多了一个32字节出来cout << " 641 *111 yuv420p case , avframe1->linesize[2]  =  " << avframe1->linesize[2] << endl;///352 由于字节对齐，多了一个32字节出来cout << " 640 *111 AV_PIX_FMT_RGB24 case , avframe1->linesize[0]  =  " << avframe1->linesize[0] << endl; //1920,这是因为640/32 是可以除尽的，因此640 * （RGB占用3个字节) = 1920cout << " 640 *111 AV_PIX_FMT_RGB24 case , avframe1->linesize[1]  =  " << avframe1->linesize[1] << endl; //0cout << " 640 *111 AV_PIX_FMT_RGB24 case , avframe1->linesize[2]  =  " << avframe1->linesize[2] << endl;//0cout << " 641 *111 AV_PIX_FMT_RGB24 case , avframe1->linesize[0]  =  " << avframe1->linesize[0] << endl; //2016, 这是因为641/32 是不能除尽的，因此 对于 多出来的这1个像素，本来占用1*3 = 3个字节就好，但是由于需要字节对齐，实际上给这1个像素要分配32个单位，因此实际分配位 32 *3 = 96字节 96+1920 = 2016个字节cout << " 641 *111 AV_PIX_FMT_RGB24 case , avframe1->linesize[1]  =  " << avframe1->linesize[1] << endl;cout << " 641 *111 AV_PIX_FMT_RGB24 case , avframe1->linesize[2]  =  " << avframe1->linesize[2] << endl;}

三。核心函数 av_frame_ref() 和 av_frame_unref(AVFrame *frame);   av_frame_free(AVFrame **frame);     av_buffer_get_ref_count(const AVBufferRef *buf);

int av_frame_ref(AVFrame *dst, const AVFrame *src);

引用计数 +1    和     引用计数 -1

void testAVframe1() {int ret = 0;AVFrame* avframe1 = av_frame_alloc();avframe1->width = 641;avframe1->height = 111;avframe1->format = AV_PIX_FMT_YUV420P;ret = av_frame_get_buffer(avframe1, 0);if (ret < 0) {//如果方法失败，会返回一个 负数，可以通过 av_strerror函数打印这个具体的信息char buf[1024] = { 0 };av_strerror(ret, buf, sizeof(buf));cout << buf << endl;}//这里有个疑问，这时 avframe没有放置具体的数据，为什么这个buf[0] 有值？if (avframe1->buf[0]){//av_buffer_get_ref_count函数打印 引用计数 为1cout << "frame1 ref count = " <<av_buffer_get_ref_count(avframe1->buf[0]); // 线程安全cout << endl;}AVFrame* avframe2 = av_frame_alloc();ret = av_frame_ref(avframe2, avframe1);if (ret <0 ) {//如果方法失败，会返回一个 负数，可以通过 av_strerror函数打印这个具体的信息char buf[1024] = { 0 };av_strerror(ret, buf, sizeof(buf));cout << buf << endl;}if (avframe1->buf[0]){//av_buffer_get_ref_count函数打印 引用计数2cout << "frame1 ref count = " <<av_buffer_get_ref_count(avframe1->buf[0]); // 线程安全cout << endl;}if (avframe2->buf[0]){//av_buffer_get_ref_count函数打印 引用计数2cout << "frame2 ref count = " <<av_buffer_get_ref_count(avframe2->buf[0]); // 线程安全cout << endl;}cout << "debug2...." << endl;av_frame_unref(avframe2);if (avframe1->buf[0]){//av_buffer_get_ref_count函数打印 引用计数1cout << "frame111111 ref count = " <<av_buffer_get_ref_count(avframe1->buf[0]); // 线程安全cout << endl;}//到这里 只是通过 av_frame_unref(avframe2); 释放了avframe2的内部数据，但是avframe2还是存在的if (avframe2->buf[0]){//走不到这一行//av_buffer_get_ref_count函数打印 引用计数cout << "frame222222 ref count = " <<av_buffer_get_ref_count(avframe2->buf[0]); // 线程安全cout << endl;}av_frame_free(&avframe2);cout << "debug3...." << endl;av_frame_free(&avframe1);
}