一、首先是运行这个程序：
拷贝sample_720p.h264到deepstream-test2
cd deepstream-test2/
make
./deepstream-test2-app sample_720p.h264
deepstream-test2相对deepstream-test1多了目标跟踪和分类。所以运行速度较慢。五个模型对视频的处理（目标跟踪模型以库的形式提供）。如下图：

运行效果如下图：

二、理解代码需要Gstreamer的知识，还需要C语言的基础
代码如下：

#include <gst/gst.h>
#include <glib.h>

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include "gstnvdsmeta.h"

#define PGIE_CONFIG_FILE  "dstest2_pgie_config.txt"
#define SGIE1_CONFIG_FILE "dstest2_sgie1_config.txt"
#define SGIE2_CONFIG_FILE "dstest2_sgie2_config.txt"
#define SGIE3_CONFIG_FILE "dstest2_sgie3_config.txt"
#define MAX_DISPLAY_LEN 64

#define TRACKER_CONFIG_FILE "dstest2_tracker_config.txt"
#define MAX_TRACKING_ID_LEN 16

#define PGIE_CLASS_ID_VEHICLE 0
#define PGIE_CLASS_ID_PERSON 2

/* The muxer output resolution must be set if the input streams will be of
 * different resolution. The muxer will scale all the input frames to this
 * resolution. */
#define MUXER_OUTPUT_WIDTH 1920
#define MUXER_OUTPUT_HEIGHT 1080

/* Muxer batch formation timeout, for e.g. 40 millisec. Should ideally be set
 * based on the fastest source's framerate. */
#define MUXER_BATCH_TIMEOUT_USEC 4000000

gint frame_number = 0;
/* These are the strings of the labels for the respective models */
gchar sgie1_classes_str[12][32] = {
     "black", "blue", "brown", "gold", "green",
  "grey", "maroon", "orange", "red", "silver", "white", "yellow"
};

gchar sgie2_classes_str[20][32] =
    {
     "Acura", "Audi", "BMW", "Chevrolet", "Chrysler",
  "Dodge", "Ford", "GMC", "Honda", "Hyundai", "Infiniti", "Jeep", "Kia",
      "Lexus", "Mazda", "Mercedes", "Nissan",
  "Subaru", "Toyota", "Volkswagen"
};

gchar sgie3_classes_str[6][32] = {
     "coupe", "largevehicle", "sedan", "suv",
  "truck", "van"
};

gchar pgie_classes_str[4][32] =
    {
     "Vehicle", "TwoWheeler", "Person", "RoadSign" };

/* gie_unique_id is one of the properties in the above dstest2_sgiex_config.txt
 * files. These should be unique and known when we want to parse the Metadata
 * respective to the sgie labels. Ideally these should be read from the config
 * files but for brevity we ensure they are same. */

guint sgie1_unique_id = 2;
guint sgie2_unique_id = 3;
guint sgie3_unique_id = 4;

/* This is the buffer probe function that we have registered on the sink pad
 * of the OSD element. All the infer elements in the pipeline shall attach
 * their metadata to the GstBuffer, here we will iterate & process the metadata
 * forex: class ids to strings, counting of class_id objects etc. */
 
 //这是我们在OSD元素的sink pad上注册的缓冲区探测函数。
 //管道中的所有推理单元都应将其元数据附加到GstBuffer，
 //在这里，我们将迭代并处理元数据，显示和统计物体个数等。
static GstPadProbeReturn
osd_sink_pad_buffer_probe (GstPad * pad, GstPadProbeInfo * info,
    gpointer u_data)
{
    
    GstBuffer *buf = (GstBuffer *) info->data;
    guint num_rects = 0;
    NvDsObjectMeta *obj_meta = NULL;
    guint vehicle_count = 0;
    guint person_count = 0;
    NvDsMetaList * l_frame = NULL;
    NvDsMetaList * l_obj = NULL;
    NvDsDisplayMeta *display_meta = NULL;

    NvDsBatchMeta *batch_meta = gst_buffer_get_nvds_batch_meta (buf);

    for (l_frame = batch_meta->frame_meta_list; l_frame != NULL;
      l_frame = l_frame->next) {
    
        NvDsFrameMeta *frame_meta = (NvDsFrameMeta *) (l_frame->data);
        int offset = 0;
        for (l_obj = frame_meta->obj_meta_list; l_obj != NULL;
                l_obj = l_obj->next) {
    
            obj_meta = (NvDsObjectMeta *) (l_obj->data);
            if (obj_meta->class_id == PGIE_CLASS_ID_VEHICLE) {
    
                vehicle_count++;
                num_rects++;//统计汽车个数
            }
            if (obj_meta->class_id == PGIE_CLASS_ID_PERSON) {
    
                person_count++;
                num_rects++;//统计人的个数
            }
        }
        display_meta = nvds_acquire_display_meta_from_pool(batch_meta);//要显示的文字类数据
        NvOSD_TextParams *txt_params  = &display_meta->text_params[0];
        display_meta->num_labels = 1;
        txt_params->display_text = g_malloc0 (MAX_DISPLAY_LEN);
        offset = snprintf(txt_params->display_text, MAX_DISPLAY_LEN, "Person = %d ", person_count);
        offset = snprintf(txt_params->display_text + offset , MAX_DISPLAY_LEN, "Vehicle = %d ", vehicle_count);

        /* Now set the offsets where the string should appear */
        txt_params->x_offset = 10;
        txt_params->y_offset = 12;

        /* Font , font-color and font-size *///字体相关的设置
        txt_params->font_params.font_name = "Serif";
        txt_params->font_params.font_size = 10;
        txt_params->font_params.font_color.red = 1.0;
        txt_params->font_params.font_color.green = 1.0;
        txt_params->font_params.font_color.blue = 1.0;
        txt_params->font_params.font_color.alpha = 1.0;

        /* Text background color */
        txt_params->set_bg_clr = 1;//背景颜色的设置
        txt_params->text_bg_clr.red = 0.0;
        txt_params->text_bg_clr.green = 0.0;
        txt_params->text_bg_clr.blue = 0.0;
        txt_params->text_bg_clr.alpha = 1.0;

        nvds_add_display_meta_to_frame(frame_meta, display_meta);//显示帧数据、图像数据、文字等
    }

    g_print ("Frame Number = %d Number of objects = %d "
            "Vehicle Count = %d Person Count = %d\n",
            frame_number, num_rects, vehicle_count, person_count);
    frame_number++;
    return GST_PAD_PROBE_OK;
}

static gboolean//针对不同的消息类型进行相应的处理
bus_call (GstBus * bus, GstMessage * msg, gpointer data)
{
    
  GMainLoop *loop = (GMainLoop *) data;
  switch (GST_MESSAGE_TYPE (msg)) {
    
    case GST_MESSAGE_EOS://文件流结束的处理
      g_print ("End of stream\n");
      g_main_loop_quit (loop);
      break;
    case GST_MESSAGE_ERROR:{
    //错误的处理
      gchar *debug;
      GError *error;
      gst_message_parse_error (msg, &error, &debug);
      g_printerr ("ERROR from element %s: %s\n",
          GST_OBJECT_NAME (msg->src), error->message);
      if (debug)
        g_printerr ("Error details: %s\n", debug);
      g_free (debug);
      g_error_free (error);
      g_main_loop_quit (loop);
      break;
    }
    default:
      break;
  }
  return TRUE;
}

/* Tracker config parsing */

#define CHECK_ERROR(error) \
    if (error) { \
        g_printerr ("Error while parsing config file: %s\n", error->message); \
        goto done; \
    }

#define CONFIG_GROUP_TRACKER "tracker"
#define CONFIG_GROUP_TRACKER_WIDTH "tracker-width"
#define CONFIG_GROUP_TRACKER_HEIGHT "tracker-height"
#define CONFIG_GROUP_TRACKER_LL_CONFIG_FILE "ll-config-file"
#define CONFIG_GROUP_TRACKER_LL_LIB_FILE "ll-lib-file"
#define CONFIG_GROUP_TRACKER_ENABLE_BATCH_PROCESS "enable-batch-process"
#define CONFIG_GPU_ID "gpu-id"

static gchar *
get_absolute_file_path (gchar *cfg_file_path, gchar *file_path)
{
    
  gchar abs_cfg_path[PATH_MAX + 1];
  gchar *abs_file_path;
  gchar *delim;

  if (file_path && file_path[0] == '/') {
    
    return file_path;
  }

  if (!realpath (cfg_file_path, abs_cfg_path)) {
    
    g_free (file_path);
    return NULL;
  }

  // Return absolute path of config file if file_path is NULL.
  if (!file_path) {
    
    abs_file_path = g_strdup (abs_cfg_path);
    return abs_file_path;
  }

  delim = g_strrstr (abs_cfg_path, "/");
  *(delim + 1) = '\0';

  abs_file_path = g_strconcat (abs_cfg_path, file_path, NULL);
  g_free (file_path);

  return abs_file_path;
}

static gboolean
set_tracker_properties (GstElement *nvtracker)
{
    
  gboolean ret = FALSE;
  GError *error = NULL;
  gchar **keys = NULL;
  gchar **key = NULL;
  GKeyFile *key_file = g_key_file_new ();

  if (!g_key_file_load_from_file (key_file, TRACKER_CONFIG_FILE, G_KEY_FILE_NONE,
          &error)) {
    
    g_printerr ("Failed to load config file: %s\n", error->message);
    return FALSE;
  }//导入配置文件

  keys = g_key_file_get_keys (key_file, CONFIG_GROUP_TRACKER, NULL, &error);
  //返回tracker的keys
  //tracker-width
  //tracker-height
  //gpu-id
  //ll-lib-file
  //ll-config-file
  //enable-batch-process
  CHECK_ERROR (error);
  //对nvtracker进行相应的设置
  for (key = keys; *key; key++) {
    
    if (!g_strcmp0 (*key, CONFIG_GROUP_TRACKER_WIDTH)) {
    
      gint width =
          g_key_file_get_integer (key_file, CONFIG_GROUP_TRACKER,
          CONFIG_GROUP_TRACKER_WIDTH, &error);
      CHECK_ERROR (error);
      g_object_set (G_OBJECT (nvtracker), "tracker-width", width, NULL);
    } else if (!g_strcmp0 (*key, CONFIG_GROUP_TRACKER_HEIGHT)) {
    
      gint height =
          g_key_file_get_integer (key_file, CONFIG_GROUP_TRACKER,
          CONFIG_GROUP_TRACKER_HEIGHT, &error);
      CHECK_ERROR (error);
      g_object_set (G_OBJECT (nvtracker), "tracker-height", height, NULL);
    } else if (!g_strcmp0 (*key, CONFIG_GPU_ID)) {
    
      guint gpu_id =
          g_key_file_get_integer (key_file, CONFIG_GROUP_TRACKER,
          CONFIG_GPU_ID, &error);
      CHECK_ERROR (error);
      g_object_set (G_OBJECT (nvtracker), "gpu_id", gpu_id, NULL);
    } else if (!g_strcmp0 (*key, CONFIG_GROUP_TRACKER_LL_CONFIG_FILE)) {
    
      char* ll_config_file = get_absolute_file_path (TRACKER_CONFIG_FILE,
                g_key_file_get_string (key_file,
                    CONFIG_GROUP_TRACKER,
                    CONFIG_GROUP_TRACKER_LL_CONFIG_FILE, &error));
      CHECK_ERROR (error);
      g_object_set (G_OBJECT (nvtracker), "ll-config-file", ll_config_file, NULL);
    } else if (!g_strcmp0 (*key, CONFIG_GROUP_TRACKER_LL_LIB_FILE)) {
    
      char* ll_lib_file = get_absolute_file_path (TRACKER_CONFIG_FILE,
                g_key_file_get_string (key_file,
                    CONFIG_GROUP_TRACKER,
                    CONFIG_GROUP_TRACKER_LL_LIB_FILE, &error));
      CHECK_ERROR (error);
      g_object_set (G_OBJECT (nvtracker), "ll-lib-file", ll_lib_file, NULL);
    } else if (!g_strcmp0 (*key, CONFIG_GROUP_TRACKER_ENABLE_BATCH_PROCESS)) {
    
      gboolean enable_batch_process =
          g_key_file_get_integer (key_file, CONFIG_GROUP_TRACKER,
          CONFIG_GROUP_TRACKER_ENABLE_BATCH_PROCESS, &error);
      CHECK_ERROR (error);
      g_object_set (G_OBJECT (nvtracker), "enable_batch_process",
                    enable_batch_process, NULL);
    } else {
    
      g_printerr ("Unknown key '%s' for group [%s]", *key,
          CONFIG_GROUP_TRACKER);
    }
  }

  ret = TRUE;
done:
  if (error) {
    
    g_error_free (error);
  }
  if (keys) {
    
    g_strfreev (keys);
  }
  if (!ret) {
    
    g_printerr ("%s failed", __func__);
  }
  return ret;
}

int
main (int argc, char *argv[])
{
    
  GMainLoop *loop = NULL;
  GstElement *pipeline = NULL, *source = NULL, *h264parser = NULL,
      *decoder = NULL, *streammux = NULL, *sink = NULL, *pgie = NULL, *nvvidconv = NULL,
      *nvosd = NULL, *sgie1 = NULL, *sgie2 = NULL, *sgie3 = NULL, *nvtracker = NULL;
  g_print ("With tracker\n");
#ifdef PLATFORM_TEGRA
  GstElement *transform = NULL;
#endif
  GstBus *bus = NULL;
  guint bus_watch_id = 0;
  GstPad *osd_sink_pad = NULL;

  /* Check input arguments */
  if (argc != 2) {
    
    g_printerr ("Usage: %s <elementary H264 filename>\n", argv[0]);
    return -1;
  }

  /* Standard GStreamer initialization */
  gst_init (&argc, &argv);
  loop = g_main_loop_new (NULL, FALSE);

  /* Create gstreamer elements */

  /* Create Pipeline element that will be a container of other elements */
  pipeline = gst_pipeline_new ("dstest2-pipeline");
  //通过gst_pipeline_new创建pipeline，参数为pipeline的名字。
  /* Source element for reading from the file */
  source = gst_element_factory_make ("filesrc", "file-source");
  //创建一个element  类型是filesrc，名字是file-source。
  //该element用于读取文件
  /* Since the data format in the input file is elementary h264 stream,
   * we need a h264parser */
  h264parser = gst_element_factory_make ("h264parse", "h264-parser");
  //创建一个element  类型是h264parse，名字是h264-parser。
  //该element用于解析h264文件
  /* Use nvdec_h264 for hardware accelerated decode on GPU */
  decoder = gst_element_factory_make ("nvv4l2decoder", "nvv4l2-decoder");
  //该element用于调用GPU硬件加速解码h264文件

  /* Create nvstreammux instance to form batches from one or more sources. */
  streammux = gst_element_factory_make ("nvstreammux", "stream-muxer");
 //该element用于把输入按照参数处理成一系列的视频帧
  if (!pipeline || !streammux) {
    
    g_printerr ("One element could not be created. Exiting.\n");
    return -1;
  }

  /* Use nvinfer to run inferencing on decoder's output,
   * behaviour of inferencing is set through config file */
  pgie = gst_element_factory_make ("nvinfer", "primary-nvinference-engine");
  //目标检测的模型，对输入图像进行推理，通过推理的配置文件
  /* We need to have a tracker to track the identified objects */
  nvtracker = gst_element_factory_make ("nvtracker", "tracker");
  //跟踪器  跟踪识别出来的目标
  /* We need three secondary gies so lets create 3 more instances of
     nvinfer */
  sgie1 = gst_element_factory_make ("nvinfer", "secondary1-nvinference-engine");
  //分类器--车的颜色
  sgie2 = gst_element_factory_make ("nvinfer", "secondary2-nvinference-engine");
  //分类器--车的厂家
  sgie3 = gst_element_factory_make ("nvinfer", "secondary3-nvinference-engine");
  //分类器--车的类型
  /* Use convertor to convert from NV12 to RGBA as required by nvosd */
  nvvidconv = gst_element_factory_make ("nvvideoconvert", "nvvideo-converter");
    //视频颜色格式转换

  /* Create OSD to draw on the converted RGBA buffer */
  nvosd = gst_element_factory_make ("nvdsosd", "nv-onscreendisplay");
  //处理RGBA buffer 绘制ROI，绘制识别对象的Bounding Box，绘制识别对象的文字标签（字体、颜色、标示框）
  
  /* Finally render the osd output */
#ifdef PLATFORM_TEGRA
  transform = gst_element_factory_make ("nvegltransform", "nvegl-transform");
    // 转换成 EGLImage instance 给nveglglessink使用

#endif
  sink = gst_element_factory_make ("nveglglessink", "nvvideo-renderer");

  if (!source || !h264parser || !decoder || !pgie ||
      !nvtracker || !sgie1 || !sgie2 || !sgie3 || !nvvidconv || !nvosd || !sink) {
    
    g_printerr ("One element could not be created. Exiting.\n");
    return -1;
  }

#ifdef PLATFORM_TEGRA
  if(!transform) {
    
    g_printerr ("One tegra element could not be created. Exiting.\n");
    return -1;
  }
#endif

  /* Set the input filename to the source element */
  g_object_set (G_OBJECT (source), "location", argv[1], NULL);
   //设置source的位置，source是视频文件

  g_object_set (G_OBJECT (streammux), "width", MUXER_OUTPUT_WIDTH, "height",
      MUXER_OUTPUT_HEIGHT, "batch-size", 1,
      "batched-push-timeout", MUXER_BATCH_TIMEOUT_USEC, NULL);
   //设置视频格式，如分辨率等

  /* Set all the necessary properties of the nvinfer element,
   * the necessary ones are : */
  g_object_set (G_OBJECT (pgie), "config-file-path", PGIE_CONFIG_FILE, NULL);//目标检测的配置文件
  g_object_set (G_OBJECT (sgie1), "config-file-path", SGIE1_CONFIG_FILE, NULL);//颜色分类的配置文件
  g_object_set (G_OBJECT (sgie2), "config-file-path", SGIE2_CONFIG_FILE, NULL);//车厂分类的配置文件
  g_object_set (G_OBJECT (sgie3), "config-file-path", SGIE3_CONFIG_FILE, NULL);//车型分类的配置文件

  /* Set necessary properties of the tracker element. */
  //设置目标跟踪器
  if (!set_tracker_properties(nvtracker)) {
    
    g_printerr ("Failed to set tracker properties. Exiting.\n");
    return -1;
  }

  /* we add a message handler */
  bus = gst_pipeline_get_bus (GST_PIPELINE (pipeline));
  bus_watch_id = gst_bus_add_watch (bus, bus_call, loop);//指定消息处理函数
  gst_object_unref (bus);

  /* Set up the pipeline */
  /* we add all elements into the pipeline */
  /* decoder | pgie1 | nvtracker | sgie1 | sgie2 | sgie3 | etc.. */
#ifdef PLATFORM_TEGRA
  gst_bin_add_many (GST_BIN (pipeline),
      source, h264parser, decoder, streammux, pgie, nvtracker, sgie1, sgie2, sgie3,
      nvvidconv, nvosd, transform, sink, NULL);//element加入到pipeline
#else
  gst_bin_add_many (GST_BIN (pipeline),
      source, h264parser, decoder, streammux, pgie, nvtracker, sgie1, sgie2, sgie3,
      nvvidconv, nvosd, sink, NULL);
#endif

  GstPad *sinkpad, *srcpad;
  gchar pad_name_sink[16] = "sink_0";
  gchar pad_name_src[16] = "src";

  sinkpad = gst_element_get_request_pad (streammux, pad_name_sink);
  if (!sinkpad) {
    
    g_printerr ("Streammux request sink pad failed. Exiting.\n");
    return -1;
  }//获取指定element中的指定pad  该element为 streammux

  srcpad = gst_element_get_static_pad (decoder, pad_name_src);
  if (!srcpad) {
    
    g_printerr ("Decoder request src pad failed. Exiting.\n");
    return -1;
  }

  if (gst_pad_link (srcpad, sinkpad) != GST_PAD_LINK_OK) {
    
      g_printerr ("Failed to link decoder to stream muxer. Exiting.\n");
      return -1;
  }

  gst_object_unref (sinkpad);
  gst_object_unref (srcpad);

  /* Link the elements together */
  if (!gst_element_link_many (source, h264parser, decoder, NULL)) {
    
    g_printerr ("Elements could not be linked: 1. Exiting.\n");
    return -1;
  }

#ifdef PLATFORM_TEGRA
  if (!gst_element_link_many (streammux, pgie, nvtracker, sgie1,
      sgie2, sgie3, nvvidconv, nvosd, transform, sink, NULL)) {
    
    g_printerr ("Elements could not be linked. Exiting.\n");
    return -1;
  }
#else
  if (!gst_element_link_many (streammux, pgie, nvtracker, sgie1,
      sgie2, sgie3, nvvidconv, nvosd, sink, NULL)) {
    
    g_printerr ("Elements could not be linked. Exiting.\n");
    return -1;
  }
#endif

  /* Lets add probe to get informed of the meta data generated, we add probe to
   * the sink pad of the osd element, since by that time, the buffer would have
   * had got all the metadata. */
  osd_sink_pad = gst_element_get_static_pad (nvosd, "sink");
  
  if (!osd_sink_pad)
    g_print ("Unable to get sink pad\n");
  else
    gst_pad_add_probe (osd_sink_pad, GST_PAD_PROBE_TYPE_BUFFER,
        osd_sink_pad_buffer_probe, NULL, NULL);
//osd_sink_pad_buffer_probe 创建探针 
//以上都是设置属性，连接Elements，设置消息等操作，先把整个的视频处理流程勾勒出来。



  /* Set the pipeline to "playing" state */
  g_print ("Now playing: %s\n", argv[1]);
  gst_element_set_state (pipeline, GST_STATE_PLAYING);//运行

  /* Iterate */
  g_print ("Running...\n");
  g_main_loop_run (loop);//bus_call 消息处理函数可以结束loop

  /* Out of the main loop, clean up nicely */
  g_print ("Returned, stopping playback\n");
  gst_element_set_state (pipeline, GST_STATE_NULL);//释放为pipeline分配的所有资源
  g_print ("Deleting pipeline\n");
  gst_object_unref (GST_OBJECT (pipeline));
  g_source_remove (bus_watch_id);
  g_main_loop_unref (loop);//销毁loop对象
  return 0;
}

参考文献
[1].https://docs.nvidia.com/metropolis/deepstream/plugin-manual/index.html
[2].https://docs.nvidia.com/metropolis/deepstream/dev-guide/DeepStream_Development_Guide/baggage/index.html