参考资料：

Constructing the Boilerplate

gstreamer插件-CSDN博客

在Constructing the Boilerplate 里面有生成插件的例子，

shell $ git clone https://gitlab.freedesktop.org/gstreamer/gst-template.git

使用里面的工具自动生成一个插件程序，比如MyFilter

shell $ cd gst-template/gst-plugin/src
shell $ ../tools/make_element MyFilter

注意，这样生成的自定义的element的名字使用起来应该是my_filter，

画一下图，就比较容易理解生成的代码结构

class的定义，init

element的定义，init

pad（sink，src）的定义

属性的设置和获取处理

chain的处理

event事件的处理

代码参考

 
 
/**
 * SECTION:element-myfilter
 *
 * FIXME:Describe myfilter here.
 *
 * <refsect2>
 * <title>Example launch line</title>
 * |[
 * gst-launch -v -m fakesrc ! myfilter ! fakesink silent=TRUE
 * ]|
 * </refsect2>
 */a
 
#ifdef HAVE_CONFIG_H
#  include <config.h>
#endif
 
 
#ifndef __GST_MYFILTER_H__
#define __GST_MYFILTER_H__
 
#include <gst/gst.h>
#include <stdio.h>
 
G_BEGIN_DECLS
 
#define GST_TYPE_MYFILTER (gst_my_filter_get_type())
G_DECLARE_FINAL_TYPE (GstMyFilter, gst_my_filter,
    GST, MYFILTER, GstElement)
 
struct _GstMyFilter
{
  GstElement element;
 
  GstPad *sinkpad, *srcpad;
  gboolean width;
  gboolean height;
  gboolean silent;
};
 
G_END_DECLS
 
#endif /* __GST_MYFILTER_H__ */
 
GST_DEBUG_CATEGORY_STATIC (gst_my_filter_debug);
#define GST_CAT_DEFAULT gst_my_filter_debug
 
#define DEFAULT_PROP_WIDTH 0    /* Original */
#define DEFAULT_PROP_HEIGHT 0   /* Original */
 
/* Filter signals and args */
enum
{
  /* FILL ME */
  LAST_SIGNAL
};
 
enum
{
  PROP_0,
  PROP_SILENT,
  PROP_WIDTH,
  PROP_HEIGHT
};
 
 
/* the capabilities of the inputs and outputs.
 *
 * describe the real formats here.
 */
static GstStaticPadTemplate sink_factory = GST_STATIC_PAD_TEMPLATE ("sink",
    GST_PAD_SINK,
    GST_PAD_ALWAYS,
    GST_STATIC_CAPS ("ANY"));
 
 
static GstStaticPadTemplate src_factory = GST_STATIC_PAD_TEMPLATE ("src",
    GST_PAD_SRC,
    GST_PAD_ALWAYS,
    GST_STATIC_CAPS ("video/x-raw, "
    "width = (int) [96, 1920], height = (int) [96, 1080]"
    )
    );
 
#define gst_my_filter_parent_class parent_class
G_DEFINE_TYPE (GstMyFilter, gst_my_filter, GST_TYPE_ELEMENT);
 
GST_ELEMENT_REGISTER_DEFINE (my_filter, "my_filter", GST_RANK_NONE,
    GST_TYPE_MYFILTER);
 
static void gst_my_filter_set_property (GObject * object,
    guint prop_id, const GValue * value, GParamSpec * pspec);
static void gst_my_filter_get_property (GObject * object,
    guint prop_id, GValue * value, GParamSpec * pspec);
 
static gboolean gst_my_filter_sink_event (GstPad * pad,
    GstObject * parent, GstEvent * event);
static GstFlowReturn gst_my_filter_chain (GstPad * pad,
    GstObject * parent, GstBuffer * buf);
/* GObject vmethod implementations */
 
/* initialize the myfilter's class */
static void
gst_my_filter_class_init (GstMyFilterClass * klass)
{
  GObjectClass *gobject_class;
  GstElementClass *gstelement_class;
 
  gobject_class = (GObjectClass *) klass;
  gstelement_class = (GstElementClass *) klass;
 
  gobject_class->set_property = gst_my_filter_set_property;
  gobject_class->get_property = gst_my_filter_get_property;
 
  g_object_class_install_property (gobject_class, PROP_SILENT,
      g_param_spec_boolean ("silent", "Silent", "Produce verbose output ?",
          FALSE, G_PARAM_READWRITE));
  g_object_class_install_property (gobject_class, PROP_WIDTH,
    g_param_spec_uint ("width", "Width",
        "Width (0 = original)",
        0, G_MAXINT, DEFAULT_PROP_WIDTH,
        G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS));
  g_object_class_install_property (gobject_class, PROP_HEIGHT,
      g_param_spec_uint ("height", "Height",
          "Height (0 = original)",
          0, G_MAXINT, DEFAULT_PROP_HEIGHT,
          G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS));
 
  gst_element_class_set_details_simple (gstelement_class,
      "MyFilter",
      "FIXME:Generic",
      "FIXME:Generic Template Element", " <<user@hostname.org>>");
 
  gst_element_class_add_pad_template (gstelement_class,
      gst_static_pad_template_get (&src_factory));
  gst_element_class_add_pad_template (gstelement_class,
      gst_static_pad_template_get (&sink_factory));
}
 
/* initialize the new element
 * instantiate pads and add them to element
 * set pad callback functions
 * initialize instance structure
 */
static void
gst_my_filter_init (GstMyFilter * filter)
{
  filter->sinkpad = gst_pad_new_from_static_template (&sink_factory, "sink");
  gst_pad_set_event_function (filter->sinkpad,
      GST_DEBUG_FUNCPTR (gst_my_filter_sink_event));
  gst_pad_set_chain_function (filter->sinkpad,
      GST_DEBUG_FUNCPTR (gst_my_filter_chain));
  GST_PAD_SET_PROXY_CAPS (filter->sinkpad);
  gst_element_add_pad (GST_ELEMENT (filter), filter->sinkpad);
 
  filter->srcpad = gst_pad_new_from_static_template (&src_factory, "src");
  GST_PAD_SET_PROXY_CAPS (filter->srcpad);
  gst_element_add_pad (GST_ELEMENT (filter), filter->srcpad);
 
  filter->silent = TRUE;
}
 
static void
gst_my_filter_set_property (GObject * object, guint prop_id,
    const GValue * value, GParamSpec * pspec)
{
  GstMyFilter *filter = GST_MYFILTER (object);
 
  switch (prop_id) {
    case PROP_SILENT:
      filter->silent = g_value_get_boolean (value);
      break;
    case PROP_WIDTH:
      filter->width = g_value_get_boolean (value);
      break;
    case PROP_HEIGHT:
      filter->height = g_value_get_boolean (value);
      break;
    default:
      G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
      break;
  }
}
 
static void
gst_my_filter_get_property (GObject * object, guint prop_id,
    GValue * value, GParamSpec * pspec)
{
  GstMyFilter *filter = GST_MYFILTER (object);
 
  switch (prop_id) {
    case PROP_SILENT:
      g_value_set_boolean (value, filter->silent);
      break;
    case PROP_WIDTH:
      g_value_set_boolean (value, filter->width);
      break;
    case PROP_HEIGHT:
      g_value_set_boolean (value, filter->height);
      break;
    default:
      G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
      break;
  }
}
 
/* GstElement vmethod implementations */
 
/* this function handles sink events */
static gboolean
gst_my_filter_sink_event (GstPad * pad, GstObject * parent,
    GstEvent * event)
{
  GstMyFilter *filter;
  gboolean ret;
 
  filter = GST_MYFILTER (parent);
 
  GST_LOG_OBJECT (filter, "Received %s event: %" GST_PTR_FORMAT,
      GST_EVENT_TYPE_NAME (event), event);
 
  switch (GST_EVENT_TYPE (event)) {
    case GST_EVENT_CAPS:
    {
      GstCaps *caps;
 
      GstStructure *structure;
      gst_event_parse_caps (event, &caps);
 
      structure = gst_caps_get_structure(caps, 0);
 
      gint width, height;
      gst_structure_get_int(structure, "width", &width);
      gst_structure_get_int(structure, "height", &height);
      filter->width = width;
      filter->height = height;
 
      /* and forward */
      ret = gst_pad_event_default (pad, parent, event);
      break;
    }
    default:
      ret = gst_pad_event_default (pad, parent, event);
      break;
  }
  return ret;
}
 
 
 
/* chain function
 * this function does the actual processing
 */
static GstFlowReturn
gst_my_filter_chain (GstPad * pad, GstObject * parent, GstBuffer * buf)
{
  GstMyFilter *filter;
 
  filter = GST_MYFILTER (parent);
  
  if (filter->silent == FALSE){
    g_print ("I'm plugged, therefore I'm in.\n");
  }
 
  GstFlowReturn ret = gst_pad_push (filter->srcpad, buf);
  
  /* just push out the incoming buffer without touching it */
  return ret;
}
 
 
/* entry point to initialize the plug-in
 * initialize the plug-in itself
 * register the element factories and other features
 */
static gboolean
myfilter_init (GstPlugin * myfilter)
{
  /* debug category for filtering log messages
   *
   * exchange the string 'Template myfilter' with your description
   */
  GST_DEBUG_CATEGORY_INIT (gst_my_filter_debug, "myfilter",
      0, "Template myfilter");
 
  return GST_ELEMENT_REGISTER (my_filter, myfilter);
}
 
/* PACKAGE: this is usually set by meson depending on some _INIT macro
 * in meson.build and then written into and defined in config.h, but we can
 * just set it ourselves here in case someone doesn't use meson to
 * compile this code. GST_PLUGIN_DEFINE needs PACKAGE to be defined.
 */
#ifndef PACKAGE
#define PACKAGE "myfirstmyfilter"
#endif
 
/* gstreamer looks for this structure to register myfilters
 *
 * exchange the string 'Template myfilter' with your myfilter description
 */
GST_PLUGIN_DEFINE (GST_VERSION_MAJOR,
    GST_VERSION_MINOR,
    myfilter,
    "my_filter",
    myfilter_init,
    PACKAGE_VERSION, GST_LICENSE, GST_PACKAGE_NAME, GST_PACKAGE_ORIGIN)

在gst_my_filter_chain方法中，

static GstFlowReturn
gst_my_filter_chain (GstPad * pad, GstObject * parent, GstBuffer * buf)
{
  GstMyFilter *filter;
 
  filter = GST_MYFILTER (parent);
  
  if (filter->silent == FALSE){
    g_print ("I'm plugged, therefore I'm in.\n");
  }
 
  GstFlowReturn ret = gst_pad_push (filter->srcpad, buf);
  
  /* just push out the incoming buffer without touching it */
  return ret;
}

可以使用Basic tutorial 8: Short-cutting the pipeline 例子的的方法来获取buffer中的内存数据，来进行操作，

  gst_buffer_map (buffer, &map, GST_MAP_WRITE);
  raw = (gint16 *)map.data;

 比如数据格式是RGB，获取到长宽后，可以进行颜色转换，画面镜像处理等等，

就比如最近欧洲杯里讨论的比较火的广告内容区域化转换处理，就可以在gst_my_filter_chain里进行