GStreamer pwg ch 4~6

 

1.     Specifying the pads

 

• Ÿ   _event() function
  • n  format 설정 함수
• Ÿ   _init() 함수에서
  • n  _class_init에서 등록된 pad template에서 pad를 생성
• Ÿ   _chain()
  • n  sinkpad에서 input dat를 받을 function pointer를 설정
• Ÿ   _event() / _query()
  • n  선택적으로 설정 가능

 

pad의 동작

• Ÿ   looping mode로 동작함
  • n  data를 스스로 pull 할 수 있음

 

static void gst_my_filter_init (GstMyFilter *filter) {   /* pad through which data comes in to the element */ filter->sinkpad = gst_pad_new_from_static_template (&sink_template, "sink");     /* pads are configured here with gst_pad_set_*_function () */   gst_element_add_pad (GST_ELEMENT (filter), filter->sinkpad);     /* pad through which data goes out of the element */ filter->srcpad = gst_pad_new_from_static_template (&src_template, "src");     /* pads are configured here with gst_pad_set_*_function () */   gst_element_add_pad (GST_ELEMENT (filter), filter->srcpad);     /* properties initial value */   filter->silent = FALSE; }

 

 

5.     The Chain function

chain function은 모든 data가 위치하게 되는 function

 

Ÿ   _chain()

• n  아래 코드에서는 linear function
• n  각 incoming buffer에 대해서 buffer 하나를 내보냄

 

static GstFlowReturn gst_my_filter_chain (GstPad    *pad,                                           GstObject *parent,                                           GstBuffer *buf);   [..]   static void gst_my_filter_init (GstMyFilter * filter) { [..]   /* configure chain function on the pad before adding    * the pad to the element */ gst_pad_set_chain_function (filter->sinkpad, gst_my_filter_chain);   [..] }   static GstFlowReturn gst_my_filter_chain (GstPad    *pad,                      GstObject *parent,              GstBuffer *buf) {   GstMyFilter *filter = GST_MY_FILTER (parent);     if (!filter->silent)     g_print ("Have data of size %" G_GSIZE_FORMAT" bytes!\n",         gst_buffer_get_size (buf));     return gst_pad_push (filter->srcpad, buf); }

 

buffer는 read-only 일 수도 있음

 

event handling function을 추가 할 수도 있음

stream-event가 보내지면 호출됨

• Ÿ   caps
• Ÿ   EOS
• Ÿ   newsegment
• Ÿ   tags
• Ÿ   …

static void gst_my_filter_init (GstMyFilter * filter) { [..]   gst_pad_set_event_function (filter->sinkpad,       gst_my_filter_sink_event); [..] }     static gboolean gst_my_filter_sink_event (GstPad    *pad,                   GstObject *parent,                   GstEvent  *event) {   GstMyFilter *filter = GST_MY_FILTER (parent);     switch (GST_EVENT_TYPE (event)) {     case GST_EVENT_CAPS:       /* we should handle the format here */       break;     case GST_EVENT_EOS:       /* end-of-stream, we should close down all stream leftovers here */       gst_my_filter_stop_processing (filter);       break;     default:       break;   }     return gst_pad_event_default (pad, parent, event); }     static GstFlowReturn gst_my_filter_chain (GstPad    *pad,              GstObject *parent,              GstBuffer *buf) {   GstMyFilter *filter = GST_MY_FILTER (parent);   GstBuffer *outbuf;     outbuf = gst_my_filter_process_data (filter, buf);   gst_buffer_unref (buf);   if (!outbuf) {     /* something went wrong - signal an error */     GST_ELEMENT_ERROR (GST_ELEMENT (filter), STREAM, FAILED, (NULL), (NULL));     return GST_FLOW_ERROR;   }     return gst_pad_push (filter->srcpad, outbuf); }

 

 

6.     The event function

data stream 상에 event가 발생하면 호출됨

• Ÿ   caps, EOS, netsegment, tags, etc.
• Ÿ   event는 upstream과 downstream으로 둘 다 흐름
  • n  즉, sink pads와 source pads에서 event를 받을 수 있음

 

sink pad 상에 설치된 간단한 event function

static gboolean gst_my_filter_sink_event (GstPad    *pad,                                           GstObject *parent,                                           GstEvent  *event);   [..]   static void gst_my_filter_init (GstMyFilter * filter) { [..]   /* configure event function on the pad before adding    * the pad to the element */   gst_pad_set_event_function (filter->sinkpad,       gst_my_filter_sink_event); [..] }   static gboolean gst_my_filter_sink_event (GstPad    *pad,                   GstObject *parent,                   GstEvent  *event) {   gboolean ret;   GstMyFilter *filter = GST_MY_FILTER (parent);     switch (GST_EVENT_TYPE (event)) {     case GST_EVENT_CAPS:       /* we should handle the format here */         /* push the event downstream */       ret = gst_pad_push_event (filter->srcpad, event);       break;     case GST_EVENT_EOS:       /* end-of-stream, we should close down all stream leftovers here */       gst_my_filter_stop_processing (filter);       ret = gst_pad_event_default (pad, parent, event);       break;     default:       /* just call the default handler */       ret = gst_pad_event_default (pad, parent, event);       break;   }   return ret; }

 

Ÿ   알려지지 않은 event에 대해서는 기본 event handler인 gst_pad_event_default()를 호출하는 것이 좋음

Ÿ   event type에 따라서, default handler는 event를 넘기던가 단순히 unref하기도 함

Ÿ   CAPS event

n  기본적으로 forward하지 않음

n  직접 처리해야 함