uridecodebin

How to use uridecodebin.

uridecodebin能够将URI数据解码成raw media，它会自动选择一个能处理uri数据的source element并将其和decodebin链接。

Github: uridecodebin

uri

 bool Create (void)
 {
     // ...
 ​
     g_object_set (G_OBJECT (m_source), "uri", m_config.src.c_str(), NULL);
 ​
     // ...
 }

signals

source-setup

 static void cb_uridecodebin_source_setup (
     GstElement* pipeline, GstElement* source, gpointer user_data)
 {
     VideoPipeline* vp = reinterpret_cast<VideoPipeline*> (user_data);
 ​
     LOG_INFO_MSG ("cb_uridecodebin_source_setup called");
 ​
     /* Configure rtspsrc
     if (g_object_class_find_property (G_OBJECT_GET_CLASS (source), "latency")) {
         LOG_INFO_MSG ("cb_uridecodebin_source_setup set %d latency",
             vp->m_config.rtsp_latency);
         g_object_set (G_OBJECT (source), "latency", vp->m_config.rtsp_latency, NULL);
     }
     */
 ​
    /* Configure appsrc
     GstCaps *m_sCaps;
     src_Caps = gst_caps_new_simple ("video/x-raw", "format", G_TYPE_STRING,
         m_config.src_format.c_str(), "width", G_TYPE_INT, m_config.src_width,
           "height", G_TYPE_INT, m_config.src_height, NULL);
     g_object_set (G_OBJECT(source), "caps", src_Caps, NULL);
     g_signal_connect (source, "need-data", G_CALLBACK (start_feed), data);
     g_signal_connect (source, "enough-data", G_CALLBACK (stop_feed), data);
     gst_caps_unref (src_Caps);
    */
 }

uridecodebin会分析uri属性的值，然后选择合适的srouce element，这个uri值必须是完整的绝对路径，由source类型开始。

child-added

 static void cb_decodebin_child_added (
     GstChildProxy* child_proxy, GObject* object, gchar* name, gpointer user_data)
 {
     LOG_INFO_MSG ("cb_decodebin_child_added called");
 ​
     VideoPipeline* vp = reinterpret_cast<VideoPipeline*> (user_data);
 ​
     LOG_INFO_MSG ("Element '%s' added to decodebin", name);
 ​
     if (g_strrstr (name, "qtdemux") == name) {
         vp->m_qtdemux = reinterpret_cast<GstElement*> (object);
     }
 ​
     if ((g_strrstr (name, "h264parse") == name)) {
         vp->m_h264parse = reinterpret_cast<GstElement*> (object);
     }
 ​
     if (g_strrstr (name, "qtivdec") == name) {
         vp->m_decoder = reinterpret_cast<GstElement*> (object);
         g_object_set (object, "turbo", vp->m_config.turbo, NULL);
         g_object_set (object, "skip-frames", vp->m_config.skip_frame, NULL);
     }
 }
 ​
 static void cb_uridecodebin_child_added (
     GstChildProxy* child_proxy, GObject* object, gchar* name, gpointer user_data)
 {
     LOG_INFO_MSG ("cb_uridecodebin_child_added called");
 ​
     LOG_INFO_MSG ("Element '%s' added to uridecodebin", name);
     VideoPipeline* vp = reinterpret_cast<VideoPipeline*> (user_data);
 ​
     if (g_strrstr (name, "decodebin") == name) {
         g_signal_connect (G_OBJECT (object), "child-added",
             G_CALLBACK (cb_decodebin_child_added), vp);
     }
 }

通过打印log可以看出uridecodebin初始化过程中自动添加到pipeline中的所有GstElement：

  filesrc: qtdemux, multiqueue, h264parse/h265parse, capfilter, aacparse, avdec_aac, qtivdec
  rtspsrc: rtph264depay/rtph265depay, h264parse/h265parse, capfilter, qtivdec

在uridecodebin只会添加decodebin一个GstElement，上述的GstElement均由decodebin构建，因此除了uridecodebin的child-added回调，还在其回调中添加了一个decodebin的child-added回调，用于设置decodebin构建的GstElement的属性。

在build pipeline中提到关于filesrc插件的解复用，需要手动进行link，但这个link在实际测试过程中有decodebin自动完成，我尝试手动去做link，由于无法获取到h264parse的sink pad程序抛出了段错误。我打印了这时候vp->m_h264parse的指针地址，是一个初始值，说明这时候还未初始化vp->m_h264parse。

pad-added

 static void cb_uridecodebin_pad_added (
     GstElement* src, GstPad* new_pad, gpointer user_data)
 {
     LOG_INFO_MSG ("cb_uridecodebin_pad_added called");
 ​
     GstPadLinkReturn ret;
     GstCaps*         new_pad_caps = NULL;
     GstStructure*    new_pad_struct = NULL;
     const gchar*     new_pad_type = NULL;
     GstPad*          v_sinkpad = NULL;
     GstPad*          a_sinkpad = NULL;
 ​
     VideoPipeline* vp = reinterpret_cast<VideoPipeline*> (user_data);
 ​
     new_pad_caps = gst_pad_get_current_caps (new_pad);
     new_pad_struct = gst_caps_get_structure (new_pad_caps, 0);
     new_pad_type = gst_structure_get_name (new_pad_struct);
 ​
     if (g_str_has_prefix (new_pad_type, "video/x-raw")) {
         LOG_INFO_MSG ("Linking video/x-raw");
         /* Attempt the link */
         v_sinkpad = gst_element_get_static_pad (
                         reinterpret_cast<GstElement*> (vp->m_videoConv), "sink");
         ret = gst_pad_link (new_pad, v_sinkpad);
         if (GST_PAD_LINK_FAILED (ret)) {
             LOG_ERROR_MSG ("fail to link video source with waylandsink");
             goto exit;
         }
     } else if (g_str_has_prefix (new_pad_type, "audio/x-raw")) {
         LOG_INFO_MSG ("Linking audio/x-raw");
         a_sinkpad = gst_element_get_static_pad (
                         reinterpret_cast<GstElement*> (vp->m_audioConv), "sink");
         ret = gst_pad_link (new_pad, a_sinkpad);
         if (GST_PAD_LINK_FAILED (ret)) {
             LOG_ERROR_MSG ("fail to link audio source and audioconvert");
             goto exit;
         }
     }
 ​
 exit:
     /* Unreference the new pad's caps, if we got them */
     if (new_pad_caps != NULL)
         gst_caps_unref (new_pad_caps);
 ​
     /* Unreference the sink pad */
     if (v_sinkpad) gst_object_unref (v_sinkpad);
     if (a_sinkpad) gst_object_unref (a_sinkpad);
 }

将uridecodebin的src pad和waylandsin的sink pad连接起来，由于这时是解码后的数据，因此caps为video/x-raw或audio/x-raw类型。

在上文中提到关于qtdemux解复用器的链接问题，事实上uridecodebin内部会自动处理好这部分链接，并且为所有能够解析的数据类型创建src-pad，每创建一种类型的src-pad，pad-added回调就会触发一次，用户需要自行完成uridecodebin的这些src-pad与后续GstElement的链接。可以结合Build Pipeline中关于pad-link的内容一起理解。

在上述代码中我将video/x-raw类型的数据和videoconvert插件链接，将audio/x-raw与audioconvert链接，videoconvert和audioconvert几乎是万能的格式转换插件能够提高代码的可移植性，使得代码能够在各种平台上正常运行，但切记这两者的转换均是使用CPU完成，因此十分消耗性能，在推流中使用会造成极大的延迟。