Gstreamer basic Knowledge 2nd

This is my second time to read GStreamer Application Development Manual

Record something important

• Element:
  
  Element is an object that can send and/or receive data, also is the most important class of objects in GStreamer
  

Type	Details	Number of pads
Source elements	do not accept data, only generate data	a source pad
Filters and filter-like elements	operate the data	at least one input and one output pads
Sink elements	accept data and do not produce anything	a sink pad

• Create an element(two methods)
  
  1. gst_element_factory_make
     
  2. gst_element_factory_find + gst_element_factory_create
     

  • gst_init has to be called frist
    
  • When you don’t need the element anymore, you need to unref it using gst_object_unref ()
    
    The following example isthe simplest way to create an element using gst_element_factory_make
    

    #include <gst/gst.h>
    int main (int argc, char *argv[])
    {
     GstElement *element;
     /* init GStreamer */
     gst_init (&argc, &argv);
     /* create element */
     element = gst_element_factory_make ("fakesrc", "source");
     if (!element) {
      g_print ("Failed to create element of type ’fakesrc’\n");
      return -1;
     }
     gst_object_unref (GST_OBJECT (element));
     return 0;
    }
    

    
    

• Pad:
  
  Pad is element’s input or output
  
  A pad can have any of three availabilities:always, sometimes and on request
  

• Bin & Pipeline:
  
  A bin is a container elements
  
  A pipeline is a special subtype of a bin that allows scheduling of the containing elements
  

• Bus:
  
  A bus is a simple system that takes care of forwarding messages from the pipeline threads to an application in its own thread context
  
  Every pipeline contains a bus by default
  

• Use a bus(two methods)
  
  1. Run a main loop and attach some kind of watch to the bus using gst_bus_add_watch () or gst_bus_add_signal_watch ()
     
  2. Get the messages on the bus using gst_bus_peek () and/or gst_bus_poll ()
     
     Message types:
     

Name	Details
Error, warning and information notifications	those are used by elements if a message should be shown to the user about the state of the pipeline
End-of-stream notification	emitted when the stream has ended
Tags	emitted when metadata was found in the stream
State-changes	emitted after a successful state change
Buffering	emitted during caching of network-streams
Element messages	these are special messages that are unique to certain elements and usually represent additional features
Application-specific messages	any information on those can be extracted by getting the message |structure (see above) and reading its fields

• Buffers & Events:
  
  The data flowing through a pipeline consists of a combination of buffers and events
  
  • Buffers contain the actual media data
    
  • Events contain control information, such as seeking information and end-of-stream notifiers
    

• A buffer consists:
  

Number	Name	Description
1	Poniter	A pointer to a piece of memory
2	Size	The size of the memory
3	Timestamp	A timestamp for the buffer
4	refcount	A refcount that indicates how many elements are using this buffer, This refcount will be used to destroy the buffer when no element has a reference to it
5	Flags	Buffer flags

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