gstreamer/docs/slides/outline

Introduction (1)
	(sorry, no cool logo/graphic yet, ideas?)
	GStreamer is a library and set of tools to build arbitrary,
	 reconfigurable filter graphs.  It derives from the OGI Pipeline
	 and DirectShow (docs, no experience), and is in its second
	 generation (first was completed/abandoned *in* Atlanta on the
	 way to the Linux Expo).
	0.1.0 release is scheduled for Oct 31, 1999.
	Will cover Background, Goals, Design, and Futures
Why do we need this?
	launch reads the command line to create the graph, from .so's
	Connections (queues) are made by launcher, lots of switchout code
	Argument handling is messy, at start-time only
	...thus there is basically only one state: running
	There is no real master process capable of seeing the whole
	 pipeline as a graph, so wiring A->B->C with some backchannel
	 (parameter, not data stream) from C to A is hard
	Channels try to do IPC, file, and network I/O, excess abstraction
Goals (1)
	Provide a clean way to both build graphs and write new elements
	Make things easier by providing auto-connect, stock sub-graphs
	Include tools sorely lacking in OGI pipeline, like editor, saves
	Enable Linux to catch up with M$'s world, allowing commercial
	 plugins to a stable API (as of 1.0) so we don't end up with N
	 wheels from N-M different people (multiple projects)
Overview (1)
	Object hierarchy capable of run-time discovery, based on GtkObject
	Deeply nested parent-child relationships enable threads, blackboxes
	Buffers can point to anything, are typed, and can carry metadata
	Plugins can be loaded at any point, and registry reduces loads
	Symbiotic editor lets you design/run/save graphs visually
What are filter graphs? (1)
	Filters take data in and spit data out, doing something to it
	Filters have N>=0 inputs and M>=0 outputs
	Filter graphs are many filters connected together, like a circuit
	The goal is typically to move data from 'left' to 'right', towards
	 some kind of user-visible conclusion, i.e. audio or video
Architecture (3?)
  - Graphs of Elements
	(here lies screen-grab from editor)
	Element is core Object, Bins hold (and are) Elements
	Pads are fundamental to an Element, are cross-wired with pointers
	Since Bins hold Elements, and Bins are Elements, Bins hold Bins
	'Ghostpads' provide interfaces for Bins without native interfaces
#	Threads are type of Bin that actually run in separate threads

  - States
	(table of states, invariants, and descriptions)
	    COMPLETE	Element has all needed information
	    RUNNING	Element has acquired resources, ready to go
	    DISCOVERY	... (unimplemented)
	    PREROLL	... (unimplemented)
	    PLAYING	Element is actively trading data
	    PAUSED	Special state where things just don't run (?..)
	States are used to keep elements in check

  - Buffers
	Buffers designed to be versatile, with arbitrary typing/metadata
	Has pointer to data, length, so can point to someone else's data
	Type system (crude so far) ensures buffers don't go stray
	Metadata can be attached, such as the audio parameters
	Ref-counting and copy-on-write avoids most copies, not complete
	Sub-buffers can be created from bigger buffer, limitting copies
Gtk+ Object System (2)
  - Pros
	C-language object system, well tested (Gtk+, Gnome...)
	Arguments of any fundamental type, read/write, built-in hooks
	Signals used for hooks into object events, overridable
	Run-time discovery of args, signals (quarks)
  - Cons
	No multiple-inheritance, though I haven't *needed* it
	There are some holes (can't attach quarks to *eveything*)

  - Design
	Classes, instances are structs; 1st item is parent class
	Type system allows clean casting, ^^^^^^
	Arguments are set/get by string, use functions to do the work,
	 thus setting an arg might trigger a redraw of the GUI
	Signals are strings, use marshallers, various firing methods
Basic GStreamer objects (1)
  - Elements
	(show class,instance structs)
	Very simple, just provides a means to handle pads, state
  - Bins
	(show class,instance structs)
	Supports children, handles group state transitions
Pads (1)
	Pad list type, direction, and chaining function ptr
	When creating a sink pad (!src) you set the chaining function
	gst_pad_connect() sets the peers, and copies chain function to src
	Passing buffer to a src pad transparently calls the chain function
	(graph goes here...)
Sources (1)
	Source provides functions to push data
	Regular push() function just takes next N bytes and sends them
	Async push_region() grabs N bytes at offset O and sends them
	EOF signal [will] reset the state from PLAYING down to !RUNNING
	"location" argument is global by convention, for filenames...URIs
Connections (1)
	Special type of Filter that 
Threads (1)
	Special case of Bin that actually creates a thread transparently
	When RUNNING, thread exists, mutex/cond used to go [!]PLAYING
	Automatically determines how to start sub-graph
		Looks for both Sources and Elements wired to Connection
	Will cooperate with Pipelines when threading is not available
Typing and Metadata (1)
  - Types
	Based on MIME types, set up as quarks, and dealt with as int
	Usable entirely at run-time, since they're registerable by plugins
  - Metadata
	Also registered as an int, but must be compile time due to structs
	Have refcounts and CoW semantics, since they travel with buffers
Plugins (1)
	Plugin architecture designed around class system
	Arguments and signals provide interface over standard base class
	Each Element defined by ElementFactory, which is queried by name
	At plugin load, any number of ElementFactories and Types registered
	Element registers against Type as source or sink
Editor (2+?)
	(show filter graph snapshot, a different one, more complex)
	Built as a parallel object hierarchy on top of GNOME Canvas
	Every object in filter graph has equivalent in editor, plus some
	Canvas is designed with groups and signal-propagation, so...
	Why not build the whole thing as subclasses of CanvasGroup?

	...because updates get messy/recursive (the way I *was* doing it)
	Solution is to modify objects so they own Group rather than being
	Relatively trivial modification, but requires lots of repointering
	Still a genealogical mess of parents and children...
XML
	The goal is to use XML heavily, with an eye towards DOM
	Used for both saving and providing pre-build components
	Both graph and editor will have namespace, they'll interleave
	A generic save function will exist for Elements, with hooks
	Saving an EditorElement will also save Element
	Also used for a plugin registry, to avoid loading all plugins




leaky bucket is trivial
applications - generic conferencing tool (repluggable codecs), mixing
 environment (data flow graphs)