gstreamer/docs/design/part-negotiation.txt

Negotiation
-----------

Negotiation happens when elements want to push buffers and need to decide
on the format. This is called downstream negotiation because the upstream
element decides the format for the downstream element. This is the most
common case.

Negotiation can also happen when a downstream element wants to receive 
another data format from an upstream element. This is called upstream
negotiation.

The basics of negotiation are as follows:

 - GstCaps (see part-caps.txt) are refcounted before they
   are attached to a buffer to describe the contents of the buffer.
   It is possible to add a NULL caps to a buffer, this means that the
   buffer type did not change relative to the previous buffer. If no
   previous buffer was received by a downstream element, it is free to
   discard the buffer.

 - Before receiving a buffer, an element must check if the datatype of
   the buffer has changed. The element should reconfigure itself to the
   new format before processing the buffer data. If the data type on
   the buffer is not acceptable, the element should refuse the buffer by 
   returning an appropriate return value from the chain function.
   The core will automatically call the set_caps function for this purpose
   when it is installed on the sink or source pad.

 - When requesting a buffer from a bufferpool, the prefered type should
   be passed to the buffer allocation function. After receiving a buffer
   from a bufferpool, the datatype should be checked again.

 - A bufferpool allocation function should try to allocate a buffer of the
   prefered type. If there is a good reason to choose another type, the
   alloc function should see if that other type is accepted by the other
   element, then allocate a buffer of that type and attach the type to the
   buffer before returning it.


The general flow for a source pad starting the negotiation.

             src              sink
              |                 |
              |  accepts?       |
  type A      |---------------->|
              |      yes        |
              |<----------------|
              |                 |
 get buffer   |  alloc_buf      |
 from pool    |---------------->| 
 with type A  |                 | Create buffer of type A.
              |                 |
 check type   |<----------------|
 and use A    |                 |
              |  push           |
 push buffer  |---------------->| Receive type A, reconfigure to
 with new type|                 | process type A.
              |                 |

 One possible implementation in pseudo code:

 [element wants to create a buffer]
 if not format
   # see what the peer can do
   peercaps = gst_pad_peer_get_caps (srcpad)
   # see what we can do
   ourcaps = gst_pad_get_caps (srcpad)

   # get common formats
   candidates = gst_caps_intersect (peercaps, ourcaps)

   foreach candidate in candidates
     # make sure the caps is fixed
     fixedcaps = gst_pad_fixate_caps (srcpad, candidate)

     # see if the peer accepts it
     if gst_pad_peer_accept_caps (srcpad, fixedcaps)
       # store the caps as the negotiated caps, this will
       # call the setcaps function on the pad
       gst_pad_set_caps (srcpad, fixedcaps)
       break
     endif
   done
 endif

 # if the type is different, the buffer will have different caps from
 # the src pad -- setcaps will get called on the pad_push
 buffer = gst_pad_alloc_buffer (srcpad, 0, size, GST_PAD_CAPS (fixedcaps));
 if buffer
   [fill buffer and push]
 elseif
   [no buffer, either no peer or no acceptable format found]
 endif


The general flow for a sink pad starting a renegotiation.

             src              sink
              |                 |
              |  accepts?       |
	      |<----------------| type B
              |      yes        |
              |---------------->|
              |                 |
 get buffer   |  alloc_buf      |
 from pool    |---------------->| 
 with type A  |                 | Create buffer of new type B.
              |                 |
 check type   |<----------------|
 and          |                 |
 reconfigure  |                 |
              |  push           |
 push buffer  |---------------->| Receive type B, reconfigure to
 with new type|                 | process type B.
              |                 |

	      

Use case:


videotestsrc ! xvimagesink

  1) Who decides what format to use?
   - src pad always decides, by convention. sinkpad can suggest a format
     by putting it high in the getcaps function GstCaps. 
   - since the src decides, it can always choose something that it can do,
     so this step can only fail if the sinkpad stated it could accept
     something while later on it couldn't.

  2) When does negotiation happen?
   - before srcpad does a push, it figures out a type as stated in 1), then 
     it calls the pad alloc function with the type. The sinkpad has to 
     create a buffer of that type, src fills the buffer and sends it to sink.
   - since the sink stated in 1) it could accept the type, it will be able to
     create a buffer of the type and handle it.
   - sink checks media type of buffer and configures itself for this type.
     
  3) How can sink request another format?
   - sink asks if new format is possible for the source.
   - sink returns buffer with new type in allocfunction.
   - src receives buffer with new type, reconfigures and pushes.
   - sink can always select something it can create and handle since it takes
     the initiative. src should be able to handle the new type since it said
     it could accept it.

videotestsrc ! queue ! xvimagesink

  - queue implements an allocfunction, proxying all calls to its srcpad peer.
  - queue proxies all accept and getcaps to the other peer pad.
  - queue contains buffers with different types.