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64 lines
2.3 KiB
Text
64 lines
2.3 KiB
Text
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Negotiation
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-----------
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Negotiation happens when elements want to push buffers and need to decide
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on the format.
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For any two linked pads there are different ways to agree on the format but
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in general the source pad will propose a format and the sink format accepts
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or rejects.
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src sink
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| accepts? |
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|---------------->|
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| yes |
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|<----------------|
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get buffer | alloc_buf |
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from pool |---------------->|
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with type | | Create buffer of type.
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check type |<----------------|
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and use | |
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| push |
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push buffer |---------------->| Receive type, reconfigure to
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with new type| | process type.
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Use case:
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videotestsrc ! xvimagesink
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1) Who decides what format to use?
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- src pad always decides, by convention. sinkpad can suggest a format
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by putting it high in the getcaps function GstCaps.
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- since the src decides, it can always choose something that it can do,
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so this step can only fail if the sinkpad stated it could accept
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something while it later on couldn't.
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2) When does negotiation happen?
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- before srcpad does a push, it figures out a type as stated in 1), then
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it calls the pad alloc function with the type. The sinkpad has to
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create a buffer of that type, src fills the buffer and sends it to sink.
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- since the sink stated in 1) it could accept the type, it will be able to
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create a buffer of the type and handle it.
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- sink checks media type of buffer and configures itself for this type.
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3) How can sink request another format?
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- sink asks if new format is possible for the source.
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- sink returns buffer with new type in allocfunction.
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- src receives buffer with new type, reconfigures and pushes.
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- sink can always select something it can create and handle since it takes
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the initiative. src should be able to handle the new type since it said
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it could accept it.
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videotestsrc ! queue ! xvimagesink
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- queue implements an allocfunction, proxying all calls to its srcpad peer.
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- queue proxies all accept and getcaps to the other peer pad.
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