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98 lines
3.4 KiB
Text
98 lines
3.4 KiB
Text
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Some notes on pad negotiation
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A "pad link" is a connection between two pads. It can be in one of
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two states, "negotiated" or "not negotiated". Pad links are created
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by gst_pad_link().
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A "pad link" is created when two pads are linked using gst_pad_link().
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When initially created, the link only specifies a src pad, a sink pad,
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and optionally a filter caps and a fixate function (both provided by
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the application).
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In order to pass data through a link, the peer pads must decide on
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what data format to use. This is called negotiation. Pads can
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describe acceptable data formats either by pad template caps or a
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pad->getcaps function.
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Steps in negotiation:
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- core checks that both pad's parent elements are in the READY or
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higher state.
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- core calls gst_pad_get_caps() on each pad, intersects the two caps,
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and intersects again with the filter caps. If the intersection is
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empty, then the pads have no formats in common, and the link fails.
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- If the intersection caps is not fixed, there are multiple possible
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formats that the link could use. If this is the case, fixate
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functions are called until the caps are fixed. The fixate functions
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are called by priority -- application fixate function, src and sink
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fixate functions, and the default core fixate function.
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- Each pad can have a pad_link function, which is called with the
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fixed caps. The pad_link function has the option of accepting
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or rejecting the caps.
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- If both pads accept the caps, the link is then negotiated.
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Steps in pad-initiated negotiation (gst_pad_try_set_caps):
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- both elements are assumed to be in a non-NULL state
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- the caps in try_set_caps() must be fixed, and are checked to be
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compatible with the peer's caps.
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- Fixate functions don't need to be called, since the caps are
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already fixed.
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- The peer's pad_link function is called.
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- If the peer's pad_link function accepts the caps, the link is then
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negotiated.
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Steps in renegotiation:
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- same as negotiation. Note that get_caps() functions should always
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ignore the currently negotiated caps of a link.
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- if renegotiation fails, the previous negotiation is still in effect.
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If the renegotiation fails in the last pad_link step, the pad_link
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functions are called with the previously negotiated caps. (FIXME:
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this is currently buggy.)
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Notes for sources:
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- sources and sinks that talk to hardware may not be able to fully
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describe their available formats, and thus need to rely on pad_link
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functions to test a particular format. FIXME: currently, the core
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completely fails negotiation if a pad_link function refuses a caps,
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instead of attempting with an alternate caps.
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Notes for decoders/demuxers:
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- Decoders will typically negotiate a sink pad, receive some data,
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determine the output format, and call try_set_caps() with the given
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format. If the output format is non-fixed, gst_pad_renegotiate()
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may be used instead, in order to have the fixate functions choose
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the optimal format. Note that this requires communication so that
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the pad's getcaps function returns the correct caps.
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Notes for converters:
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- Converters change one or more properties of the format of a data
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stream. A typical converter's getcaps function will call
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gst_pad_get_allowed_caps() for the opposite pad in the element,
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change one or more fields in the caps, and return the result.
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Notes for filters:
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- Filters can almost always use proxied getcaps and pad_link functions.
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