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Original commit message from CVS: 2005-10-03 Andy Wingo <wingo@pobox.com> * docs/design/part-gstghostpad.txt: Add a note about activation of proxy pads outside of ghost pads. * gst/gstghostpad.c: Implement the ghost pad activation design.
406 lines
13 KiB
Text
406 lines
13 KiB
Text
Ghostpads
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---------
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GhostPads are used to build complex compound elements out of
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existing elements. They are used to expose internal element pads
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on the complex element.
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Some design requirements
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- Must look like a real GstPad on both sides.
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- target of Ghostpad must be changeable
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* a GhostPad is implemented using a smaller GstProxyPad class:
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GstProxyPad
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(------------------)
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| GstPad |
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|------------------|
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| GstPad *target |
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(------------------)
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GstGhostPad
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(------------------) -\
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| GstPad | |
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|------------------| > GstProxyPad
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| GstPad *target | |
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|------------------| -/
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| GstPad *internal |
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(------------------)
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Some use case follow with a description of how the datastructure
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is modified.
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* Creating a ghostpad with a target:
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gst_ghost_pad_new (char *name, GstPad *target)
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1) create new GstGhostPad X
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2) X name set to @name
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3) X direction is the same as the target
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4) the target is set to @target
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5) internal is NULL
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6) link/unlink and activate functions are set up
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on GstGhostPad.
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(--------------
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(- X --------) |
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| | |------)
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| target *------------------> | sink |
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|------------| |------)
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| internal *---->// (--------------
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(------------)
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- Automatically takes same direction as target.
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- target is filled in automatically.
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* Creating a ghostpad without a target
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gst_ghost_pad_new_notarget (char *name, GstPadDirection dir)
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1) create new GstGhostPad X
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2) X name set to @name
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3) X direction is @dir
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4) internal is NULL
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5) link/unlink and activate functions are set up
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on GstGhostPad.
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(- X --------)
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| |
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| target *------>//
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|------------|
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| internal *---->//
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(------------)
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- allows for setting the target later
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* Setting target on an untargetted unlinked ghostpad
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gst_ghost_pad_set_target (char *name, GstPad *newtarget)
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(- X --------)
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| |
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| target *------>//
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|------------|
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| internal *---->//
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(------------)
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1) assert direction of newtarget == X direction
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2) target is set to newtarget
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(--------
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(- X --------) |
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| | |------)
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| target *------------->| sink |
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|------------| |------)
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| internal *--->// (--------
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(------------)
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* Setting target on an targetted unlinked ghostpad
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gst_ghost_pad_set_target (char *name, GstPad *newtarget)
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(--------
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(- X --------) |
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| | |------)
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| target *------------->| sink |
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|------------| |------)
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| internal *--->// (--------
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(------------)
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1) assert direction of newtarget == X direction
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2) target is set to newtarget
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(--------
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(- X --------) |
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| | |------)
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| target *------------->| sink |
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|------------| |------)
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| internal *--->// (--------
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(------------)
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* Linking a pad to an untargetted ghostpad:
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gst_pad_link (src, X)
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(- X --------)
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| |
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| target *------>//
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|------------|
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| internal *---->//
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(------------)
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-------)
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(-----|
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| src |
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(-----|
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-------)
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1) link function is called
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a) new GstProxyPad Y is created
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b) Y direction is same as peer
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c) Y target is set to peer
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d) X internal pad is set to Y (X is parent of Y)
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e) Y is activated in the same mode as X
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f) core makes link from src to X
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(- X --------)
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| |
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| target *----->//
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>|------------|
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(real pad link) / | internal * |
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/ (----------|-)
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/ |
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-------) / V
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| / (- Y ------)
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(-----|/ | |
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| src |<-------------* target |
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(-----| (----------)
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-------)
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* Linking a pad to a targetted ghostpad:
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gst_pad_link (src, X)
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(--------
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(- X --------) |
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| | |------)
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| target *------------->| sink |
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|------------| |------)
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| internal *--->// (--------
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(------------)
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-------)
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(-----|
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| src |
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(-----|
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-------)
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1) link function is called
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a) new GstProxyPad Y is created
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b) Y direction is same as peer
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c) Y target is set to peer
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d) X internal pad is set to Y (X is parent of Y)
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e) link is made from Y to X target (sink)
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f) Y is activated in the same mode as X
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g) core makes link from src to X
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(--------
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(- X --------) |
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| | |------)
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| target *------------->| sink |
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>|------------| >|------)
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(real pad link) / | internal * | / (--------
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/ (----------|-) /
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/ | /
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-------) / V / (real pad link)
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| / (- Y ------) /
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(-----|/ | |/
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| src |<-------------* target |
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(-----| (----------)
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-------)
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* Setting target on untargetted linked ghostpad:
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gst_ghost_pad_set_target (char *name, GstPad *newtarget)
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(- X --------)
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| |
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| target *----->//
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>|------------|
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(real pad link) / | internal * |
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/ (----------|-)
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/ |
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-------) / V
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| / (- Y ------)
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(-----|/ | |
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| src |<-------------* target |
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(-----| (----------)
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-------)
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1) assert direction of newtarget == X direction
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2) X target is set to newtarget
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3) Y (X internal) is linked to newtarget
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(--------
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(- X --------) |
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| | |------)
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| target *------------->| sink |
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>|------------| >|------)
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(real pad link) / | internal * | / (--------
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/ (----------|-) /
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/ | /
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-------) / V / (real pad link)
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| / (- Y ------) /
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(-----|/ | |/
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| src |<-------------* target |
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(-----| (----------)
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-------)
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* Setting target on targetted linked ghostpad:
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gst_ghost_pad_set_target (char *name, GstPad *newtarget)
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(--------
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(- X --------) |
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| | |------)
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| target *------------->| sink |
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>|------------| >|------)
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(real pad link) / | internal * | / (--------
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/ (----------|-) /
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/ | /
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-------) / V / (real pad link)
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| / (- Y ------) /
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(-----|/ | |/
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| src |<-------------* target |
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(-----| (----------)
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-------)
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1) assert direction of newtarget == X direction
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2) Y and X target are unlinked
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2) X target is set to newtarget
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3) Y (X internal) is linked to newtarget
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(--------
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(- X --------) |
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| | |------)
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| target *------------->| sink |
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>|------------| >|------)
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(real pad link) / | internal * | / (--------
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/ (----------|-) /
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/ | /
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-------) / V / (real pad link)
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| / (- Y ------) /
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(-----|/ | |/
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| src |<-------------* target |
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(-----| (----------)
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-------)
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Activation
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==========
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Sometimes ghost pads should proxy activation functions. This thingie
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attempts to explain how it should work in the different cases.
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+---+ +----+ +----+ +----+
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| A +-----+ B | | C |-------+ D |
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+---+ +---=+ +=---+ +----+
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+--=-----------------------------=-+
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| +=---+ +----+ +----+ +---=+ |
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| | a +---+ b ==== c +--+ d | |
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| +----+ +----+ +----+ +----+ |
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+----------------------------------+
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state change goes from right to left
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<-----------------------------------------------------------
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All of the labeled boxes are pads. The dashes (---) show pad links, and
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the double-lines (===) are internal connections. The box around a, b, c,
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and d is a bin. B and C are ghost pads, and a and d are proxy pads. The
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arrow represents the direction of a state change algorithm. Not counting
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the bin, there are three elements involved here -- the parent of D, the
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parent of A, and the parent of b and c.
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Now, in the state change from READY to PAUSED, assuming the pipeline
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does not have a live source, all of the pads will end up activated at
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the end. There are 4 possible activation modes:
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1) AD and ab in PUSH, cd and CD in PUSH
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2) AD and ab in PUSH, cd and CD in PULL
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3) AD and ab in PULL, cd and CD in PUSH
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4) AD and ab in PULL, cd and CD in PULL
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When activating (1), the state change algorithm will first visit the
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parent of D and activate D in push mode. Then it visits the bin. The bin
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will first change the state of its child before activating its pads.
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That means c will be activated in push mode. [*] At this point, d and C
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should also be active in push mode, because it could be that activating
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c in push mode starts a thread, which starts pushing to pads which
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aren't ready yet. Then b is activated in push mode. Then, the bin
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activates C in push mode, which should already be in push mode, so
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nothing is done. It then activates B in push mode, which activates b in
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push mode, but it's already there, then activates a in push mode as
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well. The order of activating a and b does not matter in this case.
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Then, finally, the state change algorithm moves to the parent of A,
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activates A in push mode, and dataflow begins.
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[*] Not yet implemented.
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Activation mode (2) is implausible, so we can ignore it for now. That
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leaves us with the rest.
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(3) is the same as (1) until you get to activating b. Activating b will
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proxy directly to activating a, which will activate B and A as well.
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Then when the state change algorithm gets to B and A it sees that they
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are already active, so it ignores them.
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Similarly in (4), activating D will cause the activation of all of the
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rest of the pads, in this order: C d c b a B A. Then when the state
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change gets to the other elements they are already active, and in fact
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data flow is already occuring.
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So, from these scenarios, we can distill how ghost pad activation
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functions should work:
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Ghost source pads (e.g. C):
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push:
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called by: element state change handler
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behavior: just return TRUE
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pull:
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called by: peer's activatepull
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behavior: change the internal pad, which proxies to its peer e.g. C
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changes d which changes c.
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Internal sink pads (e.g. d):
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push:
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called by: nobody (doesn't seem possible)
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behavior: n/a
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pull:
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called by: ghost pad
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behavior: proxy to peer first
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Internal src pads (e.g. a):
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push:
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called by: ghost pad
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behavior: activate peer in push mode
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pull:
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called by: peer's activatepull
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behavior: proxy to ghost pad, which proxies to its peer (e.g. a
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calls B which calls A)
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Ghost sink pads (e.g. B):
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push:
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called by: element state change handler
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behavior: change the internal pad, which proxies to peer (e.g. B
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changes a which changes b)
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pull:
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called by: internal pad
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behavior: proxy to peer
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It doesn't really make sense to have activation functions on proxy pads
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that aren't part of a ghost pad arrangement.
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