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349 lines
11 KiB
Text
349 lines
11 KiB
Text
OUTDATED
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--------
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caps negotiation
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================
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1) purpose
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----------
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The pads expose the media types they can handle using a mime
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type and a set of properties. Before the pad is created or
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used to pass buffers, we only know the global 'range' of media
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data this pad can accept. When the element has had a chance to
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look at the media data, only then it knows the exact values of the
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properties.
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example1:
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!
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! The mp3 decoder exposes the capabilities of its src pad
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! with the following caps:
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!
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! 'mpg123_src':
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! MIME type: 'audio/raw':
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! format: Integer: 16
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! depth: Integer: 16
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! rate: Integer range: 11025 - 48000
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! channels: Integer range: 1 - 2
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as you can see in example1, the padtemplate has both a range
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(for the audio rate) and a list (for the number of channels)
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for its properties.
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only when the mpg123 element has decoded the first mpeg audio
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header, it knows the exact values of the rate and channels
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properties.
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suppose that we want to connect this src pad to the sink pad
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of an audiosink with the properties given in example2:
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example2:
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!
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! 'audiosink_sink':
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! MIME type: 'audio/raw':
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! format: Integer: 16
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! depth: List:
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! Integer: 8
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! Integer: 16
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! rate: Integer range: 8000 - 44000
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! channels: Integer range: 1 - 2
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we can see that connecting the mpg123 src pad with the
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audiosinks sink pad can cause a potential problem with the
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rate property.
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When the mpg123 decoder decides to output raw audio with a
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48000Hz samplerate, the audiosink will not be able to handle
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it. The conservative approach would be to disallow the connection
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between the two incompatible pads. This rules out any potential
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problems but severely limits the amount of possible connections
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between the elements.
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Another approach would be to allow the connection (and mark it
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as dangerous) and let the two elements figure out a suitable
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media type at runtime. This procedure is called caps negotiation.
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2) a bit of history
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-------------------
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The typing of the data that was attached to a buffer used to be
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done using GstMeta* (and it still is as of 11 feb 2001). With
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the new GstCaps and GstProps system this typing is gradually moved
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to the pads and to the padtemplates. This has several advantages:
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- the typing of the data tends to be static. The type of media
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doesn't change for every buffer.
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- Moving the typing up to the pad(templates) allows us to save
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them into the registry and allows us to figure out what pads
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are compatible.
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- the current metadata implementation needs header files. this may
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change when we also use properties for metadata.
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example3:
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!
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! This is the current GstMeta structure that travels with audio buffers
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!
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! struct _MetaAudioRaw {
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! GstMeta meta;
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!
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! /* formatting information */
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! gint format;
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! gint channels;
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! gint frequency;
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! gint bps;
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! };
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The question still remains what purpose the metadata will serve
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now that we expose the media type in the pads. Some possibilities:
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- interesting information, not describing the data itself but the
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context in which the data was generated (suggested buffer size,
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timestamps, etc...)
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- user app metadata.
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In this proposal we also assume that the current use of metadata using
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GstMeta is deprecated and that we move this information to the properties
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of the pads.
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3) the pad/padtemplates caps
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----------------------------
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All elements have to provide a padtemplate for their pads.
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The padtemplates provide a range of possible media types this pad can
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src/sink. the main purpose for the padtemplates is to allow a
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rough guess at which pads are compatible before even a single buffer
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has been processed by the element.
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pads are usually created from the templates. When the pad is created
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it has no GstCaps* attached to it yet. The possible caps this pad
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can have is exposed in the padtemplate. The caps are filled in by
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the element when it knows the values for the caps.
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4) the connect function
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-----------------------
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when two pads are connected the following steps will take
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place (not sure, FIXME):
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- if both pads have caps, the caps are checked. If the caps
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are incompatible, the padtemplates are checked, if they
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are compatible, caps negotiation is performed.
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- if one of the pads has caps, the caps is checked against
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the padtemplate of the peer pad. If they are incompatible,
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the padtemplates are compared, if they are incompatible,
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caps negotiation is performed.
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- if none of the pads have caps, the padtemplates are checked,
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if they are incompatible, a warning is issued.
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5) when the element knows the media type it is handling
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-------------------------------------------------------
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When the element has received its first buffer it will know
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the media type it is handling by inspecting the buffer.
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before pushing the data out to its peer element(s), the element
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will set its src pad with the appropriate caps and properties.
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These caps must follow the following rules:
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- the caps must be compatible with the padtemplates of this
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pad.
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- the caps cannot contain ranges or lists.
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when the element wants to change the caps of a pad, it has to
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perform gst_pad_renegotiate (GstPad *pad). this will trigger
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the caps negotiation procedure.
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this will trigger the class method of the pad and calls the pads
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gst_pad_negotiate function:
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GstCaps *gst_pad_negotiate (GstPad *pad, GstCaps *caps, guint count);
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This function takes a GstCaps *structure as an argument (typically the
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current caps of the pad) and a negotiation counter. this counter can be
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used to keep track of the negotiation process.
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The pad then creates a new caps structure with the desired caps.
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If the caps are accepted, it just returns the provided input caps. the
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_renegotiate function will set the caps of both pads whenever the
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input caps are the same (pointer wise) as the input caps.
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the caps structure is checked against the padtemplate of the peer pad,
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if it is incompatible the gst_pad_negotiate function is called again
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and the element is supposed to create another caps structure.
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the gst_pad_renegotiate function then calls the gst_pad_negotiate
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function of the peer pad with the new caps as the argument. The peer
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pad can adjust or create a new caps if it doesn't accept it.
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the caps structure keeps on bouncing between the two pads until one
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of the pads negotiation functions returns the caps unmodified.
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The element can also return a NULL pointer if it has run out of
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options for the caps structure. When this happens, both pads are set
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the NULL caps again and the pad connection is broken.
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The negotiation process is stopped after a fixed number of tries,
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when the counter has reached some limit. This limit is typically
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checked by the pads negotiate function.
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6) caps negotiation function
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----------------------------
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the negotiate function of a pad is called whenever the pad or
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peer pad has performed _renegotiate.
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example5:
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!
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! this is the caps negotiation function implemented by an element on
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! one of its sink pads.
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!
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! static GstCaps*
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! gst_pad_negotiate (GstPad *pad, GstCaps *caps, guint counter)
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! {
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! /* we don't accept anything else than audio/raw */
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! if (strcmp (gst_caps_get_mime (caps), "audio/raw"))
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! return NULL;
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!
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! if (gst_caps_get_int_prop (caps, "format") != AFMT_S16_LE)
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! return NULL;
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!
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! /* we accept everything else */
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! return caps;
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! }
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When the negotiate function returns NULL (it does not accept the
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specified caps of the peer pad), the negotiation process is stopped.
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APPENDIX A: use cases
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=====================
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1) mpg123 src!sink audiosink
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----------------------------
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When the pads are connected the padtemplates are checked and it
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turns out that the pads might be incompatible (mpg123 can do
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48000Hz while audiosink can do 44000Hz). Nothing happens at
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connect time except for the user app that can mark this connection
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as possibly dangerous and keep some spare elements ready for when
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the pads turn out to be incompatible.
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both elements start out with no caps at all (NULL). mpg123 wants
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to output a buffer with specific properties. It calls
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gst_pad_renegotiate (mpg123->srcpad).
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The _renegotiate functions calls the negotiate function of the
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mpg123->srcpad. the negotiate function would look like this:
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/*
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* The mpg123 element cannot convert the decoded type into something
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* else so it has to force the caps of the src pad into the specific
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* type as defined by the mp3.
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*/
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static GstCaps*
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gst_mpeg123_src_negotiate (GstPad *pad, GstCaps *caps, guint counter)
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{
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GstMpg123 *mpg123;
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mpg123 = GST_MPG123 (gst_pad_get_parent (pad));
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/* we got caps in, check them */
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if (caps != NULL) {
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if (!strcmp (gst_caps_get_mime (caps), "audio/raw") &&
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(gst_caps_get_int_prop (caps, "format") == AFMT_S16_LE) &&
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(gst_caps_get_int_prop (caps, "depth") == 16) &&
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(gst_caps_get_int_prop (caps, "rate") == mpg123->rate) &&
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(gst_caps_get_int_prop (caps, "channels") == mpg123->channels)) {
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return caps;
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}
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}
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/* we didn't get caps, so we decide */
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else if (counter != 2) {
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GstCaps *new;
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/* fill in our desired caps */
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new = gst_caps_new_with_props (
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"src_caps", /* name */
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"audio/raw", /* mime */
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gst_props_new (
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"format", GST_PROPS_INT (AFMT_S16_LE),
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"depth", GST_PROPS_INT (16),
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"rate", GST_PROPS_INT (mpg123->rate),
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"channels", GST_PROPS_INT (mpg123->channels),
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NULL
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)
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);
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return caps;
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}
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/* too many attempts at nogotiation, bail out */
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return NULL;
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}
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The audiosink pad negotiate function would look like this:
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/*
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* The audiosink has a wide range of possible parameters for
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* its sink pad, based on the audio card capabilities and
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* possibly the element configuration.
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* we assume the audiosink element can be both the initiator of
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* the negotiations and the negotiated one.
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*/
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static GstCaps*
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gst_audiosink_sink_negotiate (GstPad *pad, GstCaps *caps, guint counter)
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{
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GstAudiosink *audiosink;
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gboolean accepted = TRUE;
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audiosink = GST_AUDIOSINK (gst_pad_get_parent (pad));
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/* we got caps in, we know they will match the padtemplate */
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if (caps != NULL) {
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return caps;
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}
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/* we didn't get caps, so we decide */
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else if (counter != 2) {
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GstCaps *new;
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/* fill in our desired caps */
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new = gst_caps_new_with_props (
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"sink_caps", /* name */
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"audio/raw", /* mime */
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gst_props_new (
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"format", GST_PROPS_INT (audiosink->format),
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"depth", GST_PROPS_INT (audiosink->depth),
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"rate", GST_PROPS_INT (audiosink->rate),
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"channels", GST_PROPS_INT (audiosink->channels),
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NULL
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)
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);
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return caps;
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}
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/* too many attempts at nogotiation, bail out */
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return NULL;
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}
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