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266 lines
9.9 KiB
Text
266 lines
9.9 KiB
Text
In this document we describe how we can obtain various properties
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of the pipeline we are running.
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we have 5 possible ways to get information, each one of these
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methods focus on one particular property of the pipeline.
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- caps: this is a description of the media type that flows between
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2 pads.
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- metadata: non essential extra (human readable) information about
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the stream, like author, copyright, name etc..
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- streaminfo: information about the stream as encoded into the
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stream itself.
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- pad/element queries: information about the stream as it is being
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processed.
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- pad/element convert: information about the relation between formats
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in a streams as it is being processed.
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note that element properties are not included in this list. Element
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properties are only used to configure the codec/element.
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caps, metadata, streaminfo are supposed to remain fairly static during
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the stream. The queries/converts can be done on demand. The reason
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for this is that caps/metadata/streaminfo is quite expensive and
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could degrade pipeline performance.
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Caps.
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-----
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Caps are automatically set on pads by the core when two pads agree
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on a media type. This automatically means that the caps are fixed.
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Since caps is a property of the pad and the g_object_notify() mechanism
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is used to signal a change, the user can either connect a "notify"
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signal handler to the pad or connect to the "deep_notify" signal on
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a parent pipeline.
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The caps notifications are useful if you want to know what kind of
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media is passing through pads. You can, for example, report to the
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user how the video or audiosink is configured or what kind of
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media some plugin is producing/accepting.
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So, always use the caps to find out the channels/samplerate/size
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of the media.
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Metadata
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--------
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Metadata is a GstCaps element property (named "metadata") that contains
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additional information encoded into the stream that doesn't say anything
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about the media type of the stream itself.
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Metadata are typically human readable information like author, copyright,
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title, ... and can be displayed by the application as-is.
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An element with a "metadata" property is supposed to g_object_notify that
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property when the metadata changes so that the app can connect a signal
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handler to the property or use the "deep_notify" signal to get the
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notification.
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Streaminfo
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----------
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Streaminfo is a GstCaps element property (named "streaminfo") that contains
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additional information about the stream that is not stricly required to
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specify the media type of the stream.
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Streaminfo is typically the length, bitrate, framerate, flags, ... of the
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stream.
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It is important to note that this information should be derived from the
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stream itself and might not be correct for the pipeline being processed.
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Let's illustrate this with an example:
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- an mp3 stream has an id3 tags that contains the length of the stream
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(TLEN).
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- we cut the mp3 stream in half
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The actual length doesn't match the stated length as encoded in the id3
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tags, so the TLEN tag should be put in the streaminfo and the actual
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length should be queried with a pad_query.
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So, be careful when showing streaminfo as-is in an app.
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Queries
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-------
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Queries can be performed on pads and elements and are used to get
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information about the current stream. The value is a single gint64
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in a specific format.
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example:
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- the query (GST_QUERY_TOTAL, GST_FORMAT_TIME) will return the total
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amount of time this pad/element will run.
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- the query (GST_QUERY_POSITION, GST_FORMAT_UNITS) will return the
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current position of the stream expressed in units (units are samples,
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frames, bytes, ... depending on the media type of the stream)
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two methods exist to perform a query:
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- gboolean gst_pad_query (GstPad *pad, GstQueryType type,
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GstFormat *format, gint64 *value);
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and:
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- gboolean gst_element_query (GstElement *element, GstQueryType type,
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GstFormat *format, gint64 *value);
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if you want to get the total duration of a stream or the current position,
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you need to use a pad query. A pad query can fail (method returns FALSE),
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this usually means that the duration is not known or that not enough data
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has been processed to report the correct value.
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the possible queries that can be performed on a pad/element can be obtained
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with
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- const GstQueryType* gst_pad_get_query_types (GstPad *pad);
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and
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- const GstQueryType* gst_element_get_query_types (GstElement *element);
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These functions return an array of GstQueryTypes (last element == 0). you
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can loop over the array to see what is supported or do
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- gboolean gst_queries_contains (const GstQueryType *types, GstQueryType type);
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to see if a specific format is contained in the list.
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The value reported by a pad query can be considered correct or
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as-good-as-can-be. Elements that support pad queries must do the best
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they can to report correct values.
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Convert
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-------
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The convert functions are used to query an element/pad for the relationship
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between two formats that it supports. For example:
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suppose we want to know how many frames a particular video decoder will
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produce in one second, we ask it to convert its concept of 1 SECOND into
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1 UNIT (frames in the context of video). so,
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GstFormat format = GST_FORMAT_UNITS;
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res = gst_pad_convert (pad, GST_FORMAT_TIME, GST_SECOND,
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&format, &value);
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if res == TRUE, value will contain the framerate of the video, of course
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this framerate will only contain the integral part. If you want more
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accuracy, use 1000 * GST_SECOND and divide the result by 1000 to get
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a fractional part.
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All other neat things can be done too, look at the typical cdplayer
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plugin for example. It defines a new format "track". Now you can
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ask it to convert a TRACK to TIME like this:
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GstFormat format = GST_FORMAT_TIME;
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track_format = gst_format_get_by_nick ("track");
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res = gst_pad_convert (pad, track_format, 1,
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&format, &value);
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This will convert 1 track to a time. of course we didn't (couldn't) specify
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which track, but that's not a problem if you understand how the stream is
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divided into different formats:
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Take the total stream as containing bytes (indicated with + in figure) we
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can subdivide the (byte)stream in different formats.
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++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ... bytes
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! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ... samples (units)
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! ! ! ! ... buffers
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! ! ! ... time
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! ! ... track
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The raw bytestream can be grouped in samples for example (for 16 bit stereo
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int audio, 4 bytes == 1 sample) or we can divide it into time
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(44100 samples == 176400 bytes == 1 second) or into tracks
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(1 track could be 17640000 bytes or 100 seconds or 4410000 samples)
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It is important to know that the stream starts at position 0 (for all formats)
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and ends at position X (expressed in a specific format). now take this stream
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divided into the track format:
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0 200 500 700 1000
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++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ .. bytes
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! track0 ! track1 ! track2 ! track3 ! ...
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if we now perform a pad convert from 100 bytes to the track format, we
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get the value 0, as the region 0-100 bytes is contained in track0.
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if we perform a pad convert from 600 bytes to track, we get the value
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2, as track0->track2 contains the bytes 0-500.
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We can also do: convert track1 to bytes, then we get 200. If we do
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convert track2 to bytes, we get 500. Note that the conversions are
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always performed relative to 0, so if we convert track2 to bytes, we
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always get the number of bytes from track0->track2.
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If we want to get the number of bytes of one particular track, we have
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to substract two convert values. Look at the folowing figure to understand
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this. The --- defines the region we want to convert.
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0 200 500 700 1000
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++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ .. bytes
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! track0 ! track1 ! track2 ! track3 ! ...
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----------- track1 -> bytes (1) (size of track0)
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---------------------- track2 -> bytes (2) (size of track0 and track1)
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----------- (2) - (1) = total bytes for track 1
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Another example would be to get the bitrate of a decoder plugin, consider the
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following example:
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(------------)
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! mad !
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- sink src -
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(------------)
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The element has a sinkpad that will take N bytes as input to produce M
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samples (units) on its srcpad.
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The rate at which it takes bytes is defined as the byterate of the
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stream (bitrate == byterate * 8). So, we do:
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GstFormat format = GST_FORMAT_BYTES;
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gint64 value;
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gst_pad_convert (mad->sinkpad, GST_FORMAT_TIME, GST_SECOND,
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&format, &value);
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..and we get the number of bytes this plugin takes in each second.
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Again, note that this value is relative to 0, you can get an average
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of a specific period by using the same substract trick as above.
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Element Properties
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------------------
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Element properties are used to configure an element. They should not be
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used to describe media info, metadata fields, streaminfo fields or anything
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other that doesn't involve codec configuration.
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Several reasons:
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- metadata requires dynamic properties (one for each tag). This cannot be done
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with GObject properties.
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- you cannot signal a logical group of related properties (exposing stuff like
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samplerate/channels/encoding/... in different element properties is not a
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good idea.
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- stuff like length an position depend on the pads of the element, you cannot
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sanely expose a property for each pad to describe this.
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- element properties can only report stuff with one type. If your property
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exposes somthing like "total_length", you cannot make it both report this
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in time/bytes/samples/frames...
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- impossible to sanely implement convert with element properties.
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