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242 lines
8.2 KiB
Text
242 lines
8.2 KiB
Text
I'll use this doc to describe how I think metadata should work from the
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perspective of the application developer and end user, and from that
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extrapolate what we need to provide that.
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RATIONALE
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---------
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One of the strong points of GStreamer is that it abstracts library dependencies
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away. A user is free to choose whatever plug-ins he has, and a developer
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can code to the general API that GStreamer provides without having to deal
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with the underlying codecs.
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It is important that GStreamer also handles metadata well and efficiently,
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since more often than not the same libraries are needed to do this. So
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to avoid applications depending on these libs just to do the metadata,
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we should make sure GStreamer provides a reasonable and fast abstraction
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for this as well.
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GOALS
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-----
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- quickly read and write content metadata
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- quickly read stream metadata
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- cache both kinds of data transparently
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- (possibly) provide bins that do this
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- provide a simple API to do this
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DEFINITION OF TERMS
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-------------------
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The user or developer using GStreamer is interested in all information that
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describes the stream. The library handles these two types differently
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however, so I will use the following terms to describe this :
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- content metadata
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every kind of information that is tied to the "concept" of the stream,
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and not tied to the actual encoding or representation of the stream.
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- it can be altered without transcoding the stream
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- it would stay the same for different encodings of the file
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- describes properties of the information encoded into the stream
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- examples:
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- artist, title, author
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- year, track order, album
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- comments
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- stream metadata
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every kind of information that is tied to the "codec" or "representation"
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used.
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- cannot be altered without transcoding
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- is the set of parameters the stream has been encoded with
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- describes properties of the stream itself
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- examples:
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- samplerate, bit depth/width, channels
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- bitrate, encoding mode (e.g. joint stereo)
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- video size, frames per second, colorspace used
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- length in time
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EXAMPLE PIPELINES
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-----------------
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reading content metadata : filesrc ! id3v1
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- would read metadata from file
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- id3v1 immediately causes filesrc to seek until it has found
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- the (first) metadata
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- that there is no metadata present
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resetting and writing content metadata :
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filesrc ! id3v1 reset=true artist="Arid" ! filesink
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- effect: clear the current tag and reset it to only have Arid as artist
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- id3v1 seeks to the right location, clears the tag, and writes the new one
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- filesrc might not be necessary here
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- this probably only works when doing an in-place edit
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COST
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----
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Querying metadata can be expensive.
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Any application querying for metadata should take this into account and
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make sure that it doesn't block the app unnecessarily while the querying
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happens.
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In most cases, querying content data should be fast since it doesn't involve
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decoding
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Technical data could be harder and thus might be better done only when needed.
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CACHE
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-----
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Getting metadata can be an expensive operation. It makes sense to cache
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the metadata queried on-disk to provide rapid access to this data.
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It is important however that this is done transparently - the system should
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be able to keep working without it, or keep working when you delete this cache.
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The API would provide a function like
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gst_metadata_content_read_cached (location)
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or even
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gst_metadata_read_cached (location, GST_METADATA_CONTENT, GST_METADATA_READ_CACHED)
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to try and get the cached metadata.
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- check if the file is cached in the metadata cache
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- if no, then read the metadata and store it in the cache
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- if yes, then check the file against it's timestamp (or (part of) md5sum ?)
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- if it was changed, force a new read and store it in the cache
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- if it wasn't changed, just return the cached metadata
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For optimizations, it might also make sense to do
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GList * gst_metadata_read_many (GList *locations, ...)
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which would allow the back-end to implement this more efficiently.
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Suppose an application loads a playlist, for example, then this playlist
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could be handed to this function, and a GList of metadata types could
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be returned.
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Possible implementations :
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- one large XML file : would end up being too heavy
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- one XML file per dir on system : good compromise; would still make sense
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to keep this in memory instead of reading and writing it all the time
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Also, odds are good that users mostly use files from same dir in one app
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(but not necessarily)
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Possible extra niceties :
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- matching of moved files, and a similar move of metadata (through user-space
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tool ?)
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!!! For speed reasons, it might make sense to somehow keep the cache in memory
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instead of reparsing the same cache file each time.
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!!! For disk space reasons, it might make sense to have a system cache.
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Not sure if the complexity added is worth it though.
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!!! For disk space reasons, we might want to add an upper limit on the size of
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the cache. For that we might need a timestamp on last retrieval of metadata,
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so that we can drop the old ones.
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The cache should use standard glibc.
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FIXME: is it worth it to use gnome-vfs for this ?
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STANDARDIZATION OF METADATA
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---------------------------
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Different file formats have different "tags". It is not always possible
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to map metadata to tags. Some level of agreement on metadata names is also
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required.
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For technical metadata, the names or properties should be fairly standard.
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We also use the same names as used for properties and capabilities in
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GStreamer.
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This means we use
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- encoded audio
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- "bitrate" (which is bits per second - use the most correct one,
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ie. average bitrate for VBR for example)
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- raw audio
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- "samplerate" - sampling frequency
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- "channels"
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- "bitwidth" - how wide is the audio in bits
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- encoded video
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- "bitrate"
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- raw video
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(FIXME: I don't know enough about video, are these correct)
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- "width"
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- "height"
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- "colordepth"
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- "colorspace"
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- "fps"
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- "aspectratio"
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We must find a way to avoid collision. A system stream can contain both
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audio and video (-> bitrate) or multiple audio or video streams. One way
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to do this might be to make a metadata set for a stream a GList of metadata
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for elementary streams.
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For content metadata, the standards are less clear.
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Some nice ones to standardize on might be
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- artist
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- title
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- author
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- year
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- genre (touchy though)
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- RMS, inpoint, outpoint (calculated through some formula, used for mixing)
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TESTING
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-------
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It is important to write a decent testsuite for this and do speed comparisons
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between the library used and the GStreamer implementation.
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API
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---
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struct GstMetadata
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{
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gchar *location;
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GstMetadataType type;
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GList *streams;
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GHashtable *values;
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};
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(streams would be a GList of (again) GstMetadata's.
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"location" would then be reused to indicate an identifier in the stream.
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FIXME: is that evil ?)
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GstMetadataType - technical, content
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GstMetadataReadType - cached, raw
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GstMetadata *
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gst_metadata_read (const char *location,
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GstMetadataType type,
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GstMetadataReadType read_type);
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GstMetadata *
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gst_metadata_read_props (const char *location,
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GList *names,
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GstMetadataType type,
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GstMetadataReadType read_type);
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GstMetadata *
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gst_metadata_read_cached (const char *location,
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GstMetadataType type,
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GstMetadataReadType read_type);
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GstMetadata *
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gst_metadata_read_props_cached (...)
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GList *
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gst_metadata_read_cached_many (GList *locations,
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GstMetadataType type,
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GstMetadataReadType read_type);
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GList *
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gst_metadata_read_props_cached_many (GList *locations,
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GList *names,
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GstMetadataType type,
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GstMetadataReadType read_type);
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GList *
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gst_metadata_content_write (const char *location,
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GstMetadata *metadata);
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SOME USEFUL RESOURCES
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---------------------
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http://www.chin.gc.ca/English/Standards/metadata_multimedia.html
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- describes multimedia data for images
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distinction between content (descriptive), technical and
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administrative metadata
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