topical media & game development

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# feature extraction

Manual content annotation is laborious, and hence costly. As a consequence, content annotation will often not be done and search access to multimedia object willnot be optimal, if it is provided for at all. An alternative to manual content annotation is (semi) automatic feature extraction, which allows for obtaining a description of a particular media object using media specific analysis techniques.

The Multimedia Database Research group at CWI has developed a framework for feature extraction to support the Amsterdam Catalogue of Images (ACOI). The resulting framework for feature extraction is known as the ACOI framework,  [ACOI]. The ACOI framework is intended to accomodate a broad spectrum of classification schemes, manual as well as (semi) automatic, for the indexing and retrieval of arbitrary multimedia objects. What is stored are not the actual multimedia objects themselves, but structural descriptions of these objects (including their location) that may be used for retrieval.

The ACOI model is based on the assumption that indexing an arbitrary multimedia object is equivalent to deriving a grammatical structure that provides a namespace to reason about the object and to access its components. However there is an important difference with ordinary parsing in that the lexical and grammatical items corresponding to the components of the multimedia object must be created dynamically by inspecting the actual object. Moreover, in general, there is not a fixed sequence of lexicals as in the case of natural or formal languages. To allow for the dynamic creation of lexical and grammatical items the ACOI framework supports both black-box and white-box (feature) detectors. Black-box detectors are algorithms, usually developed by a specialist in the media domain, that extract properties from the media object by some form of analysis. White-box detectors, on the other hand, are created by defining logical or mathematical expressions over the grammar itself. Here we will focus on black-box detectors only.

The information obtained from parsing a multimedia object is stored in a database. The feature grammar and its associated detector further result in updating the data schemas stored in the database.

#### formal specification

Formally, a feature grammar G may be defined as $G = \left(V,T,P,S\right)$, where V is a collection of variables or non-terminals, T a collection of terminals, P a collection of productions of the form $V -> \left(V \union T\right)$ and S a start symbol. A token sequence ts belongs to the language $L\left(G\right)$ if $S -*-> ts$. Sentential token sequences, those belonging to $L\left(G\right)$ or its sublanguages $L\left(G_v\right) = \left(V_v,T_v,P_v,v\right)$ for $v \e \left(T \union V\right)$, correspond to a complex object $C_v$, which is the object corresponding to the parse tree for v. The parse tree defines a hierarchical structure that may be used to access and manipulate the components of the multimedia object subjected to the detector. See  [Features] for further details.

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## anatomy of a feature detector

As an example of a feature detector, we will look at a simple feature detector for (MIDI encoded) musical data. A special feature of this particular detector,that I developed while being a guest at CWI, is that it uses an intermediate representation in a logic programming language (Prolog) to facilitate reasoning about features.

The hierarchical information structure that we consider is defined in the grammar below. It contains only a limited number of basic properties and must be extended with information along the lines of some musical ontology, see  [AI].

feature grammar

detector song; ## to get the filename
detector lyrics; ## extracts lyrics
detector melody; ## extracts melody
detector check;  ## to walk the tree

atom str name;
atom str text;
atom str note;

midi: song;

song: file lyrics melody check;

file: name;

lyrics: text*;
melody: note*;

The start symbol is a song. The detector that is associated with song reads in a MIDI file. The musical information contained in the MIDI file is then stored as a collection of Prolog facts. This translation is very direct. In effect the MIDI file header information is stored, and events are recorded as facts, as illustrated below for a note_on and note_off event.

event('twinkle',2,time=384, note_on:[chan=2,pitch=72,vol=111]).
event('twinkle',2,time=768, note_off:[chan=2,pitch=72,vol=100]).

After translating the MIDI file into a Prolog format, the other detectors will be invoked, that is the composer, lyrics and melody detector, to extract the information related to these properties.

To extract relevant fragments of the melody we use the melody detector, of which a partial listing is given below.

melody detector

int melodyDetector(tree *pt, list *tks ){
char buf[1024]; char* _result;
void* q = _query;
int idq = 0;

idq = query_eval(q,"X:melody(X)");
while ((_result = query_result(q,idq)) ) {
putAtom(tks,"note",_result);
}
return SUCCESS;
}

The embedded logic component is given the query X:melody(X), which results in the notes that constitute the (relevant fragment of the) melody. These notes are then added to the tokenstream. A similar detector is available for the lyrics.

Parsing a given MIDI file, for example twinkle.mid, results in updating the database.

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#### implementation

The embedded logic component is part of the hush framework,  [OO]. It uses an object extension of Prolog that allows for the definition of native objects to interface with the MIDI processing software written in C++. The logic component allows for the definition of arbitrary predicates to extract the musical information, such as the melody and the lyrics. It also allows for further analysis of these features to check for, for example, particular patterns in the melody.

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## example(s) -- modern art: who cares?

The artworks shown above are taken from  [Modern], which bundles the experiences and insights resulting from studying the preservation of contemprary art, under the title: modern art, who cares? This project was a precursor to the INCCA that provided the input to our multimedia casus, which is introduced in chapter 10.

Both the INCCA project and the related Open Archives Initiative, focus on making meta-information available on existing resources for the preservation of contemporary art and cultural heritage in general, including reports, case studies and recordings of artworks, that is images, videos and artists interviews.

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## research directions -- media search

There is a wealth of powerful search engines on the Web. Technically, search engines rely either on classification schemes (as for example Yahoo) or content-based (keyword) indexing (as for example Excite or AltaVista). Searching on the Web, nowadays, is moderately effective when text-based documents are considered. For multimedia objects (such as images or music) existing search facilities are far less effective, simply because indexing on category or keywords can not so easily be done automatically. In the following we will explore what search facilities there are for music (on the web).

We will first give some examples of search based on keywords and categories, then some examples of content-based search and finally we will discuss a more exhaustive list of musical databases and search facilities on the Web. All search facilities mentioned are listed online under musical resources.

#### keywords and categories

For musical material, in particular MIDI, there are a number of sites that offer search over a body of collected works. One example is the Aria Database, that allows to search for an aria part of an opera based on title, category and even voice part. Another example is the MIDI Farm, which provides many MIDI-related resources, and also allows for searching for MIDI material by filename, author, artist and ratings. A category can be selected to limit the search. The MIDI Farm employs voting to achieve collaborative filtering on the results for a query.

Search indexes for sites based on categories and keywords are usually created by hand, sometimes erreonously. For example, when searching for a Twinkle fragment, Bach's variations for Twinkle were found, whereas to the best of our knowledge there exist only Twinkle variations by Mozart  [Mozart].

The Digital Tradition Folksong Database provides in addition a powerful lyrics (free text) search facility based on the AskSam search engine.

An alternative way of searching is to employ a meta-search engine. Meta-search engines assist the user in formulating an appropriate query, while leaving the actual search to (possibly multiple) search engines. Searching for musical content is generally restricted to the lyrics, but see below (and section Match).

#### content-based search

Although content-based search for images and sound have been a topic of interest for over a decade, few results have been made available to the public. As an example, the MuscleFish Datablade for Informix, allows for obtaining information from audio based on a content analysis of the audio object.

As far as content-based musical search facilities for the Web are concerned, we have for example, the Meldex system of the New Zealand Digital Library initiative, an experimental system that allows for searching tunes in a folksong database with approximately 1000 records,  [Meldex]. Querying facilities for Meldex include queries based on transcriptions from audio input, that is humming a tune! We will discuss the approach taken for the Meldex system in more detail in research directions section, to assess its viability for retrieving musical fragments in a large database.

#### music databases

In addition to the sites previously mentioned, there exist several databases with musical information on the Web. We observe that these databases do not rely on DBMS technology at all. This obviously leads to a plethora of file formats and re-invention of typical DBMS facilities.

Without aiming for completeness, we have for example the MIDI Universe, which offers over a million MIDI file references, indexed primarily by composer and file length. It moreover keeps relevant statistics on popular tunes, as well as a hot set of MIDI tunes. It further offers access to a list of related smaller MIDI databases.

Another example is the aforementioned Meldex system that offers a large collection of tunes (more than 100.000), of which a part is accessible by humming-based retrieval. In addition text-based search is possible against file names, song titles, track names and (where available) lyrics.

The Classical MIDI Archive is an example of a database allowing text-based search on titles only. Results are annotated with an indication of "goodness" and recency.

The Classical Themefinder Database allows extensive support for retrieval based on (optional) indications of meter, pitch, pitch-class, interval, semi-tone interval and melodic contour, within a fixed collection of works arranged according to composer and category. The index is clearly created and maintained manually. The resulting work is delivered in the MuseData format, which is a rich (research-based) file format from which MIDI files can be generated,  [Beyond].

A site which collects librarian information concerning music resources is the International Inventory of Music Resources (RISM), which offers search facilities over bibliographic records for music manuscripts, librettos and secondary sources for music written after c.a. 1600. It also allows to search for libraries related to the RISM site.

Tune recognition is apparently offered by the Tune Server. The user may search by offering a WAV file with a fragment of the melody. However, the actual matching occurs against a melodic outline, that is indications of rising or falling in pitch. The database contains approx. 15.000 records with such pitch contours, of which one third are popular tunes and the rest classical themes. The output is a ranked list of titles about which the user is asked to give feedback.

#### discussion

There is great divergence in the scope and aims of music databases on the Web. Some, such as the RISM database, are the result of musicological investigations, whereas others, such as the MIDI Farm, are meant to serve an audience looking for popular tunes. With regard to the actual search facilities offered, we observe that, with the exception of Meldex and the Tune Server, the query facilities are usually text-based, although for example the Classical Themefinder allows for encoding melodic contour in a text-based fashion.

(C) Æliens 04/09/2009

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