of slides and related annotations: vector- and raster- based approaches. With vector-based recording, objects shown at the computer screen (and projected onto the wall for the audience of a live presentation) are captured in an abstract representation of geometrical primitives
(points, lines, polygons, etc.). The resulting file is an
object-based representation of visual elements together with the respective timing information. In contrast to this, raster-based recording takes snapshots, that is, images of the screen output of a computer monitor or
data projector. The resulting file contains a temporally
ordered sequence of single images or bitmaps, that is rectangular grids of pixels with the respective color information.
Vector-based recording is generally done directly on the machine used by the lecturer during the pre- sentation. In most cases, the recording functionality is directly integrated into the presentation software. As a consequence, all material has to be prepared with this (often proprietary) software, and external applica- tions (such as short animations or video clips replayed with separate programs) cannot be recorded. To cope
with the first problem, some approaches use import filters for common slide formats, such as PowerPoint,
LaTeX, or PDF (Hürst, Müller, & Ottmann, 2006). The dependency of a particular application, that is the presentation software and recording tool, is one of the main disadvantages of this approach.
In contrast to this, raster-based recording can be realized with a separate program running independently of any particular presentation program. Normally, it captures anything that is represented on the screen. Hence, external applications can be recorded as well. Alternatively to such a software-based solution, ad- ditional hardware can be used to record the visual information not on the lecturer’s computer but at the interface between the presenter’s machine and the monitor. Such a technique is often called VGA cap- turing or VGA grabbing because it grabs the signal at the video output of the computer. In contrast to this, the software-based solution is normally referred to as screen capturing or screen recording, because the information is grabbed directly from the internal memory of the system. The main advantage of a hard- ware-based capturing solution is obvious: Since the signal is grabbed at a standardized interface, that is, the
computer’s video output, the recording is not only done independently from any application, but also from the operating system. Its main disadvantage compared to screen capturing, that is, the software-based solution, is the necessity to use additional hardware. Although solutions exist (Schillings & Meinel, 2002), which get by with a rather mobile set of equipment, a pure software-based solution where nothing other than the lecturer’s PC or laptop is needed might come in handier in many situations.
Being able to perform an application or even plat- form independent automatic presentation recording
is a significant advantage of raster-based approaches
compared to vector-based techniques. However, this advantage comes at the expense of compromises that
generally have to be made related to quality of the file and flexibility in its usage and post-processing. For
example, while vector-based images and animations
can basically be scaled indefinitely without degradation, raster-based recordings can have a significant loss in
quality if scaled to higher or lower resolutions. Hence, a VGA capturing achieves platform independence on
the lecturer’s machine, but the resulting file might be
represented in different qualities depending on the end user’s (i.e., the student’s) computer. In addition, vec-
tor-based file formats are generally assumed to have
a lower data rate, although the actual compression rate depends on the used codec, and therefore general statements about this issue should be treated with care.
Distribution of the final recordings to the students might
require to produce different versions of the recorded data in various media formats, for example, streaming media, such as Real Media or Windows Media, MPEG-
2 or MPEG-4 files, and so forth. Transferring bitmap
data produced with raster-based recording might again result in a loss of quality, while on the other hand vec-
tor-based data can generally be transferred into other file
formats much easier. While approaches for indexing of raster-based data streams exist (e.g., based on Optical Character Recognition [Ziewer, 2004]), indexing of vector-based data streams can generally be done much easier, with less effort, and higher reliability. In a similar way, Meta information such as a structured overview
of a file’s content can often be produced easily from
vector-based data. Of course, techniques such as image analysis can be used to create such information from raster-based data streams as well (Welte, Eschbach, & Becker, 2005), but again, much more effort is required, and the result is usually less reliable. In addition, it is
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generally much easier to add annotations to data that is stored in a vector-based way (Fiehn et al., 2003; Lienhard & Lauer, 2002).
systems
Figure 3 summarizes and classifies the main charac- teristics of the three different approaches described previously. An example for an actual system based on vector-based recording is the Authoring on the Fly (AOF) approach (Hürst, Müller, & Ottmann, 2006). Research projects realizing a software- and hardware- based raster recording are, for example, the TeleTeach- ingTool (http://teleteaching.uni-trier.de; Ziewer & Seidl, 2002) and the tele-TASK system (http://www. tele-task.de/en/index.php; Schillings & Meinel, 2002), respectively. Meanwhile, commercial systems based on all three different recording approaches exist as well. For example, LECTURNITY (http://www.lecturnity. de) is an automatic lecture and presentation recording software, which actually evolved from the AOF project. The tele-TASK system is now available commercially as well. An example for a software-based screen captur- ing system that is often used for presentation recording is TechSmith’s Camtasia Studio software (http://www. camtasia.com).
future trends
Different approaches for authoring of educational mul- timedia documents via automatic lecture recording have been studied for several years now. The results of the respective projects have led to numerous commercial products. However, various questions related to such an authoring process remain and are the focus of current, ongoing research. For example, different projects are dealing with the question of how the interface for the presenter can be improved. Related issues include the design of better interfaces for common presentation software (Hürst, Mohamed, & Ottmann, 2005), digital blackboards and lecture halls (Friedland, Knipping, Tapia, & Rojas, 2004; Mohamed & Ottmann, 2005; Mühlhäuser & Trompler, 2002), as well as gesture-based interaction with wall-mounted digital whiteboards (Mohamed, 2005). Another area of particular interest is approaches for automatic analysis and indexing of lecture recordings, including speech retrieval (Hürst 2003; Park, Hazen, & Glass, 2005) and indexing of the content of the slides and annotation stream (Welte, Eschbach, & Becker, 2005; Ziewer, 2004). Finally, different projects are concerned with the usage of lec- ture recordings in different learning scenarios, such as peer assessment (Trahasch, 2004) or anchored group discussions (Fiehn et al., 2003) and online annotations (Lienhard & Lauer, 2002).
conclusIon
Lightweight production of educational multimedia data via automatic lecture recording is an idea that has been implemented and studied in different developments
over the last decade. In this article, we identified the most significant characteristics of these approaches
and discussed the resulting advantages and limitations for the whole production process. It turned out that there is no “best” technique, but that the advantages and disadvantages compensate each other. The suc- cessful and ongoing usage of systems based on both approaches, vector- as well as raster-based recording,
in different realizations confirms this statement. It
also makes clear, why it is particularly important to be aware of not only the advantages and possibilities of- fered by a particular system, but also its limitations and shortcomings. Although there is a common consensus Figure 3. Characterization of different approaches for
automatic lecture recording
VECTOR-BASED RECORDING INTEGRATED INTO THE
PRESENTATION SOFTWARE TOOL RECORDING VIA SCREEN CAPTURING USING SPECIAL SOFTWARE TOOLS
RECORDING VIA VGA- CAPTURING USING SPECIAL HARDWARE A PP LIC . D EP EN D EN T PLATFORM DEPEND. A PP LIC . IN D P EN D EN T PLATFORM INDEPEND. VE C TO R -G R AP H IC S BI TM AP S, R AS TE R GR AP H. SOFTWARE HARDWARE
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that no best recording approach does or will ever exist but rather all developed techniques will most likely co-exist in the future, many interesting open research questions in the area of automatic lecture recording remain, mostly related to interface issues during the recording, the post-processing phase, and the further usage of the produced documents.