4.1). However, every image needs to be requested by a separate connection [89], which results in a higher transfer overhead and thus in a slower possible frame rate (see Section 4.1.2). In contrast, mJPEG just pushes a new image on the initially opened connection and thus has a lower network overhead. Many of the existing web-based visualization applications use this transfer technique, as e.g. ParaViewWeb [59].
The technique analysis in Section 4.1.1 shows that mJPEG is a favorable and well performing choice. However, it is currently not usable on all web browsers, because Internet Explorer and Opera do not support mJPEG by default. Providing interactive visualization with best perfor-mance and quality on all web browsers would therefore require to provide different methods. The need of providing fallback mechanisms is a big disadvantage of several web technologies, e.g. video streaming [221]. Regardless, a specific fallback mechanism without added software as substitute to mJPEG (Safari, Chrome and Firefox) could be pulling single JPEG images (Internet Explorer, Opera) as described by ParaViewWeb or, alternatively, the transfer via WebSockets [60]. The specific image transfer technique is independent of the stereoscopic presentation discussed in Sec-tion 8.2 and the automatic quality adjustment discussed in SecSec-tion 8.4.
The requirement of using pure web browsers reduces the choice of image codecs to JPEG [257]
and Portable Network Graphics (PNG) [256] (see Section 4.1.3). PNG is ideal to compress text or line drawings with hard edges and lossy JPEG to compress images with smooth transitions, to get low file sizes [288]. The typical volume visualization of the human body shows muscles, organs, and bones, each usually with smooth transitions. JPEG is therefore a good choice to compress the visualization. Another image format is JPEG 2000 [259], which utilizes wavelet compression instead of the discrete cosine transformation of JPEG, and results in a higher quality/file size ratio. Since this is ideal for remote visualization, it is used in many projects that utilize native applications. But since JPEG 2000 and other newer image compression formats, e.g. WebP [261]
and H.264 intra frame compression [262], are not supported by default on any web browser, they can not be used.
8.2 Support of multiple participants at different locations with different stereoscopic systems simultaneously
The previous discussion in Section 8.1 highlights the importance of the easy accessibility of inter-active visualization and discusses available remote visualization approaches and implementations using web browsers. The described pure web-based systems were mostly developed for a mono-scopic usage. Stereomono-scopic remote visualization, in contrast, is less frequently used and mainly based on native applications, e.g. the special Access Grid-based deployment of the previous vir-tual class setup [108]. The general screen sharing applications (e.g. Guacamole [58]) might only
be able to provide a well performing interactive remote stereoscopic visualization with special con-figurations or additional developments. The specific web-based applications (e.g. ParaViewWeb [59]) are used together with a base application and, thus, are bound to the stereoscopic capabilities and visualization algorithms of their base application. Only Vitrall [65] provides a specific single stereoscopic content type. The simultaneous provision of multiple stereoscopic content types to serve different stereoscopic systems simultaneously would require additional development in each of the described cases. No literature was found that describes such an easy to use system for stereoscopic volume visualization.
CoWebViz fills this gap and provides the same ease of use for stereoscopic remote visualization on different stereoscopic display systems via a pure web browser as it does for monoscopic vi-sualization. Its feasibility is shown by its usage in different practical scenarios for stereoscopic but also monoscopic visualization (see Section 6.1 to 6.3), which is underpinned by performance tests in Section 7.1.2 (see also discussion in Section 8.4). As described in Section 3.2, various stereoscopic display systems exist, which partially utilize differing content types. Thus, in order to provide ad-hoc usability in a remotely scattered group with different stereoscopic systems, multiple content types need to be provided simultaneously. CoWebViz provides three different stereoscopic content types (two-view, side-by-side and anaglyph), which can be used on different classes of stereoscopic display devices (projector-based setups, three-dimensional (3D) displays and standard two-dimensional (2D) displays). This is a range from standard desktop and tablet computers to large-scale projection setups.
The projector-based setup was used successfully during the anatomy classes. Due to its large presentation, this display technique provides a high degree of visual immersion.
The side-by-side stereoscopic visualization on a 3DTV was successfully used in multiple scenarios.
At first it was used for several visualization demonstration sessions in a hospital conference room.
But to be highlighted is the stereoscopic usage within an operating room to inform a plastic surgeon during reconstructive surgery, which shows the feasibility of CoWebViz’s usage in a real practical medical scenario. The usage on 3DTVs is an important use case, because of the low cost of such TVs and their simple setup.
A content type that requires no stereoscopic hardware deployment at all, is anaglyph stereoscopic visualization, which only requires low-cost glasses. Its big disadvantage is the color shift caused by the colorized lenses, which, however, is still under research and can be considerable minimized [289, 290]. But its ubiquitous usability makes this technique still interesting for simple use cases [291]. Because of this simple accessibility, it was implemented to be used for self-directed learning on-campus, but was only tested in test sessions.
Monoscopic visualization was also used for different scenarios, e.g. an ad-hoc demonstration of CoWebViz to collaborators with a parallel teleconference and to provide researchers with interactive visualization rendered on a remote visualization cluster.
8.2 Multiple participants at different locations with different stereoscopic systems
Auto stereoscopic systems are increasingly often used, because of their advantage of not requiring any personal gear (e.g. polarized glasses). Such systems, however, are currently not supported by CoWebViz, because they require a specific content type that has all stereoscopic views merged into a single image. This means that any two neighboring pixels or sub-pixels of an image presented on most auto stereoscopic 3DTVs represent another stereoscopic view. Such a merged image cannot be transferred with image compression and would require client side procession, which would entail further research.
Stereoscopic displays are not yet available as widely as standard monoscopic displays, which are provided with every computer workstation. But stereoscopic display devices are increasingly often deployed at special visualization centers [161, 292], conference rooms/lecture halls [293], or directly at the demanding departments [236]. An important factor that influences the availability of stereoscopic devices is the movie industries’ initiative of producing more stereoscopic movies in the past years, which entailed an increased production and wider availability of stereoscopic devices. Whereas the hardware is more often available, the specific software for interactive visu-alization still needs to be deployed, which is eased by CoWebViz.
CoWebViz can be used simultaneously by multiple participants from different remote locations.
The scalability analysis (see Section 7.1.3) shows that the system has a good performance with up to the maximal tested six simultaneously accessing participants. It is further shown that the server’s Central Processing Unit (CPU) usage and network throughput increases with every additional user. Thus, the maximal possible number of simultaneously accessing users highly depends on the environment in which the system is used. But it is expected that most use cases are not requiring more than about four simultaneously accessing groups, which is discussed by the following examples.
The proof of concept conduction in the class involved two participating groups, each with a two-view stereoscopic visualization. While this could be slightly increased to provide visualization to more than two groups, a real contrast would be the usage for self-directed learning by students.
This could potentially require the synchronous access by 10 to 100 students, depending on the class size. This scenario, however, would not be viable, since only one student would be able to modify the visualization at any time. Providing each student with a freely controllable visualization would require a separate server instance for every student. This, however, would be a whole another use case with only one participant per instance. It is therefore estimated that a small group of up to four remote participants is realistic, at least for education use cases. Four participants with a two-view stereoscopic visualization (each view 1024x768 and a JPEG quality of 85 and a peak frame rate of 15 fps) would result in a peak network usage of about 25 Mbits. This is high, but currently viable with a high-performing network on the server side. A possible solution to provide visualization to more participants would be the usage of multicast networks. Multicast however requires special network arrangements between the server and each client, which would reduce the ad-hoc usability. An alternative that does not require any specific client adjustments
but more effort on the server side would be the addition of load-balancing servers. This issue, however, was not a central topic of this work, because the proof of concept conduction did not require large amounts of simultaneously accessing users. The central issue rather was the network throughput optimization to each single client as discussed in Section 8.4.
The sharing of monoscopic visualization is not new compared to the other web-based and non web-based systems. However, while developing such a system, the pattern of how remote users interact with each other needs to be considered. Interactive systems with multiple users have been used with different interaction methods. Prominent methods are to give only one per-son the right to modify and the others only the right to view [270], to give all participants the right to modify after a request [271], and to let every participant freely modify after oral coor-dination [160]. The idea behind the proof of concept was to provide a most simple system with least required user knowledge. Thus, CoWebViz’s method is to inform the participants about the current state and to give everyone the instantaneous right to just modify, if nobody else is modifying. Because of the few direct users in the proof of concept conduction, these methods could not be evaluated against each other. But this method seems to be as natural as possible without any system restrains. It requires mutual oral coordination, which is always necessary for synchronous collaborative working and can be done via video and/or teleconferencing. No barriers regarding this method were observed during the proof of concept conduction.
CoWebViz’s stereoscopic and collaborative functionality can be seen as a stereoscopic remote visualization service, which is a mix of a stereoscopic player [129] (taking stereoscopic input formats and providing desired output formats) and a video-streaming server (taking input and streaming to multiple participants). By requiring no special software deployment, CoWebViz allows for various advanced use cases with the utilization of interactive 3D and stereoscopic visualization on remote computers and multiple participants.