International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B4, 2012
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
developed with the main goal to be platform independent,
extendable, and to work well over low-bandwidth internet
connections. Important geospatial features such as accurate
coordinate system support and a certain level of scalability were
added by GeoVRML in the late 90ies (Reddy et al., 2000).
(Geo)VRML and its successor X3D are text based and do not
provide the capability to access graphics hardware and to create
custom graphics engines. In parallel to the VRML efforts,
numerous approaches for interactively streaming large 3d
virtual environments were being developed. However, they all
required the installation of a proprietary application or a
specific browser plugin. Among the earliest technologies for
generating and interactively exploiting very large 3d landscape
models over the Internet using browser plugins were DILAS /
G-VISTA (Nebiker, 2003) and LandExplorer (Dóllner et al.,
2003).
Another mechanism for 3d scene description and integration of
complex, explorable 3d scenes into webpages is XML3D (Sons
et al, 2010). XML3D is a HTMLS extension and allows the
integration of these scene descriptions directly into the HTML5
object model. Modern browsers support XML3D natively and
often WebGL is used for rendering within a website. For web-
design purposes, CSS3 3D (Jackson et al., 2009) offers some
functionality for 3d visualization of CSS elements (e.g.
perspective transformations, translations etc.) CSS 3D is an
extension of the CSS3 standard and is supported by many
modern browsers.
The creation and recent release of WebGL has spurred a
number of projects and activities with the goal of exploiting 3d
contents directly within the web browser. WebGL is a low level
API, this means that WebGL provides functionality to access
graphics hardware such as creating textures, create shader
programs on the GPU, and vertex buffers. Therefore new high-
level engines can be created using the WebGL standard by
adding functionality such as loading 3d models, providing
vector math functionality, ray-picking, texture atlases, or scene
graphs. One very popular WebGL engine is three.js which is a
lightweight 3d engine with low level of complexity (Three.js,
2012). Other noteable WebGL based high-level 3d engines are
SpiderGL (Di Benedetto, 2010), SceneJS (SceneJS, 2012),
Copperlicht (Ambiera, 2012) and Processing.js (Processing,
2012).
For visualization of large-scale geospatial 3d contents various
virtual globes running directly in the web browser have been
developed. They can be categorized in three types: First, there
are plugin-based globes requiring the previous installation of a
browser plugin. For every browser and operating system a
separate plugin must be developed to achieve cross
browser/cross platform support. Examples of such globes are
Google Earth Plugin (Google Earth Plugin, 2012), Nokia Maps
3D (Nokia, 2012) and Bing Maps 3D, which was discontinued
in November 2010 (Bing Maps 3D, 2010). The second type
uses Java Applets, such as OSM-3D (Schilling and Zipf, 2011)
or NASA World Wind (World Wind, 2007). The third type of
web browser based globes use WebGL for rendering. Examples
of such globes are Nokia Maps for WebGL (Beta) (Nokia
WebGL, 2012), WebGLEarth (WebGLEarth, 2012), ReadyMap
(Pelican Mapping, 2012) based on the osgjs engine (Osgjs,
2012), and OpenWebGlobe (OpenWebGlobe, 2012).
3. THE OPENWEBGLOBE PROJECT
The OpenWebGlobe project (www.openwebglobe.org) was
initiated by the Institute of Geomatics Engineering of the
FHNW University of Applied Sciences and Arts Northwestern
Switzerland (IVGI). It started in April 2011 as an open source
project following nearly a decade of 3d geobrowser
development at the institute. Together with developers from
industry and from other universities, the functionality of the
SDK is being extended continuously.
The development is based on the earlier i3D virtual globe
technology, which was also developed at the IVGI and which
was used for several research and industry projects (Christen &
Nebiker, 2010). Unlike the i3D technology, the OpenWebGlobe
SDK is fully open source and released under MIT license. All
source code is freely available at github
(http://github.com/OpenWebGlobe) and can be viewed, adapted
or extended even for commercial use.
The OpenWebGlobe SDK consists of two main parts: first, the
OpenWebGlobe Viewer part (as described in section 3.2), it
consists of a JavaScript library which allows the integration of
the OpenWebGlobe into custom web-applications. Second, the
OpenWebGlobe Processing Tools (introduced in Section 3.3), a
bundle of tools for bulk data processing, e.g. tiling or
resampling of large geospatial data sets. This pre-processing is
required by the viewer part to enable scalable fragment-based,
streamed download and visualization of data.
Further information about the project, tutorials, a function
reference and a support forum are available at
http://www.openwebglobe.org.
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With the OpenW'ebGlobe SDK you can create your owm virtual globe applications running in the web browser!
OpeniwebGlobe consists of a high-performance 3d geobrowser and it also encompasses the software for processing very
large volumes of geospatiat data in highly parallel and scalable computing environments. The WebGL Version runs in the
browser without plugin.
Figure 1. Screenshot of the project homepage
www.openwebglobe.org
3.1 Geospatial Foundations of OpenWebGlobe
Great emphasis was placed on providing a sound geospatial
reference. This is crucial, especially if OpenWebGlobe is used
as a basis for accurate virtual or mixed reality applications.
An ellipsoidal geodetic reference model is employed, in order
to minimize geometric transformation errors and to enable
position accuracies within the virtual globe at the sub-meter
level (Christen & Nebiker, 2011b). The default spatial reference
system and reference ellipsoid in OpenWebGlobe is WGS84.
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