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MRTSpace is based on MRT, a Minimal Rendering Tool [Fel96], developed at the University of Bonn.
MRTSpace builds on the modeling functionality and the structure of geometric objects maintained by MRT to
represent all of its internal data, handle the interaction between the different actors and do all the visibility
computations, including collision detection. This toolkit also resolves some of the major problems of developing
a platform independent program: it has a platform independent graphical user interface which uses most of the
existing graphics hardware to display 3D objects.
2 Improving the 3D perception in VRML worlds
2.1 Navigating along solid objects
A general problem of 3D viewers is the handling of solid objects (like walls) when the user walks through a scene.
The minimum level of user support is the detection of solid obstacles and the prevention of moving the user’s
camera into ’black holes’, i.e. 3D areas surrounded by non-transparent planes thus blocking all light sources and
thus resulting in a completely black screen. Some viewers provide a head-light feature to circumvent this problem
of getting lost in the dark. Nevertheless, light is not the only issue as the user is still confronted with a uniformly
colored screen.
A better navigation control can be built on functional elements commonly used for ray-tracing algorithms
[Gla89]: most of the time in the ray-tracing process is spent on casting rays onto the scene and determining which
is the closest object. The considerable research invested in ray acceleration techniques provides us with a rich set
of tools (e.g. octree or regular space subdivision) which can be applied to various aspects of navigation control.
MRTSpace uses the object-oriented data structure of MRT to represent all VRML objects. Consequently,
collisions can be detected by using the ray-tracing functionality of MRT. The built-in ray-tracing modules compute
the intersection of a given ray with a scene, returning the distance to the closest object hit.
So if the user moves into a given direction MRTSpace casts a ray along the trajectory and checks whether the
returned distance is less than a given value. In this case the user was about to run ‘into’ an object and MRTSpace
takes the normal vector of this object at the intersection point of the trajectory and the object to determine how
to change the user’s direction in order to avoid a collision or disappearance in ‘outer space’. This feature has the
advantage that a user who is too close to an obstacle (assuming the starting position of the camera has been chosen
inappropriately by the designer of this scene), and therefore sees nothing but a solid color, just needs to go ahead
and will automatically be repositioned at a more appropriate point.
Figure 1: Using rays cast ahead of the moving avatar to guide the user
Implementing this feature in MRT can be done with the following code (which is much shorter than its
description):