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Figure 2: Adjusting the up vector of the avatar to the normal of the walking plane.
// find closest object ’hitObject’ intersecting the ray emanating from
// ’eyePoint’ in direction ’dir’ returning the distance to the intersected
// object in variable ’dist’. In case that no intersection occurs
// if ’hit’ equals FALSE all other variables remain unchanged.
hit = allObjs->intersect(eyePoint,dir,NULL,dist,hitObject);
if (hit && dist-HEAD_DISTANCEsurfaceNormal(eyePoint+dir*dist);
// change user’s direction accordingly
if ( dot(dir*norm,up) < 0 )
TurnRight();
else
TurnLeft();
}
2.2 Climbing Stairs
Another problem frequently encountered with 3D viewers is the inadequate support for navigating over uneven
terrain or for climbing stairs. Having taken care of the collision problem moving on level ground can be mastered
by most users without significant problems. But navigating on a slope or climbing stairs adds an extra degree of
navigational complexity and typically gives the user a hard time.
An easy solution of keeping the user at a constant distance over the ground is implemented in MRTSpace by
casting two rays down to the ground: one ray before and one ray after each step. Both intersections return a
distance. If the difference of the distances is zero the user is considered to walk on flat ground and no special action
is taken. If the difference is within a small range the user is considered to take a step and the difference is added
to the position of the user. If the difference is negative and within a rather large range the user is considered to fall
and the position of the user is decremented in several steps to the new level, giving the user the impression of a
fall. If the distance of the new position towards the ground is too large, the user is either set to the fly mode or just
stopped depending on some setup parameter. If the user is in fly mode, the casting of rays towards the ground is
used to automatically switch back to the walk mode when the user lands on the ground.
Once the scene structure is consistently exploited staying upright is not too difficult either. In this mode the
viewer casts a ray down towards the object the user is moving on and calculates the angle between the surface
and the up vector. If this angle exceeds a given threshold value the up vector is continuously set to the normal
vector to the ground position. The most impressive example in terms of user comfort for this feature is a big ball
representing a planet where the user has the ability to walk on this ball, the feet always staying on the ground and
the gaze vector always parallel to the tangent plane.