Lehrveranstaltungsinhalt aus - Institute for Computer Graphics and ...

0.5. GEOMETRIC DETAIL 13
in the courtyards and so they shown black or grey here. When this occurs, the representation is
without photographic texture, and is instead in the form of a flat shaded representation.
Slide 0.37 looks at the roof scape and we see that perhaps we should model the chimneys as shown
here. However, the skylights were not modeled. What can we do with these data? We can walk or
fly through the cities. We can assess changes for example by removing a building and replacing
it by a new one. We call this “virtual reality”, but scientists often prefer the expression “virtual
environment”, since “virtual” and “reality” represent a contradiction in terms. This differs of
course from photographic reality, which is more detailed and more realistic by showing great
geometric detail, showing wires, dirt on the road, cars, the effect of weather. There is yet another
type of reality, namely “physical reality”, when we are out there in a city and we feel the wetness
in our shoes, we feel the cold in the air, we hear the noise of birds, the screeching of cars. So we
see various levels of reality: physical, photographic and virtual reality.
0.5 Geometric Detail
What geometric detail do we need when we model a city? Lets take the example of a roof. Slide
0.44 is a roofshape extracted for the Vienna example, We have not applied to the roof photographic
texture, but instead some generic computer texture. We will talk later of course about texture
and I will try to explain different types of texture for use in rendering for computer graphics. If
we apply this kind of generic texture we loose all information about the specific characteristics
of this roof. What we would like to have is the roof shown with chimneys. Maybe we need
skylights as well far the fire-guard in order to direct people to an exit through the roof in the case
of a catastrophy. There is a topic here for a Diplomarbeit and Dissertation theme to study the
amount of geometric detail needed in the presence of photographic texture: the trade-off between
photographic texture and geometry detail. To illustrate this further let us take a look at the same
roof with its skylights and chimneys and now use photographic texture to illustrate how this roof
looks like. If we take photographic texture, and if we have some chimneys, and if we render this
roof from another perspective than that from which the photograph was taken, the chimneys will
look very unnatural. So we need to do some work and create the geometric model of the chimneys.
If we employ that model and we now superimpose the photographic texture over it, we see that
we have sunshine casting shadows and we have certain areas of the roof that are covered by pixels
from the shadows left by the chimneys. If the sunshine is from another side, say in the morning,
but the picture was taken in the afternoon, we have wrong shadows. So we need to fix this by
eliminating the shadows in the texture. We introduce the shadow in a proper rendering by a
computation. We also need to fill in those pixels that are covered by the perspective distortion of
the chimneys, and use generic pixels of the roof to fill in the areas where no picture exists. Slide
0.50 is the final result: we have removed the shadow, we have filled in the pixels. We now have
the best representation of that roof with its chimneys and we can render this now correctly in the
morning and in the afternoon, with rain or with sunshine.
0.6 Automation
All of this modeling of cities is expensive, because it is based on manual work. In order to
reduce the cost of creating such models one needs to automate their creation. Automation is a
large topic and is available for many Diplomarbeiten and many Dissertations. Let me illustrate
automation for about our city-models in Graz. There already exist 2-dimensional descriptions
so the task of automating here is to achieve the transition from two to three dimensions. Slide
0.52 is a two-dimensional so-called geographic information system (GIS) of a certain area around
the Schlossberg in Graz. Lets take a look at this particular building in Slide 0.53. We have a
total of five aerial photographs, 3 of them are shown of that particular building in Slide 0.54.