Geography Reference
In-Depth Information
are no natural boundaries, where the shape of the fortification can be changed in
different LODs.
The fortification is an “unimportant” object and it just completes the rest of the
model. There is no need to display much of details. Making an impression of a
fortress is enough.
Because of the problems arising from using multiple LODs and only low
requirements on details, the fortification is modelled in one LOD only.
Texturing the Model of the Fortification
The prevailing surfaces in fortification are walls, grass and paving. To be consistent
with the rest of the model, not a photorealistic but an artificial textures should be
used for them. Up to now, mainly the grass texture has been dealt with, because it is
considered to be the most ingenious.
The fortification is modeled in one LOD only, so the grass-like texture is the
same for wide ranges of observer distance. This put some special requirements on
the texture:
There must be enough details in the texture so that it looks good when the
observer is reasonably close.
The texture must be at least apparent for a wide range of observer distance.
Pouring together all colors in the texture should be avoided for as long distance
as possible.
There should not be visible any texture discontinuities on edges connecting
surfaces with the same texture.
The effect of a repeated pattern should be suppressed as much as possible.
Besides that, the texture should be seamless and its size should be kept small.
There are sophisticated computational methods of creating an artificial texture
(for example Chang and Hsiao
1994
), but it does not meet our requirements.
Because the grass does not create any regular pattern, a simple texture made by
random noise added to the green base-color was tested (see Fig.
1d
). It looks good
for small distances of the observer, where the pixels (or small groups of them) are
distinguishable. For farther distances of the observer the pixels pours together and
the texture fades out to a non-textured one-color surface only. However, this is the
only disadvantage of the simple texture. All other requirements are met.
To avoid pouring colors together for longer distances of observer, an advanced
texture had been computed. The advanced texture is based on the random noise
also, but the noise is added in multiple scales. The results can be seen in Fig.
1e
,
where the advanced texture with size of 20
864 pixels) is used. It is
obvious that the advanced texture is apparent in much larger distance than the
simple texture. There are almost no texture discontinuities on the edges and the
effect of repeated pattern is minimized.
20 m (864
