It "is “necessary now to render the 
terrain, using Computer Graphic 
techniques. It must be made a choice 
between a fast methods and methods that 
produce more realistic effects. 
Figures 3.7 to 3.9 show the sequence to 
the rendering of the terrain. For hidden 
lines it was used the Z-buffer algorithm, 
and for illumination the Gouraud 
technique. The illumination was computed 
by the following formula: 
Vetor 
Normal 
  
Vetor de 
Visualizacáo 
Ia Ci. cos (a) Ic + Ta, 
where Ia is the light intensity reflected 
in each RGB color, C; is the reflectance 
coefficient for the material for each RGB 
color, cos(a) is the cosine of the angle 
formed by the normal and the incident 
light, 1: is the intensity of the Light 
source in each RGB color, and I, is the 
ambient light. Using Gouraud, the 
intensities inside each point in the 
triangles is the linear interpolation of 
the vertices intensities. 
  
Fig. 3.7 - Vector Normal to the plane. 
Ponto de 
lluminacáo 
lluminaçäo 
Ambiente 
   
  
Fig. 3.8 - Vectors between a illumination point and triangle 
vertices. 
  
Fig. 3.9 - Painting sequence with the Z-Buffer algorithm de o Z- 
Buffer. 
The last step is the painting of each 
polygon, through gray levels. Tt is 
; : | possible to use more sophisticated 
Next, the conic perspective projection methods as fractals, or an airplane or 
has to be performed. Figure 3.10 presents satellite image, however due to the need 
the steps for the calculation of a for speed in the computer calculations in 
conical perspective projection, with the this work the gray levels were painted 
reduction from three to two dimensions. 
911 
according to their light intensity, from 
0 (black) to 255 (white) in each RGB 
color. 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996