Graphics Systems (LEEC)
1.
Illumination
model
The aim
of this exercise is to study local illumination models, mainly by changing
material (diffuse and specular characteristics) and light properties.
Fig. 1 - 3D scene to create
using OpenGL
The zip
file G1_Ilum it is supplied a C
program that produces part of the scene shown in figure 1. For now the
attention should be given for the global variable declarations and the function display() and inicializacao(). In the display() function we put the
code that should be executed in every screen refresh (several times per
second), while in the inicializacao()function we put the
instruction that can be executed just once.
Compile
and execute the code given. As you can see, the initial scene contains:
- A punctual
light source, positioned in the space at the coordinates given by light0_position[]. Note that the light source in not visible so that there is a
sphere for you to identity it better;
- A colored axis
system (x, y, z = R, G, B) each
one obtained by gluCylinder()
- An horizontal
plane divided in small polygons (GL_TRIANGLE_STRIP);
- Two faces of a
box located below the former plane (GL_POLYGON, inside the function myBox() );
The
punctual light source properties are declared in the variables light0_xxx and take effect by calling the
function glLightfv(). The ambient component light_ambient[]is activated by calling glLightModelfv().
The
plane and the box have the same material properties (mat1_xxx); the 3 axis have no material
defined, instead their color is directly defined with glColor3f().
Note:
it is advised to read the chapter 6 of OpenGL
Programming Guide that describes
the main function of the OpenGL light model. The chapters 1-Introduction to
OpenGL and 2-Drawing
Geometric Objects are also
considered indispensable in order to have a good understanding of the OpenGL
technology. The OpenGL
Reference Manual and OpenGL
Online Manual contain the
functions syntax and describe their meaning.
1. Complete the scene as shown in
figure 1, by finishing the box and by putting the white sphere at the origin.
a)
The
box is drawn in myBox() and is a collection of individual
polygons (GL_POLYGON); the polygon front is defined by
the counter clockwise order of vertices. The surface normal used by the
illumination model is defined by glNormal3d().
b)
The
white sphere that is at the origin is drawn by gluSphere() from the GLU-OpenGL Utility
Library (see OpenGL
Online Manual).
2. Turn off LIGHT0, so that only ambient light is on.
Analyze the illumination value obtained in each object.
3. Turn on LIGHT0 and turn off ambient light. Analize
the image and identify, on the horizontal plane, the diffuse and specular reflection effects.
4. Turn on ambient light and compare
with the last result. Take particular attention to the lateral and inferior
faces of the box.
5. Try to answer to the following
questions before experimenting them on the program.
a) What is the effect, on the plane
surface, if the LIGHT0 y coordinate is increased?
b)
What
happens to the specular reflection if the parameter mat1_shininess[]is changed? For example, if it
assumes values like 4, 16, 64, 128.
c)
What
happens to the specular reflection if the parameter mat1_shininess[r,g,b]is changed? Experiment the
following values {0.0,0.0,0.0}, {0.2,0.2,0.2}, {0.4,0.4,0.4}, {0.6,0.6,0.6}, {0.8,0.8,0.8}, {1.0,1.0,1.0} with mat1_shininess=128.
6. Modify the RGB coefficients of the ambient
and diffuse reflection of the material mat1_xxx so that R and G component are equal to zero. What
happens in the following conditions:
a)
With
the initial specular coefficients;
b)
By
zeroing the R and G components of the specular reflectiom;
c)
By
putting also LIGHT0 with pure yellow color.
7. Insert, to the LIGHT0 declaration, the parameters light0_kc, light0_kl e light0_kq
used by the
illumination model to attenuate the light intensity with distance. The
following instruction should be inserted also in the inicializacao():
glLightf(GL_LIGHT0, GL_CONSTANT_ATTENUATION, light0_kc);
glLightf(GL_LIGHT0, GL_LINEAR_ATTENUATION, light0_kl);
glLightf(GL_LIGHT0, GL_QUADRATIC_ATTENUATION, light0_kq);
a)
Make
light0_kc=1.0, light0_kl=0.0 and light0_kq=0.0; comment the plane illumination obtained
(diffuse and specular illumination, vertices
illumination, general illumination aspect …), for the following situations light0_y=1, light0_y=3 e light0_y=6.
b)
Repeat the
former question with light0_kc=0.0, light0_kl=1.0 e light0_kq=0.0.
c)
Also with light0_kc=0.0, light0_kl=0.0 e light0_kq=1.0
8.
How do
you confirm that there is no shadow projections?
9.
Justify
why the plane is built with many and small polygons instead of a single polygon
like the box.
AAS/JGB/JVV