Real World and OpenGL Lighting - Introduction to Computer Graphics - Lecture Notes, Study notes for Computer Graphics. COMSATS Institute of Information Technology, Abbottabad
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taariq10 November 2012

Real World and OpenGL Lighting - Introduction to Computer Graphics - Lecture Notes, Study notes for Computer Graphics. COMSATS Institute of Information Technology, Abbottabad

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Real World and OpenGL Lighting, Normal Vectors for Each Vertex of Every Object, Material Properties for the Objects in the Scene, Creating Light Sources, Position and Attenuation are key learning points in this lecture h...
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Introduction to Computer Graphics Lecture 43

Real-World and OpenGL Lighting When we look at a physical surface, our eye's perception of the color depends on the distribution of photon energies that arrive and trigger our cone cells. Those photons come from a light source or combination of sources, some of which are absorbed and some are reflected by the surface. In addition, different surfaces may have very different properties - some are shiny and preferentially reflect light in certain directions, while others scatter incoming light equally in all directions. Most surfaces are somewhere in between. OpenGL approximates light and lighting as if light can be broken into red, green, and blue components. Thus, the color of light sources is characterized by the amount of red, green, and blue light they emit, and the material of surfaces is characterized by the percentage of the incoming red, green, and blue components that is reflected in various directions. The OpenGL lighting equations are just an approximation but one that works fairly well and can be computed relatively quickly. If we desire a more accurate (or just different) lighting model, we have to do our own calculations in software. Such software can be enormously complex, as a few hours of reading any optics textbook should convince us. In the OpenGL lighting model, the light in a scene comes from several light sources that can be individually turned on and off. Some light comes from a particular direction or position, and some light is generally scattered about the scene. For example, when we turn on a light bulb in a room, most of the light comes from the bulb, but some light comes after bouncing off one, two, three, or more walls. This bounced light (called ambient) is assumed to be so scattered that there is no way to tell its original direction, but it disappears if a particular light source is turned off. Finally, there might be a general ambient light in the scene that comes from no particular source, as if it had been scattered so many times that its original source is impossible to determine. In the OpenGL model, the light sources have an effect only when there are surfaces that absorb and reflect light. Each surface is assumed to be composed of a material with various properties. A material might emit its own light (like headlights on an automobile), it might scatter some incoming light in all directions, and it might reflect some portion of the incoming light in a preferential direction like a mirror or other shiny surface. The OpenGL lighting model considers the lighting to be divided into four independent components: emissive, ambient, diffuse and specular. All four components are computed independently and then added together.

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A Simple Example: Rendering a Lit Sphere These are the steps required to add lighting to our scene. Define NORMAL vectors for each vertex of all the objects. These NORMALS determine the orientation of the object relative to the light sources.

1. Create, select, and position one or more light sources. 2. Create and select a lighting model, which defines the level of global

ambient light and the effective location of the viewpoint (for the purposes of lighting calculations)

3. Define material properties for the objects in the scene.

Example 1 accomplishes these tasks. It displays a sphere illuminated by a single light source, as shown earlier in Figure 1. Example 1 : Drawing a Lit Sphere: #include <GL/gl.h> #include <GL/glu.h> #include <GL/glut.h> void init(void) {

GLfloat mat_specular[] = { 1.0, 1.0, 1.0, 1.0 }; GLfloat mat_shininess[] = { 50.0 }; GLfloat light_position[] = { 1.0, 1.0, 1.0, 0.0 }; glClearColor (0.0, 0.0, 0.0, 0.0); glShadeModel (GL_SMOOTH); glMaterialfv(GL_FRONT, GL_SPECULAR, mat_specular); glMaterialfv(GL_FRONT, GL_SHININESS, mat_shininess); glLightfv(GL_LIGHT0, GL_POSITION, light_position); glEnable(GL_LIGHTING); glEnable(GL_LIGHT0); glEnable(GL_DEPTH_TEST);

} void display(void) {

glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); glutSolidSphere (1.0, 20, 16); glFlush ();

} void reshape (int w, int h) {

glViewport (0, 0, (GLsizei) w, (GLsizei) h); glMatrixMode (GL_PROJECTION); glLoadIdentity(); if (w <= h)

glOrtho (-1.5, 1.5, -1.5*(GLfloat)h/(GLfloat)w, 1.5*(GLfloat)h/(GLfloat)w, -10.0, 10.0);

else

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glOrtho (-1.5*(GLfloat)w/(GLfloat)h, 1.5*(GLfloat)w/(GLfloat)h, -1.5, 1.5, -10.0, 10.0);

glMatrixMode(GL_MODELVIEW); glLoadIdentity();

} int main(int argc, char** argv) {

glutInit(&argc, argv); glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB | GLUT_DEPTH); glutInitWindowSize (500, 500); glutInitWindowPosition (100, 100); glutCreateWindow (argv[0]); init (); glutDisplayFunc(display); glutReshapeFunc(reshape); glutMainLoop(); return 0;

} The lighting-related calls are in the init() command; they're discussed briefly in the following paragraphs and in more detail later in the chapter. One thing to note about Example 1 is that it uses RGBA color mode, not color-index mode. The OpenGL lighting calculation is different for the two modes, and in fact the lighting capabilities are more limited in color-index mode. Thus, RGBA is the preferred mode when doing lighting. Define Normal Vectors for Each Vertex of Every Object An object's NORMALS determine its orientation relative to the light sources. For each vertex, OpenGL uses the assigned normal to determine how much light that particular vertex receives from each light source. In this example, the NORMALS for the sphere are defined as part of the glutSolidSphere() routine. (recall "Normal Vectors") Create, Position, and Enable One or More Light Sources Example 1 uses only one, white light source; its location is specified by the glLightfv() call. This example uses the default color for light zero (GL_LIGHT0), which is white; if we want a differently colored light, use glLight*() to indicate this. We can include at least eight different light sources in our scene of various colors; the default color of these other lights is black. (The particular implementation of OpenGL we're using might allow more than eight.) we can also locate the lights wherever we desire - we can position them near the scene, as a desk lamp would be, or an infinite distance away, like the sun. In addition, we can control whether a light produces a narrow, focused beam or a wider beam. Remember that each light source adds significantly to the calculations needed to

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render the scene, so performance is affected by the number of lights in the scene. After we've defined the characteristics of the lights we want, we have to turn them on with the glEnable() command. We also need to call glEnable() with GL_LIGHTING as a parameter to prepare OpenGL to perform lighting calculations. Select a Lighting Model As we might expect, the glLightModel*() command describes the parameters of a lighting model. In Example 1, the only element of the lighting model that's defined explicitly is the global ambient light. The lighting model also defines whether the viewer of the scene should be considered to be an infinite distance away or local to the scene, and whether lighting calculations should be performed differently for the front and back surfaces of objects in the scene. Example 1 uses the default settings for these two aspects of the model - an infinite viewer and one-sided lighting. Using a local viewer adds significantly to the complexity of the calculations that must be performed, because OpenGL must calculate the angle between the viewpoint and each object. With an infinite viewer, however, the angle is ignored, and the results are slightly less realistic. Further, since in this example, the back surface of the sphere is never seen (it's the inside of the sphere), one-sided lighting is sufficient. Define Material Properties for the Objects in the Scene An object's material properties determine how it reflects light and therefore what material it seems to be made of. Because the interaction between an object's material surface and incident light is complex, specifying material properties so that an object has a certain desired appearance is an art. We can specify a material's ambient, diffuse, and specular colors and how shiny it is. In this example, only these last two material properties - the specular material color and shininess - are explicitly specified (with the glMaterialfv() calls). Some Important Notes As we write our own lighting program, remember that we can use the default values for some lighting parameters; others need to be changed. Also, don't forget to enable whatever lights we define and to enable lighting calculations. Finally, remember that we might be able to use display lists to maximize efficiency as we change lighting conditions. Creating Light Sources Light sources have a number of properties, such as color, position, and direction. The following sections explain how to control these properties and what the resulting light looks like. The command used to specify all properties of lights is glLight*(); it takes three arguments: to identify the light whose property is being specified, the property, and the desired value for that property. void glLight{if}(GLenum light, GLenum pname, TYPEparam);

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void glLight{if}v(GLenum light, GLenum pname, TYPE *param); Creates the light specified by light, which can be GL_LIGHT0, GL_LIGHT1, ... , or GL_LIGHT7. The characteristic of the light being set is defined by pname, which specifies a named parameter (see Table 1). param indicates the values to which the pname characteristic is set; it's a pointer to a group of values if the vector version is used, or the value itself if the nonvector version is used. The nonvector version can be used to set only single-valued light characteristics. Table 1 : Default Values for pname Parameter of glLight*()

Note: The default values listed for GL_DIFFUSE and GL_SPECULAR in Table 1 apply only to GL_LIGHT(). For other lights, the default value is (0.0, 0.0, 0.0, 1.0) for both GL_DIFFUSE and GL_SPECULAR. Example 2 shows how to use glLight*(): Example 2 : Defining Colors and Position for a Light Source GLfloat light_ambient[] = { 0.0, 0.0, 0.0, 1.0 }; GLfloat light_diffuse[] = { 1.0, 1.0, 1.0, 1.0 };

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GLfloat light_specular[] = { 1.0, 1.0, 1.0, 1.0 }; GLfloat light_position[] = { 1.0, 1.0, 1.0, 0.0 }; glLightfv(GL_LIGHT0, GL_AMBIENT, light_ambient); glLightfv(GL_LIGHT0, GL_DIFFUSE, light_diffuse); glLightfv(GL_LIGHT0, GL_SPECULAR, light_specular); glLightfv(GL_LIGHT0, GL_POSITION, light_position); As we can see, arrays are defined for the parameter values, and glLightfv() is called repeatedly to set the various parameters. In this example, the first three calls to glLightfv() are superfluous, since they're being used to specify the default values for the GL_AMBIENT, GL_DIFFUSE, and GL_SPECULAR parameters. Note: Remember to turn on each light with glEnable(). All the parameters for glLight*() and their possible values are explained in the following sections. These parameters interact with those that define the overall lighting model for a particular scene and an object's material properties. Color OpenGL allows we to associate three different color-related parameters - GL_AMBIENT, GL_DIFFUSE, and GL_SPECULAR - with any particular light. The GL_AMBIENT parameter refers to the RGBA intensity of the ambient light that a particular light source adds to the scene. As we can see in Table 1, by default there is no ambient light since GL_AMBIENT is (0.0, 0.0, 0.0, 1.0). This value was used in Example 1. If this program had specified blue ambient light as GLfloat light_ambient[] = { 0.0, 0.0, 1.0, 1.0}; glLightfv(GL_LIGHT0, GL_AMBIENT, light_ambient); The GL_DIFFUSE parameter probably most closely correlates with what we naturally think of as "the color of a light." It defines the RGBA color of the diffuse light that a particular light source adds to a scene. By default, GL_DIFFUSE is (1.0, 1.0, 1.0, 1.0) for GL_LIGHT0, which produces a bright. The default value for any other light (GL_LIGHT1, ... , GL_LIGHT7) is (0.0, 0.0, 0.0, 0.0). The GL_SPECULAR parameter affects the color of the specular highlight on an object. Typically, a real-world object such as a glass bottle has a specular highlight that's the color of the light shining on it (which is often white). Therefore, if we want to create a realistic effect, set the GL_SPECULAR parameter to the same value as the GL_DIFFUSE parameter. By default, GL_SPECULAR is (1.0, 1.0, 1.0, 1.0) for GL_LIGHT0 and (0.0, 0.0, 0.0, 0.0) for any other light. Note: The alpha component of these colors is not used until blending is enabled. Position and Attenuation As previously mentioned, we can choose whether to have a light source that's treated as though it's located infinitely far away from the scene or one that's nearer to the scene. The first type is referred to as a directional light source; the effect of an infinite location is that the rays of light can be considered parallel by the time they reach an object. An example of a real-world directional light source is the sun. The second type is called a positional light source, since its exact position within the scene determines the effect it has on a scene and, specifically,

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the direction from which the light rays come. A desk lamp is an example of a positional light source. The light used in Example 1 is a directional one: GLfloat light_position[] = { 1.0, 1.0, 1.0, 0.0 }; glLightfv(GL_LIGHT0, GL_POSITION, light_position); As shown, we supply a vector of four values (x, y, z, w) for the GL_POSITION parameter. If the last value, w, is zero, the corresponding light source is a directional one, and the (x, y, z) values describe its direction. This direction is transformed by the modelview matrix. By default, GL_POSITION is (0, 0, 1, 0), which defines a directional light that points along the negative z-axis. (Note that nothing prevents we from creating a directional light with the direction of (0, 0, 0), but such a light won't help we much.) If the w value is nonzero, the light is positional, and the (x, y, z) values specify the location of the light in homogeneous object coordinates. This location is transformed by the modelview matrix and stored in eye coordinates. Also, by default, a positional light radiates in all directions, but we can restrict it to producing a cone of illumination by defining the light as a spotlight. Note: Remember that the colors across the face of a smooth-shaded polygon are determined by the colors calculated for the vertices. Because of this, we probably want to avoid using large polygons with local lights. If we locate the light near the middle of the polygon, the vertices might be too far away to receive much light, and the whole polygon will look darker than we intended. To avoid this problem, break up the large polygon into smaller ones. For real-world lights, the intensity of light decreases as distance from the light increases. Since a directional light is infinitely far away, it doesn't make sense to attenuate its intensity over distance, so attenuation is disabled for a directional light. However, we might want to attenuate the light from a positional light. OpenGL attenuates a light source by multiplying the contribution of that source by an attenuation factor:

where d = distance between the light's position and the vertex kc = GL_CONSTANT_ATTENUATION kl = GL_LINEAR_ATTENUATION kq = GL_QUADRATIC_ATTENUATION By default, kc is 1.0 and both kl and kq are zero, but we can give these parameters different values: glLightf(GL_LIGHT0, GL_CONSTANT_ATTENUATION, 2.0); glLightf(GL_LIGHT0, GL_LINEAR_ATTENUATION, 1.0); glLightf(GL_LIGHT0, GL_QUADRATIC_ATTENUATION, 0.5); Note that the ambient, diffuse, and specular contributions are all attenuated. Only the emission and global ambient values aren't attenuated. Also note that since

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attenuation requires an additional division (and possibly more math) for each calculated color, using attenuated lights may slow down application performance. Spotlights As previously mentioned, we can have a positional light source act as a spotlight - that is, by restricting the shape of the light it emits to a cone. To create a spotlight, we need to determine the spread of the cone of light we desire. (Remember that since spotlights are positional lights, we also have to locate them where we want them. Again, note that nothing prevents us from creating a directional spotlight, but it won't give us the result we want.) To specify the angle between the axis of the cone and a ray along the edge of the cone, use the GL_SPOT_CUTOFF parameter. The angle of the cone at the apex is then twice this value, as shown in Figure 2.

Figure 2 : GL_SPOT_CUTOFF Parameter Note that no light is emitted beyond the edges of the cone. By default, the spotlight feature is disabled because the GL_SPOT_CUTOFF parameter is 180.0. This value means that light is emitted in all directions (the angle at the cone's apex is 360 degrees, so it isn't a cone at all). The value for GL_SPOT_CUTOFF is restricted to being within the range [0.0,90.0] (unless it has the special value 180.0). The following line sets the cutoff parameter to 45 degrees: glLightf(GL_LIGHT0, GL_SPOT_CUTOFF, 45.0); We also need to specify a spotlight's direction, which determines the axis of the cone of light: GLfloat spot_direction[] = { -1.0, -1.0, 0.0 }; glLightfv(GL_LIGHT0, GL_SPOT_DIRECTION, spot_direction); The direction is specified in object coordinates. By default, the direction is (0.0, 0.0, -1.0), so if we don't explicitly set the value of GL_SPOT_DIRECTION, the light points down the negative z-axis. Also, keep in mind that a spotlight's

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direction is transformed by the modelview matrix just as though it were a normal vector, and the result is stored in eye coordinates. In addition to the spotlight's cutoff angle and direction, there are two ways we can control the intensity distribution of the light within the cone. First, we can set the attenuation factor described earlier, which is multiplied by the light's intensity. We can also set the GL_SPOT_EXPONENT parameter, which by default is zero, to control how concentrated the light is. The light's intensity is highest in the center of the cone. It's attenuated toward the edges of the cone by the cosine of the angle between the direction of the light and the direction from the light to the vertex being lit, raised to the power of the spot exponent. Thus, higher spot exponents result in a more focused light source. Multiple Lights As mentioned, we can have at least eight lights in our scene (possibly more, depending on our OpenGL implementation). Since OpenGL needs to perform calculations to determine how much light each vertex receives from each light source, increasing the number of lights adversely affects performance. The constants used to refer to the eight lights are GL_LIGHT0, GL_LIGHT1, GL_LIGHT2, GL_LIGHT3, and so on. In the preceding discussions, parameters related to GL_LIGHT0 were set. If we want an additional light, we need to specify its parameters; also, remember that the default values are different for these other lights than they are for GL_LIGHT0, as explained in Table 1. Example 3 defines a white attenuated spotlight. Example 3 : Second Light Source GLfloat light1_ambient[] = { 0.2, 0.2, 0.2, 1.0 }; GLfloat light1_diffuse[] = { 1.0, 1.0, 1.0, 1.0 }; GLfloat light1_specular[] = { 1.0, 1.0, 1.0, 1.0 }; GLfloat light1_position[] = { -2.0, 2.0, 1.0, 1.0 }; GLfloat spot_direction[] = { -1.0, -1.0, 0.0 }; glLightfv(GL_LIGHT1, GL_AMBIENT, light1_ambient); glLightfv(GL_LIGHT1, GL_DIFFUSE, light1_diffuse); glLightfv(GL_LIGHT1, GL_SPECULAR, light1_specular); glLightfv(GL_LIGHT1, GL_POSITION, light1_position); glLightf(GL_LIGHT1, GL_CONSTANT_ATTENUATION, 1.5); glLightf(GL_LIGHT1, GL_LINEAR_ATTENUATION, 0.5); glLightf(GL_LIGHT1, GL_QUADRATIC_ATTENUATION, 0.2); glLightf(GL_LIGHT1, GL_SPOT_CUTOFF, 45.0); glLightfv(GL_LIGHT1, GL_SPOT_DIRECTION, spot_direction); glLightf(GL_LIGHT1, GL_SPOT_EXPONENT, 2.0); glEnable(GL_LIGHT1); If these lines were added to Example 1, the sphere would be lit with two lights, one directional and one spotlight. Try This Modify Example 1 in the following manner:

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Change the first light to be a positional colored light rather than a directional white one.

Add an additional colored spotlight. Hint: Use some of the code shown in the preceding section.

Measure how these two changes affect performance. Controlling a Light's Position and Direction OpenGL treats the position and direction of a light source just as it treats the position of a geometric primitive. In other words, a light source is subject to the same matrix transformations as a primitive. More specifically, when glLight*() is called to specify the position or the direction of a light source, the position or direction is transformed by the current modelview matrix and stored in eye coordinates. This means we can manipulate a light source's position or direction by changing the contents of the modelview matrix. (The projection matrix has no effect on a light's position or direction.) This section explains how to achieve the following three different effects by changing the point in the program at which the light position is set, relative to modeling or viewing transformations:

A light position that remains fixed A light that moves around a stationary object A light that moves along with the viewpoint

Keeping the Light Stationary In the simplest example, as in Example 1, the light position remains fixed. To achieve this effect, we need to set the light position after whatever viewing and/or modeling transformation we use. In Example 4, the relevant code from the init() and reshape() routines might look like this. Example 4 : Stationary Light Source glViewport (0, 0, (GLsizei) w, (GLsizei) h); glMatrixMode (GL_PROJECTION); glLoadIdentity(); if (w <= h)

glOrtho (-1.5, 1.5, -1.5*h/w, 1.5*h/w, -10.0, 10.0); else

glOrtho (-1.5*w/h, 1.5*w/h, -1.5, 1.5, -10.0, 10.0); glMatrixMode (GL_MODELVIEW); glLoadIdentity(); /* later in init() */ GLfloat light_position[] = { 1.0, 1.0, 1.0, 1.0 }; glLightfv(GL_LIGHT0, GL_POSITION, position); As we can see, the viewport and projection matrices are established first. Then, the identity matrix is loaded as the modelview matrix, after which the light position is set. Since the identity matrix is used, the originally specified light position (1.0, 1.0, 1.0) isn't changed by being multiplied by the modelview matrix. Then, since neither the light position nor the modelview matrix is modified after this point, the direction of the light remains (1.0, 1.0, 1.0).

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Independently Moving the Light Now suppose we want to rotate or translate the light position so that the light moves relative to a stationary object. One way to do this is to set the light position after the modeling transformation, which is itself changed specifically to modify the light position. We can begin with the same series of calls in init() early in the program. Then we need to perform the desired modeling transformation (on the modelview stack) and reset the light position, probably in display(). Example 5 shows what display() might be. Example 5 : Independently Moving Light Source static GLdouble spin; void display(void) {

GLfloat light_position[] = { 0.0, 0.0, 1.5, 1.0 }; glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); glPushMatrix(); gluLookAt (0.0, 0.0, 5.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0); glPushMatrix(); glRotated(spin, 1.0, 0.0, 0.0); glLightfv(GL_LIGHT0, GL_POSITION, light_position); glPopMatrix(); glutSolidTorus (0.275, 0.85, 8, 15); glPopMatrix(); glFlush();

} spin is a global variable and is probably controlled by an input device. display() causes the scene to be redrawn with the light rotated spin degrees around a stationary torus. Note the two pairs of glPushMatrix() and glPopMatrix() calls, which are used to isolate the viewing and modeling transformations, all of which occur on the modelview stack. Since in Example 5 the viewpoint remains constant, the current matrix is pushed down the stack and then the desired viewing transformation is loaded with gluLookAt(). The matrix stack is pushed again before the modeling transformation glRotated() is specified. Then the light position is set in the new, rotated coordinate system so that the light itself appears to be rotated from its previous position. (Remember that the light position is stored in eye coordinates, which are obtained after transformation by the modelview matrix.) After the rotated matrix is popped off the stack, the torus is drawn. Example 6 is a program that rotates a light source around an object. When the left mouse button is pressed, the light position rotates an additional 30 degrees. A small, unlit, wireframe cube is drawn to represent the position of the light in the scene.

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Example 6 : Moving a Light with Modeling Transformations: #include <GL/gl.h> #include <GL/glu.h> #include "glut.h" static int spin = 0; void init(void) {

glClearColor (0.0, 0.0, 0.0, 0.0); glShadeModel (GL_SMOOTH); glEnable(GL_LIGHTING); glEnable(GL_LIGHT0); glEnable(GL_DEPTH_TEST);

} /* Here is where the light position is reset after the modeling * transformation (glRotated) is called. This places the * light at a new position in world coordinates. The cube * represents the position of the light. */ void display(void) {

GLfloat position[] = { 0.0, 0.0, 1.5, 1.0 }; glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); glPushMatrix (); glTranslatef (0.0, 0.0, -5.0); glPushMatrix (); glRotated ((GLdouble) spin, 1.0, 0.0, 0.0); glLightfv (GL_LIGHT0, GL_POSITION, position); glTranslated (0.0, 0.0, 1.5); glDisable (GL_LIGHTING); glColor3f (0.0, 1.0, 1.0); glutWireCube (0.1); glEnable (GL_LIGHTING); glPopMatrix (); glutSolidTorus (0.275, 0.85, 8, 15); glPopMatrix (); glFlush ();

} void reshape (int w, int h) {

glViewport (0, 0, (GLsizei) w, (GLsizei) h); glMatrixMode (GL_PROJECTION); glLoadIdentity(); gluPerspective(40.0, (GLfloat) w/(GLfloat) h, 1.0, 20.0); glMatrixMode(GL_MODELVIEW); glLoadIdentity();

}

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void mouse(int button, int state, int x, int y) {

switch (button) { case GLUT_LEFT_BUTTON:

if (state == GLUT_DOWN) { spin = (spin + 30) % 360; glutPostRedisplay();

} break;

default: break;

} } int main(int argc, char** argv) {

glutInit(&argc, argv); glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB | GLUT_DEPTH); glutInitWindowSize (500, 500); glutInitWindowPosition (100, 100); glutCreateWindow (argv[0]); init (); glutDisplayFunc(display); glutReshapeFunc(reshape); glutMouseFunc(mouse); glutMainLoop(); return 0;

} Selecting a Lighting Model The OpenGL notion of a lighting model has three components:

The global ambient light intensity Whether the viewpoint position is local to the scene or whether it should

be considered to be an infinite distance away Whether lighting calculations should be performed differently for both the

front and back faces of objects This section explains how to specify a lighting model. It also discusses how to enable lighting - that is, how to tell OpenGL that we want lighting calculations performed. The command used to specify all properties of the lighting model is glLightModel*(). glLightModel*() has two arguments: the lighting model property and the desired value for that property. void glLightModel{if}(GLenum pname, TYPEparam);

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void glLightModel{if}v(GLenum pname, TYPE *param); Sets properties of the lighting model. The characteristic of the lighting model being set is defined by pname, which specifies a named parameter (see Table 2). param indicates the values to which the pname characteristic is set; it's a pointer to a group of values if the vector version is used, or the value itself if the nonvector version is used. The nonvector version can be used to set only single-valued lighting model characteristics, not for GL_LIGHT_MODEL_AMBIENT. Table 2 : Default Values for pname Parameter of glLightModel*()

Global Ambient Light As discussed earlier, each light source can contribute ambient light to a scene. In addition, there can be other ambient light that's not from any particular source. To specify the RGBA intensity of such global ambient light, use the GL_LIGHT_MODEL_AMBIENT parameter as follows: GLfloat lmodel_ambient[] = { 0.2, 0.2, 0.2, 1.0 }; glLightModelfv(GL_LIGHT_MODEL_AMBIENT, lmodel_ambient); In this example, the values used for lmodel_ambient are the default values for GL_LIGHT_MODEL_AMBIENT. Since these numbers yield a small amount of white ambient light, even if we don't add a specific light source to our scene, we can still see the objects in the scene. Enabling Lighting With OpenGL, we need to explicitly enable (or disable) lighting. If lighting isn't enabled, the current color is simply mapped onto the current vertex, and no calculations concerning normals, light sources, the lighting model, and material properties are performed. Here's how to enable lighting: glEnable(GL_LIGHTING); To disable lighting, call glDisable() with GL_LIGHTING as the argument. We also need to explicitly enable each light source that we define, after we've specified the parameters for that source. Example 1 uses only one light, GL_LIGHT0: glEnable(GL_LIGHT0); Defining Material Properties We've seen how to create light sources with certain characteristics and how to define the desired lighting model. This section describes how to define the

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material properties of the objects in the scene: the ambient, diffuse, and specular colors, the shininess, and the color of any emitted light. Most of the material properties are conceptually similar to ones we've already used to create light sources. The mechanism for setting them is similar, except that the command used is called glMaterial*(). void glMaterial{if}(GLenum face, GLenum pname, TYPEparam); void glMaterial{if}v(GLenum face, GLenum pname, TYPE *param); Specifies a current material property for use in lighting calculations. face can be GL_FRONT, GL_BACK, or GL_FRONT_AND_BACK to indicate which face of the object the material should be applied to. The particular material property being set is identified by pname and the desired values for that property are given by param, which is either a pointer to a group of values (if the vector version is used) or the actual value (if the nonvector version is used). The nonvector version works only for setting GL_SHININESS. The possible values for pname are shown in Table 3. Note that GL_AMBIENT_AND_DIFFUSE allows we to set both the ambient and diffuse material colors simultaneously to the same RGBA value. Table 3 : Default Values for pname Parameter of glMaterial*()

As discussed in "Selecting a Lighting Model," we can choose to have lighting calculations performed differently for the front- and back-facing polygons of objects. If the back faces might indeed be seen, we can supply different material properties for the front and the back surfaces by using the face parameter of glMaterial*(). Note that most of the material properties set with glMaterial*() are (R, G, B, A) colors. Regardless of what alpha values are supplied for other parameters, the alpha value at any particular vertex is the diffuse-material alpha value (that is, the

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alpha value given to GL_DIFFUSE with the glMaterial*() command, as described in the next section). Also, none of the RGBA material properties apply in color- index mode. Diffuse and Ambient Reflection The GL_DIFFUSE and GL_AMBIENT parameters set with glMaterial*() affect the color of the diffuse and ambient light reflected by an object. Diffuse reflectance plays the most important role in determining what we perceive the color of an object to be. It's affected by the color of the incident diffuse light and the angle of the incident light relative to the normal direction. (It's most intense where the incident light falls perpendicular to the surface.) The position of the viewpoint doesn't affect diffuse reflectance at all. Ambient reflectance affects the overall color of the object. Because diffuse reflectance is brightest where an object is directly illuminated, ambient reflectance is most noticeable where an object receives no direct illumination. An object's total ambient reflectance is affected by the global ambient light and ambient light from individual light sources. Like diffuse reflectance, ambient reflectance isn't affected by the position of the viewpoint. For real-world objects, diffuse and ambient reflectance are normally the same color. For this reason, OpenGL provides we with a convenient way of assigning the same value to both simultaneously with glMaterial*(): GLfloat mat_amb_diff[] = { 0.1, 0.5, 0.8, 1.0 }; glMaterialfv(GL_FRONT_AND_BACK, GL_AMBIENT_AND_DIFFUSE, mat_amb_diff); In this example, the RGBA color (0.1, 0.5, 0.8, 1.0) - a deep blue color - represents the current ambient and diffuse reflectance for both the front- and back-facing polygons. Specular Reflection Specular reflection from an object produces highlights. Unlike ambient and diffuse reflection, the amount of specular reflection seen by a viewer does depend on the location of the viewpoint - it's brightest along the direct angle of reflection. To see this, imagine looking at a metallic ball outdoors in the sunlight. As we move our head, the highlight created by the sunlight moves with us to some extent. However, if we move our head too much, we lose the highlight entirely. OpenGL allows us to set the effect that the material has on reflected light (with GL_SPECULAR) and control the size and brightness of the highlight (with GL_SHININESS). We can assign a number in the range of [0.0, 128.0] to GL_SHININESS - the higher the value, the smaller and brighter (more focused) the highlight.

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Twelve spheres, each with different material parameters. The row properties are as labeled above. The first column uses a blue diffuse material color with no specular properties. The second column adds white specular reflection with a low shininess exponent. The third column uses a high shininess exponent and thus has a more concentrated highlight. The fourth column uses the blue diffuse color and, instead of specular reflection, adds an emissive component. In above figure, the spheres in the first column have no specular reflection. In the second column, GL_SPECULAR and GL_SHININESS are assigned values as follows: GLfloat mat_specular[] = { 1.0, 1.0, 1.0, 1.0 }; GLfloat low_shininess[] = { 5.0 }; glMaterialfv(GL_FRONT, GL_SPECULAR, mat_specular); glMaterialfv(GL_FRONT, GL_SHININESS, low_shininess); In the third column, the GL_SHININESS parameter is increased to 100.0. Emission By specifying an RGBA color for GL_EMISSION, we can make an object appear to be giving off light of that color. Since most real-world objects (except lights) don't emit light, we'll probably use this feature mostly to simulate lamps and other light sources in a scene. GLfloat mat_emission[] = {0.3, 0.2, 0.2, 0.0}; glMaterialfv(GL_FRONT, GL_EMISSION, mat_emission); Notice that the spheres appear to be slightly glowing; however, they're not actually acting as light sources. We would need to create a light source and position it at the same location as the sphere to create that effect. Changing Material Properties

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Example 1 uses the same material properties for all vertices of the only object in the scene (the sphere). In other situations, we might want to assign different material properties for different vertices on the same object. More likely, we have more than one object in the scene, and each object has different material properties. For example, the code that produced "above figure” has to draw twelve different objects (all spheres), each with different material properties. Example 8 shows a portion of the code in display(). Example 8 : Different Material Properties: GLfloat no_mat[] = { 0.0, 0.0, 0.0, 1.0 }; GLfloat mat_ambient[] = { 0.7, 0.7, 0.7, 1.0 }; GLfloat mat_ambient_color[] = { 0.8, 0.8, 0.2, 1.0 }; GLfloat mat_diffuse[] = { 0.1, 0.5, 0.8, 1.0 }; GLfloat mat_specular[] = { 1.0, 1.0, 1.0, 1.0 }; GLfloat no_shininess[] = { 0.0 }; GLfloat low_shininess[] = { 5.0 }; GLfloat high_shininess[] = { 100.0 }; GLfloat mat_emission[] = {0.3, 0.2, 0.2, 0.0}; glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); /* draw sphere in first row, first column * diffuse reflection only; no ambient or specular */ glPushMatrix(); glTranslatef (-3.75, 3.0, 0.0); glMaterialfv(GL_FRONT, GL_AMBIENT, no_mat); glMaterialfv(GL_FRONT, GL_DIFFUSE, mat_diffuse); glMaterialfv(GL_FRONT, GL_SPECULAR, no_mat); glMaterialfv(GL_FRONT, GL_SHININESS, no_shininess); glMaterialfv(GL_FRONT, GL_EMISSION, no_mat); glutSolidSphere(1.0, 16, 16); glPopMatrix(); /* draw sphere in first row, second column * diffuse and specular reflection; low shininess; no ambient */ glPushMatrix(); glTranslatef (-1.25, 3.0, 0.0); glMaterialfv(GL_FRONT, GL_AMBIENT, no_mat); glMaterialfv(GL_FRONT, GL_DIFFUSE, mat_diffuse); glMaterialfv(GL_FRONT, GL_SPECULAR, mat_specular); glMaterialfv(GL_FRONT, GL_SHININESS, low_shininess); glMaterialfv(GL_FRONT, GL_EMISSION, no_mat); glutSolidSphere(1.0, 16, 16); glPopMatrix(); /* draw sphere in first row, third column * diffuse and specular reflection; high shininess; no ambient */

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glPushMatrix(); glTranslatef (1.25, 3.0, 0.0); glMaterialfv(GL_FRONT, GL_AMBIENT, no_mat); glMaterialfv(GL_FRONT, GL_DIFFUSE, mat_diffuse); glMaterialfv(GL_FRONT, GL_SPECULAR, mat_specular); glMaterialfv(GL_FRONT, GL_SHININESS, high_shininess); glMaterialfv(GL_FRONT, GL_EMISSION, no_mat); glutSolidSphere(1.0, 16, 16); glPopMatrix(); /* draw sphere in first row, fourth column * diffuse reflection; emission; no ambient or specular refl. */ glPushMatrix(); glTranslatef (3.75, 3.0, 0.0); glMaterialfv(GL_FRONT, GL_AMBIENT, no_mat); glMaterialfv(GL_FRONT, GL_DIFFUSE, mat_diffuse); glMaterialfv(GL_FRONT, GL_SPECULAR, no_mat); glMaterialfv(GL_FRONT, GL_SHININESS, no_shininess); glMaterialfv(GL_FRONT, GL_EMISSION, mat_emission); glutSolidSphere(1.0, 16, 16); glPopMatrix(); As we can see, glMaterialfv() is called repeatedly to set the desired material property for each sphere. Note that it only needs to be called to change a property that needs to be respecified. The second, third, and fourth spheres use the same ambient and diffuse properties as the first sphere, so these properties do not need to be respecified. Since glMaterial*() has a performance cost associated with its use, Example 8 could be rewritten to minimize material- property changes. Another technique for minimizing performance costs associated with changing material properties is to use glColorMaterial(). void glColorMaterial(GLenum face, GLenum mode); Causes the material property (or properties) specified by mode of the specified material face (or faces) specified by face to track the value of the current color at all times. A change to the current color (using glColor*()) immediately updates the specified material properties. The face parameter can beGL_FRONT, GL_BACK, or GL_FRONT_AND_BACK (the default). The mode parameter can be GL_AMBIENT, GL_DIFFUSE, GL_AMBIENT_AND_DIFFUSE (the default), GL_SPECULAR, or GL_EMISSION. At any given time, only one mode is active. glColorMaterial() has no effect on color-index lighting. Note that glColorMaterial() specifies two independent values: the first specifies which face or faces are updated, and the second specifies which material property or properties of those faces are updated. OpenGL does not maintain separate mode variables for each face. After calling glColorMaterial(), we need

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to call glEnable() with GL_COLOR_MATERIAL as the parameter. Then, we can change the current color using glColor*() (or other material properties, using glMaterial*()) as needed as we draw: glEnable(GL_COLOR_MATERIAL); glColorMaterial(GL_FRONT, GL_DIFFUSE); /* now glColor* changes diffuse reflection */ glColor3f(0.2, 0.5, 0.8); /* draw some objects here */ glColorMaterial(GL_FRONT, GL_SPECULAR); /* glColor* no longer changes diffuse reflection */ /* now glColor* changes specular reflection */ glColor3f(0.9, 0.0, 0.2); /* draw other objects here */ glDisable(GL_COLOR_MATERIAL); We should use glColorMaterial() whenever we need to change a single material parameter for most vertices in our scene. If we need to change more than one material parameter, as was the case for "in above figure”, use glMaterial*(). When we don't need the capabilities of glColorMaterial() anymore, be sure to disable it so that we don't get undesired material properties and don't incur the performance cost associated with it. The performance value in using glColorMaterial() varies, depending on our OpenGL implementation. Some implementations may be able to optimize the vertex routines so that they can quickly update material properties based on the current color. Example 9 shows an interactive program that uses glColorMaterial() to change material parameters. Pressing each of the three mouse buttons changes the color of the diffuse reflection. Example 9 : Using glColorMaterial(): #include <GL/gl.h> #include <GL/glu.h> #include "glut.h" GLfloat diffuseMaterial[4] = { 0.5, 0.5, 0.5, 1.0 }; void init(void) {

GLfloat mat_specular[] = { 1.0, 1.0, 1.0, 1.0 }; GLfloat light_position[] = { 1.0, 1.0, 1.0, 0.0 }; glClearColor (0.0, 0.0, 0.0, 0.0); glShadeModel (GL_SMOOTH); glEnable(GL_DEPTH_TEST); glMaterialfv(GL_FRONT, GL_DIFFUSE, diffuseMaterial); glMaterialfv(GL_FRONT, GL_SPECULAR, mat_specular); glMaterialf(GL_FRONT, GL_SHININESS, 25.0); glLightfv(GL_LIGHT0, GL_POSITION, light_position); glEnable(GL_LIGHTING);

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glEnable(GL_LIGHT0); glColorMaterial(GL_FRONT, GL_DIFFUSE); glEnable(GL_COLOR_MATERIAL);

} void display(void) {

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); glutSolidSphere(1.0, 20, 16); glFlush ();

} void reshape (int w, int h) {

glViewport (0, 0, (GLsizei) w, (GLsizei) h); glMatrixMode (GL_PROJECTION); glLoadIdentity(); if (w <= h)

glOrtho (-1.5, 1.5, -1.5*(GLfloat)h/(GLfloat)w, 1.5*(GLfloat)h/(GLfloat)w, -10.0, 10.0);

else glOrtho (-1.5*(GLfloat)w/(GLfloat)h, 1.5*(GLfloat)w/(GLfloat)h, -1.5, 1.5, -10.0, 10.0);

glMatrixMode(GL_MODELVIEW); glLoadIdentity();

} void mouse(int button, int state, int x, int y) {

switch (button) { case GLUT_LEFT_BUTTON:

if (state == GLUT_DOWN) { /* change red */ diffuseMaterial[0] += 0.1; if (diffuseMaterial[0] > 1.0)

diffuseMaterial[0] = 0.0; glColor4fv(diffuseMaterial); glutPostRedisplay(); } break;

case GLUT_MIDDLE_BUTTON: if (state == GLUT_DOWN) { /* change green */

diffuseMaterial[1] += 0.1; if (diffuseMaterial[1] > 1.0)

diffuseMaterial[1] = 0.0; glColor4fv(diffuseMaterial); glutPostRedisplay(); } break;

case GLUT_RIGHT_BUTTON:

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if (state == GLUT_DOWN) { /* change blue */ diffuseMaterial[2] += 0.1; if (diffuseMaterial[2] > 1.0)

diffuseMaterial[2] = 0.0; glColor4fv(diffuseMaterial); glutPostRedisplay();

} break;

default: break;

} } int main(int argc, char** argv) {

glutInit(&argc, argv); glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB | GLUT_DEPTH); glutInitWindowSize (500, 500); glutInitWindowPosition (100, 100); glutCreateWindow (argv[0]); init (); glutDisplayFunc(display); glutReshapeFunc(reshape); glutMouseFunc(mouse); glutMainLoop(); return 0;

} Try This Modify Example 8 in the following manner:

Change the global ambient light in the scene. Hint: Alter the value of the GL_LIGHT_MODEL_AMBIENT parameter.

Change the diffuse, ambient, and specular reflection parameters, the shininess exponent, and the emission color. Hint: Use the glMaterial*() command, but avoid making excessive calls.

Use two-sided materials and add a user-defined clipping plane so that we

can see the inside and outside of a row or column of spheres. if we need to recall user-defined clipping planes.) Hint: Turn on two-sided lighting with GL_LIGHT_MODEL_TWO_SIDE, set the desired material properties, and add a clipping plane.

Remove all the glMaterialfv() calls, and use the more efficient glColorMaterial() calls to achieve the same lighting.

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