opengl 光照资料

OpenGL - Lighting (光照)

Independently Moving the Light

Now suppose you 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. You can begin with the same series of calls in init() early in the program. Then you need to perform the desired modeling transformation (on the modelview stack) and reset the light position, probably in display(). Example 5-5 shows what display() might be.

Example 5-5 : 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();

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 ();
}

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 you 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 you 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);

Chapter 5 Lighting