1. 光照模型     

环境光——经过多次反射而来的光称为环境光，无法确定其最初的方向，但当特定的光源关闭后，它们将消失.

全局环境光——每个光源都能对场景提供环境光。此外，还有一个环境光，它不来自任何特定的光源，称之为全局环境光。这也就是为什么我们不加任何自定义光源，也能看见绘制的物体的原因。
散射光——来自同一方向，照射到物体表面后，将沿各个方向均匀反射，因此，无论从哪个方向观察，表面的亮度都相同. 主要通过设置GLfloat light_diffuse[] = { 0.0, 1.0, 0.0, 1.0 }来发射绿光
镜面反射光——来自特定方向，也被反射到特定方向.镜面反射度与之相关. 
材质发射光——用于模拟发光物体.在OpenGL光照模型中，表面的发射光增加了物体的亮度，它不受光源的影响，另外，发射光不会给整个场景中增加光线.  

当光照射到一个物体表面上时，会出现三种情形。首先，光可以通过物体表面向空间反射， 产生反射光。其次，对于透明体，光可以穿透该物体并从另一端射出，产生透射光。最后，部分光将被物体表面吸收而转换成热。在上述三部分光中，仅仅是透射光和反射光能够进入人眼产生视觉效果。这里介绍的简单光照模型只考虑被照明物体表面的反射光影响，假定物体表面光滑不透明且由理想材料构成，环境假设为由白光照明。

一般来说，反射光可以分成三个分量，即环境反射、漫反射和镜面反射。环境反射分量假定入射光均匀地从周围环境入射至景物表面并等量地向各个方向反射出去，通常物体表面还会受到从周围环境来的反射光（如来自地面、天空、墙壁等的反射光）的照射，这些光常统称为环境光(Ambient Light)；漫反射分量表示特定光源在景物表面的反射光中那些向空间各方向均匀反射出去的光，这些光常称为漫射光(Diffuse Light)；镜面反射光为朝一定方向的反射光，如一个点光源照射一个金属球时会在球面上形成一块特别亮的区域，呈现所谓“高光(Highlight)”，它是光源在金属球面上产生的镜面反射光(Specular Light)。对于较光滑物体，其镜面反射光的高光区域小而亮；相反，粗糙表面的镜面反射光呈发散状态，其高光区域大而不亮。

例1. 光照GL_LIGHT0的使用，绿光

#include <GL/glut.h>
#include <stdlib.h>

/*  Initialize material property, light source, lighting model,
 *  and depth buffer.
 */
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 }; //最后一个参数为0表示该光源是directional的
 //  GLfloat light_position[] = { 1.0, 1.0, 1.0, 1.0 }; //最后一个参数非0表示该光源是positional的

   GLfloat light_ambient[] = { 0.0, 1.0, 0.0, 1.0 };
   GLfloat light_diffuse[] = { 0.0, 1.0, 0.0, 1.0 };
   GLfloat light_specular[] = { 0.0, 1.0, 0.0, 1.0 };

 //  GLfloat Light_Model_Ambient[] = { 0.0 , 0.0 , 0.0 , 1.0 }; //全局环境光设为黑色
   GLfloat Light_Model_Ambient[] = { 0.2 , 0.2 , 0.2 , 1.0 }; //默认的全局环境光
   glLightModelfv( GL_LIGHT_MODEL_AMBIENT , Light_Model_Ambient ); //设置全局环境光的方法

   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_AMBIENT, light_ambient);
   glLightfv(GL_LIGHT0, GL_DIFFUSE, light_diffuse);
   glLightfv(GL_LIGHT0, GL_SPECULAR, light_specular);
   glLightfv(GL_LIGHT0, GL_POSITION, light_position);

   glEnable(GL_LIGHTING);
   glEnable(GL_LIGHT0); //把全局环境光设置为黑色,然后注释掉本句可以发现"黑屏"
   glDepthFunc(GL_LESS); //glDepthFunc()来设置深度测试函数
   glEnable(GL_DEPTH_TEST);

/*The default depth-comparison function is GL_LESS, which draws a pixel only if its z value is less than the z value in the depth buffer

*/
}
 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 keyboard(unsigned char key, int x, int y)
{
 switch (key) {
 case 27:
  exit(0);
  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);
 glutKeyboardFunc(keyboard);
 glutMainLoop();
 return 0;
}

例2. 材质的使用

#include "windows.h"
#include <GL/gl.h>
#include <GL/glaux.h>
 
void myinit(void);
void CALLBACK myReshape(GLsizei w, GLsizei h);
void CALLBACK display(void);
 
void myinit(void)
{
   /* 设置材质的各种光的颜色成分反射比率*/
    GLfloat mat_ambient[]={0.8,0.8,0.8,1.0};
    GLfloat mat_diffuse[]={0.8,0.0,0.8,1.0};    /* 紫色 */
    GLfloat mat_specular[] = { 1.0, 0.0, 1.0, 1.0 };  /* 亮紫色 */
    GLfloat mat_shininess[] = { 50.0 };
 
    GLfloat light_position[] = { 1.0, 1.0, 1.0, 0.0 };
 
    glMaterialfv(GL_FRONT, GL_AMBIENT, mat_ambient);
    glMaterialfv(GL_FRONT, GL_DIFFUSE, mat_diffuse);
    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);
    glDepthFunc(GL_LESS);
    glEnable(GL_DEPTH_TEST);
}
 
void CALLBACK display(void)
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    auxSolidSphere(1.0);
    glFlush();
}
 
void CALLBACK myReshape(GLsizei w, GLsizei h)
{
    glViewport(0, 0, w, 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 main(void)
{
    auxInitDisplayMode (AUX_SINGLE | AUX_RGBA | AUX_DEPTH16);
    auxInitPosition (0, 0, 500, 500);
    auxInitWindow ("Lighting_1 ");
    myinit();
    auxReshapeFunc (myReshape);
    auxMainLoop(display);
}

例3. 光源的移动

#include "windows.h"
#include <GL/gl.h>
#include <GL/glu.h>
#include <GL/glaux.h>
 
void myinit(void);
void CALLBACK movelight (AUX_EVENTREC *event);
void CALLBACK display(void);
void CALLBACK myReshape(GLsizei w, GLsizei h);
 
static int step = 0;
 
void CALLBACK movelight (AUX_EVENTREC *event)
{
    step = (step + 15) % 360;
}
 
void myinit (void)
{
    GLfloat mat_diffuse[]={0.0,0.5,1.0,1.0};
    GLfloat mat_ambient[]={0.0,0.2,1.0,1.0};
    GLfloat light_diffuse[]={1.0,1.0,1.0,1.0};
    GLfloat light_ambient[]={0.0,0.5,0.5,1.0};
 
    glMaterialfv(GL_FRONT_AND_BACK,GL_DIFFUSE,mat_diffuse);
    glLightfv(GL_LIGHT0,GL_DIFFUSE,light_diffuse);
    glLightfv(GL_LIGHT0,GL_AMBIENT,light_ambient);
    glEnable(GL_LIGHTING);
    glEnable(GL_LIGHT0);
 
    glDepthFunc(GL_LESS);
    glEnable(GL_DEPTH_TEST);
 
 }
 
void CALLBACK 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);  //  或gluLookAt (0.0, 0.0, 5.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0); 
 
    glPushMatrix ();
    glRotated ((GLdouble) step, -1.0, 1.0, 1.0);
    glRotated (0.0, 1.0, 0.0, 0.0);
    glLightfv (GL_LIGHT0, GL_POSITION, position); //光源的本质是几何物体
     glTranslated (0.0, 0.0, 1.5);
    glDisable (GL_LIGHTING);
    glColor3f (1.0, 1.0, 0.0);

    auxWireSphere (0.1);
    glEnable (GL_LIGHTING);
    glPopMatrix ();
 
    auxSolidTorus (0.275, 0.85);
    glPopMatrix ();
    glFlush ();
}
 
void CALLBACK myReshape(GLsizei w, GLsizei h)
{
    glViewport(0, 0, w, h);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective(40.0, (GLfloat) w/(GLfloat) h, 1.0, 20.0);
    glMatrixMode(GL_MODELVIEW);
}
 
void main(void)
{
    auxInitDisplayMode (AUX_SINGLE | AUX_RGBA);
    auxInitPosition (0, 0, 500, 500);
    auxInitWindow ("Moving Light");
    myinit();
    auxMouseFunc (AUX_LEFTBUTTON, AUX_MOUSEDOWN, movelight);
    auxReshapeFunc (myReshape);
    auxMainLoop(display);
}

例4 光源的移动-glut

/*
 *  movelight.c
 *  This program demonstrates when to issue lighting and
 *  transformation commands to render a model with a light
 *  which is moved by a modeling transformation (rotate or
 *  translate).  The light position is reset after the modeling
 *  transformation is called.  The eye position does not change.
 *
 *  A sphere is drawn using a grey material characteristic.
 *  A single light source illuminates the object.
 *
 *  Interaction:  pressing the left mouse button alters
 *  the modeling transformation (x rotation) by 30 degrees.
 *  The scene is then redrawn with the light in a new position.
 */

#include <GL/glut.h>
#include <stdlib.h>

static int spin = 0;

/*  Initialize material property, light source, lighting model,
 *  and depth buffer.
 */
void init(void) 
{
   glClearColor (0.0, 0.0, 0.0, 0.0);
   glShadeModel (GL_SMOOTH);
   glEnable(GL_LIGHTING);
   glEnable(GL_LIGHT0);  //这里使用了LIGHT0的默认值

 
   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 ();
 gluLookAt (0.0, 0.0, 5.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);  // 效果和glTranslatef (0.0, 0.0, -5.0);一样
 
 glPushMatrix (); // 光源与WireCube模型

 glRotated ((GLdouble) spin, 1.0, 0.0, 0.0); //同时旋转光源以及WireCube
 glLightfv (GL_LIGHT0, GL_POSITION, position);   //光源是几何物体，通过ModelView变换可以移动旋转它

 glTranslated (0.0, 0.0, 1.5);
 glDisable (GL_LIGHTING);
 glColor3f (0.0, 1.0, 1.0);
 glutWireCube (0.1);  // WireCube模拟代表光源的位置

 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();
}

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;
 }
}

void keyboard(unsigned char key, int x, int y)
{
 switch (key) {
 case 27:
  exit(0);
  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);
 glutKeyboardFunc(keyboard);
 glutMainLoop();
 return 0;
}

openGL光源概念