OpenGL光照

基本光照模型

OpenGL中有四种光照模型

• 放射光（自发光）    $emissive$
• 环境光     $ambient$
• 漫反射光      $diffuse$
• 镜面反射光    $specular$

基本光照模型
  $SurfaceColor=emissive+ambient+diffuse+specular$
表面颜色 = 放射光+环境光+漫反射光+镜面反射光

下面我将以一个标准球体来演示各个光照的效果

1.放射光

$emissive=k_e$

• $k_e$材质的放射系数(颜色)
  
  上图为 $k_e=(0,0.2,0.1,1)$

	GLfloat green[] = { 0.0f,0.2f,0.1f,1.0f };
	glMaterialfv(GL_FRONT, GL_EMISSION, green);

2.环境光

-$ambient=k_a\ast GlobalAmbient$

• $k_a$材质的环境光反射系数
• $GlobalAmbient$ 全局环境光颜色
  
  $k_a=(0.5,0.5,0.5,1)$
  $GlobalAmbient=(0.3,0.3,0.3,1.0)$

	GLfloat gray[] = {0.5f, 0.5f, 0.5f, 1.0f};
	glMaterialfv(GL_FRONT, GL_AMBIENT, gray);

3.漫反射光

$diffuse=k_d\ast LightColor\ast max(N\cdot L,0)$

• $k_a$ 材质漫反射系数
• $LightColor$ 漫反射光照颜色
• $N$ 规范化的表面法向量
• $L$ 规范化的指向光源的向量
  
  $k_d=(0.7,0.6,0.1,1)$
  $LightColor=(0.7,0.7,0.7,1)$

	GLfloat yellow[] = { 0.7f,0.6f,0.1f,1.0f };
	glMaterialfv(GL_FRONT, GL_DIFFUSE , yellow);

4.镜面反射光

$specular=k_s\ast LightColor\ast facing\ast (max(N\cdot H,0){)}^{shininess}$

• $k_s$材质的镜面反射系数
• $LightColor$ 镜面反射光颜色
• $N$ 规范化表面法向量
• $V$规范化指向视点向量
• $H$规范化$N$和$V$的中间向量
• $facing$ 为1若$N\cdot L>0$ ，否则为0
• $shininess$ 材质的光泽系数
  
  $k_s=(1,1,1,1)$
  $LightColor=(0.5,0.5,0.5,1)$
  $shininess=10$

	GLfloat white[] = { 1.0f,1.0f,1.0f,1.0f };
	glMaterialfv(GL_FRONT, GL_SPECULAR, white);
	glMaterialf(GL_FRONT, GL_SHININESS, 10.0f);

光源类型

OpenGL中光源分为三类

• 平行光
• 点光源
• 聚光灯

1.平行光

$LightPosition=(1,0,0,1)$
注意第四项为1，代表一个向量(1,0,0)的平行光源

	glEnable(GL_LIGHTING);
	glEnable(GL_LIGHT0);

	GLfloat ambient[] = { .3f,.3f,.3f,1.f };
	GLfloat diffuse[] = { .7f,.7f,.7f,1.f };
	GLfloat specular[] = { .5f,.5f,.5f,1.f };

	glLightfv(GL_LIGHT0, GL_AMBIENT, ambient);
	glLightfv(GL_LIGHT0, GL_DIFFUSE, diffuse);
	glLightfv(GL_LIGHT0, GL_SPECULAR, specular);

	GLfloat position[] = { 1.0f,0.0f,0.f,1.f };
	glLightfv(GL_LIGHT0, GL_POSITION, position);

其中衰减系数
$attenuation=\frac{1}{A_0+A_1\ast D+A_2\ast D^2}$
$A_0$ 为常量系数
$A_1$ 为线性系数
$A_2$ 为平方系数
$D$ 为光源到目标距离

	glLightf(GL_LIGHT0, GL_CONSTANT_ATTENUATION, 1);
	glLightf(GL_LIGHT0, GL_LINEAR_ATTENUATION, 0);
	glLightf(GL_LIGHT0, GL_QUADRATIC_ATTENUATION, 0);

2.点光源

$LightPosition=(1,0,0,0)$
注意第四项为0，代表一个位置在(1,0,0)处的点光源

3.聚光灯

代码

球体绘制

void sphere_entry(int du, int dv){

	float gu = 3.1415926535 / du , gv = 2.0*3.1415926535 / (dv-1);

	float x, y, z ,r ;
	float p1[3], p2[3],p3[3],p4[3],v1[3],v2[3],v3[3],a;

	glPushMatrix();
#define MAKE_R(z2) sqrt(1.0 - (z2))
#define MAKE_Z(z_step) cos((z_step) *gu)
#define MAKE_Y(x_step,r) (r)*sin((x_step)* gv)
#define MAKE_X(x_step,r) (r)*cos((x_step)* gv)
	// 球体
	glBegin(GL_QUADS);
	for (int z_step = 0 ; z_step < du; ++z_step) {
		for (int x_step = 0; x_step < dv; ++x_step) {

			z = MAKE_Z(z_step);
			r = MAKE_R(z*z);
			x = MAKE_X(x_step, r);
			y = MAKE_Y(x_step, r);
			p1[0] = x;
			p1[1] = y;
			p1[2] = z;

			z = MAKE_Z(z_step+1);
			r = MAKE_R(z*z);
			x = MAKE_X(x_step, r);
			y = MAKE_Y(x_step, r);
			p2[0] = x;
			p2[1] = y;
			p2[2] = z;

			z = MAKE_Z(z_step + 1);
			r = MAKE_R(z*z);
			x = MAKE_X(x_step + 1,r);
			y = MAKE_Y(x_step+1, r);
			p3[0] = x;
			p3[1] = y;
			p3[2] = z;

			z = MAKE_Z(z_step);
			r = MAKE_R(z*z);
			x = MAKE_X(x_step + 1,r);
			y = MAKE_Y(x_step + 1, r);
			p4[0] = x;
			p4[1] = y;
			p4[2] = z;

			v1[0] = p2[0] - p1[0];
			v1[1] = p2[1] - p1[1];
			v1[2] = p2[2] - p1[2];
			 
			// 极点矫正
			if (z_step == du - 1) {
				v2[0] = p4[0] - p2[0];
				v2[1] = p4[1] - p2[1];
				v2[2] = p4[2] - p2[2];
			}
			else {
				v2[0] = p3[0] - p2[0];
				v2[1] = p3[1] - p2[1];
				v2[2] = p3[2] - p2[2];
			}

			// 向量积
			v3[0] = (v1[1] * v2[2]) - (v2[1] * v1[2]);
			v3[1] = (v1[2] * v2[0]) - (v2[2] * v1[0]);
			v3[2] = (v1[0] * v2[1]) - (v2[0] * v1[1]);

			// 单位化
			a = 1.0/ sqrt(v3[0] * v3[0] + v3[1] * v3[1] + v3[2] * v3[2]);

			glNormal3f(v3[0] * a, v3[1] * a, v3[2] * a);

			glVertex3f(p1[0], p1[1], p1[2]);
			glVertex3f(p2[0], p2[1], p2[2]);
			glVertex3f(p3[0], p3[1], p3[2]);
			glVertex3f(p4[0], p4[1], p4[2]);

		}

	}

	glEnd();
#undef MAKE_Y
#undef MAKE_Z
#undef MAKE_X
#undef MAKE_R
	glPopMatrix();
}

渲染

float angle;

void changed_size(int w, int h) {
	// 修改视窗size
	glViewport(0, 0, w, h);

	glMatrixMode(GL_PROJECTION);
	glLoadIdentity();

	gluPerspective(45.0f, double(w) / double(h), 1.0f, 1000.0f);

	glMatrixMode(GL_MODELVIEW);
	glLoadIdentity();
	
	gluLookAt(0.0f, 0.0f, 5.0f,
		0.0f, 0.0f, 0.0f,
		0.0f, 1.0f, 0.0f);
}

// 初始化渲染环境
void init_render_context() {

	// 蓝色背景
	glClearColor(0.f, 0.f, .3f, 1.f);

	angle = 0.0f;
	
	//glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);

	glEnable(GL_CULL_FACE);

	glEnable(GL_LIGHTING);
	glEnable(GL_LIGHT0);

	GLfloat ambient[] = { .3f,.3f,.3f,1.f };
	GLfloat diffuse[] = { .7f,.7f,.7f,1.f };
	GLfloat specular[] = { .5f,.5f,.5f,1.f };

	glLightfv(GL_LIGHT0, GL_AMBIENT, ambient);
	glLightfv(GL_LIGHT0, GL_DIFFUSE, diffuse);
	glLightfv(GL_LIGHT0, GL_SPECULAR, specular);

	GLfloat position[] = { 2.0f,2.0f,2.f,1.f };
	glLightfv(GL_LIGHT0, GL_POSITION, position);

	//glLightf(GL_LIGHT0, GL_CONSTANT_ATTENUATION, 1);
	//glLightf(GL_LIGHT0, GL_LINEAR_ATTENUATION, 0);
	//glLightf(GL_LIGHT0, GL_QUADRATIC_ATTENUATION, 0);

	const char* renderer = (const char*)glGetString(GL_RENDERER);
	const char* version = (const char*)glGetString(GL_VERSION);
	
	char title[0x256];
	memset(title, 0, 0x256);

	strcpy_s(title, "OpenGL");
	if (renderer != nullptr) {
		strcat_s(title, " | R - ");
		strcat_s(title, renderer);
	}
	if (version != nullptr) {
		strcat_s(title, " | V - ");
		strcat_s(title, version);
		strcat_s(title, " |");
	}

	glutSetWindowTitle(title);
}

// 渲染
void render(void) {

	// 清除颜色缓冲、深度缓冲、模板缓冲
	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);

	glLoadIdentity();
	glTranslatef(0.0f, 0.0f, -4.0f);
	glRotatef(angle, 0.0f, 1.0f, 1.0f);

	GLfloat green[] = { 0.0f,0.2f,0.1f,1.0f };
	GLfloat yellow[] = { 0.7f,0.6f,0.1f,1.0f };
	GLfloat white[] = { 1.0f,1.0f,1.0f,1.0f };
	GLfloat gray[] = {0.2f, 0.2f, 0.2f, 1.0f};

	// 自发光
	//glMaterialfv(GL_FRONT, GL_EMISSION, green);
	// 环境光
	glMaterialfv(GL_FRONT, GL_AMBIENT, gray);
	// 漫反射光
	glMaterialfv(GL_FRONT, GL_DIFFUSE , yellow);
	// 镜面反射光
	glMaterialfv(GL_FRONT, GL_SPECULAR, white);
	glMaterialf(GL_FRONT, GL_SHININESS, 10.0f);

	sphere_entry(72,144);

	angle += .02f;
	angle = angle > 360.0f ? 0.0f : angle;

	// 交换缓冲区
	glutSwapBuffers();
}

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

	gltSetWorkingDirectory(*argv);

	glutInit(&argc, argv);
	glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA | GLUT_DEPTH | GLUT_STENCIL);
	glutInitWindowSize(800, 600);
	glutCreateWindow("OpenGL");
	glutReshapeFunc(changed_size);
	glutDisplayFunc(render);
	glutIdleFunc(render);

	GLenum err = glewInit();
	if (GLEW_OK != err) {
		std::cerr << "GLEW Error : " << glewGetErrorString(err) << std::endl;
		return 1;
	}

	init_render_context();

	glutMainLoop();

	return 0;
}