OpenGL: OpenGL中的平面圆绘制与贴图

两种画圆的方法，DrawCircle2()为一般的做法，利用GL_LINE_STRIP实现，

void DrawCircle2(float cx, float cy, float r, int num_segments)  
{  
    glBegin(GL_LINE_STRIP);  
    for (int i=0; i<=num_segments; i++)  
    {  
        glVertex2f( cx+r*cos( (2*M_PI*i)/num_segments ),   
            cy+r*sin( (2*M_PI*i)/num_segments ) );  
    }  
    glEnd();  
}

DrawCircle1()则是利用GL_LINES_LOOP实现，

void DrawCircle1(float cx, float cy, float r, int num_segments) 
{ 
	float theta = 2 * M_PI / float(num_segments); 
	float tangetial_factor = tanf(theta);//calculate the tangential factor 
	float radial_factor = cosf(theta);//calculate the radial factor 
	float x = r;//we start at angle = 0 
	float y = 0; 

	glBegin(GL_LINE_LOOP); 
	for(int ii = 0; ii < num_segments; ii++) 
	{ 
		glVertex2f(x + cx, y + cy);//output vertex 
		//calculate the tangential vector 
		//remember, the radial vector is (x, y) 
		//to get the tangential vector we flip those coordinates and negate one of them 
		float tx = -y; 
		float ty = x; 
		//add the tangential vector 
		x += tx * tangetial_factor; 
		y += ty * tangetial_factor; 
		//correct using the radial factor 
		x *= radial_factor; 
		y *= radial_factor; 
	} 
	glEnd(); 
}

但是上面两个函数都只是画出了两个圆圈，想要给circle贴图，必须画出的是一个区域，所以可以利用GL_TRIANGLE_FAN绘制，要实现纹理映射，关键是纹理坐标的分配：圆心纹理坐标为：(0.5, 0.5)选取图片的中心。圆圈上的点的分配：纹理坐标必须在0，1之间，而且这些纹理坐标和圆的半径没有关系，只和圆心角有关。因为-1< cos(delta_angle*i) <1,则==> 0 <= (cos(delta_angle*i) + 1.0)*0.5 <= 1同理：0 <= (sin(delta_angle*i) + 1.0)*0.5 <= 1

GLvoid draw_circle(const GLfloat radius,const GLuint num_vertex)
{
	GLfloat vertex[4]; 
	GLfloat texcoord[2];

	const GLfloat delta_angle = 2.0*M_PI/num_vertex;

	glEnable(GL_TEXTURE_2D);
	glBindTexture(GL_TEXTURE_2D,texName);
	glTexEnvi(GL_TEXTURE_ENV,GL_TEXTURE_ENV_MODE,GL_REPLACE);
	glBegin(GL_TRIANGLE_FAN);

	//draw the vertex at the center of the circle
	texcoord[0] = 0.5;
	texcoord[1] = 0.5;
	glTexCoord2fv(texcoord);
	vertex[0] = vertex[1] = vertex[2] = 0.0;
	vertex[3] = 1.0;        
	glVertex4fv(vertex);

	//draw the vertex on the contour of the circle
	for(int i = 0; i < num_vertex ; i++)
	{
		texcoord[0] = (std::cos(delta_angle*i) + 1.0)*0.5;
		texcoord[1] = (std::sin(delta_angle*i) + 1.0)*0.5;
		glTexCoord2fv(texcoord);

		vertex[0] = std::cos(delta_angle*i) * radius;
		vertex[1] = std::sin(delta_angle*i) * radius;
		vertex[2] = 0.0;
		vertex[3] = 1.0;
		glVertex4fv(vertex);
	}

	texcoord[0] = (1.0 + 1.0)*0.5;
	texcoord[1] = (0.0 + 1.0)*0.5;
	glTexCoord2fv(texcoord);

	vertex[0] = 1.0 * radius;
	vertex[1] = 0.0 * radius;
	vertex[2] = 0.0;
	vertex[3] = 1.0;
	glVertex4fv(vertex);
	glEnd();

	glDisable(GL_TEXTURE_2D);
}

最终的效果见下图：

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