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：