空间圆弧作图算法，基于OpenGL实现！

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int CGLBasic::DrawCircle(XPoint ps, XPoint pe, XPoint pc, ColorRGB clr)
{
const float inc_a=0.001f;			// 精度系数

ps.x *= (m_ZoomRate*ONE_OF_HUNDRED);
ps.y *= (m_ZoomRate*ONE_OF_HUNDRED);
ps.z *= (m_ZoomRate*ONE_OF_HUNDRED);

pe.x *= (m_ZoomRate*ONE_OF_HUNDRED);
pe.y *= (m_ZoomRate*ONE_OF_HUNDRED);
pe.z *= (m_ZoomRate*ONE_OF_HUNDRED);

pc.x *= (m_ZoomRate*ONE_OF_HUNDRED);
pc.y *= (m_ZoomRate*ONE_OF_HUNDRED);
pc.z *= (m_ZoomRate*ONE_OF_HUNDRED);

float as_xy,as_zy,ae_xy,ae_zy,as_xz,ae_xz;	// 保存角度
size_t count;						//
float xy_inc;						// xy平面上角度增长步长
float zy_inc;						// zy平面上角度增长步长
float xz_inc;						// xz平面上的角度增长步长
float R;							// 圆弧半径

float x,y,z;

// 计算圆弧半径
R=sqrt((ps.x-pc.x)*(ps.x-pc.x)
+(ps.y-pc.y)*(ps.y-pc.y)
+(ps.z-pc.z)*(ps.z-pc.z));

// xy平面上起点、终点的角度
as_xy = CalAngle(ps.x, ps.y, pc.x, pc.y);
ae_xy = CalAngle(pe.x, pe.y, pc.x, pc.y);
if ( ae_xy <= as_xy )
{
ae_xy += 2*PI;
}

// 计算zy平面上起点、终点的角度，此时z轴相当于xy平面上的x轴
as_zy = CalAngle(ps.z, ps.y ,pc.z, pc.y);
ae_zy = CalAngle(pe.z, pe.y, pc.z, pc.y);
if ( ae_zy <= as_zy )
{
ae_zy += 2*PI;
}

// 计算xz平面上起点、终点的角度
as_xz = CalAngle(ps.x, ps.z, pc.x, pc.z);
ae_xz = CalAngle(pe.x, pe.z, pc.x, pc.z);
if ( ae_xz < as_xz )
{
ae_xz += 2*PI;
}

// xy平面为主平面的情况
if ( (pe.x-ps.x) && (pe.y-ps.y) )
{
// 分别计算两平面上角度增长的步长
if ( (ae_xy-as_xy) > (ae_zy-as_zy) )
{
xy_inc = inc_a;
count  = (size_t)(ae_xy-as_xy)/xy_inc;
zy_inc = (ae_zy-as_zy)/count;
}
else
{
zy_inc = inc_a;
count  = (size_t)(ae_zy-as_zy)/zy_inc;
xy_inc = (ae_xy-as_xy)/count;
}

// 微分思想计算画圆弧所需的点坐标
glBegin(GL_LINE_STRIP);
glColor3f(clr.r, clr.g, clr.b);
int i=0;
for ( float a_xy=as_xy, a_zy=as_zy;
a_xy<=ae_xy && a_zy<=ae_zy;
a_xy+=xy_inc,a_zy+=zy_inc,i++ )
{
x = pc.x + R*cos(a_xy);
y = pc.y + R*sin(a_xy);
z = pc.z + R*cos(a_zy);
glVertex3f(x,y,z);
}
glEnd();
glFlush();
}
// xz为主平面的情况
else if ( (pe.x-ps.x) && (pe.z-ps.z) )
{
if ( (ae_xz-as_xz) > (ae_zy-as_zy) )
{
xz_inc = inc_a;
count  = (size_t)(ae_xz-as_xz)/xz_inc;
zy_inc = (ae_zy-as_zy)/count;
}
else
{
zy_inc = inc_a;
count  = (size_t)(ae_zy-as_zy)/zy_inc;
xz_inc = (ae_xz-as_xz)/count;
}

glBegin(GL_LINE_STRIP);
glColor3f(clr.r, clr.g, clr.b);
int i=0;
for ( float a_xz=as_xz,a_zy=as_zy;
a_xz<=ae_xz && a_zy<=ae_zy;
a_xz+=xz_inc,a_zy+=zy_inc,i++ )
{
x = pc.x + R*cos(a_xz);
y = pc.y + R*sin(a_zy);
z = pc.z + R*sin(a_xz);
glVertex3f(x,y,z);
}
glEnd();
glFlush();
}
// zy为主平面的情况
else if ( (pe.y-ps.y) && (pe.z-ps.z) )
{
if ( (ae_zy-as_zy) > (ae_xy-as_xy) )
{
zy_inc = inc_a;
count  = (size_t)(ae_zy-as_zy)/zy_inc;
xy_inc = (ae_xy-as_xy)/count;
}
else
{
xy_inc = inc_a;
count  = (size_t)(ae_xy-as_xy)/xy_inc;
zy_inc = (ae_zy-as_zy)/count;
}

glBegin(GL_LINE_STRIP);
glColor3f(clr.r,clr.g,clr.b);
int i=0;
for ( float a_xy=as_xy,a_zy=as_zy;
a_xy<=ae_xy && a_zy<=ae_zy;
a_xy+=xy_inc,a_zy+=zy_inc,i++ )
{
x = pc.x + R*cos(a_xy);
y = pc.y + R*sin(a_zy);
z = pc.z + R*cos(a_zy);
glVertex3f(x,y,z);
}
glEnd();
glFlush();
}
else
{
return 0;		// 不能在任何平面构成圆弧则返回
}

return 0;
}