surface
st_circle()
{
float radius = 0.2; 
float opacity = 0.5;

if(	asin(s/radius - (1/radius/2)) < acos(t/radius - (1/radius/2)) &&
	asin((1-s)/radius - (1/radius/2)) < acos(t/radius - (1/radius/2)) &&
	asin((1-s)/radius - (1/radius/2)) < acos((1-t)/radius - (1/radius/2)) &&
	asin(s/radius - (1/radius/2)) < acos((1-t)/radius - (1/radius/2))){
	opacity = 1;
}


Oi = Os * opacity;
Ci = Cs * Oi;
}


surface
st_quadrant()
{

float opacity = 0;

if(asin(s) < acos(t)){
opacity = 1;
}

Oi = Os * opacity;
Ci = Cs * Oi;

}



surface
st_diagonal()
{

float opacity = 0;

if(1-s < t){
	opacity = 1;
}

Oi = Os * opacity;
Ci = Cs * Oi;

}


surface
st_cross(float crossWidth = 0.5)
{

float opacity = 0;

float tmp = 1-crossWidth;
if(s < (1-tmp + tmp/2) 
	&& s > (tmp - tmp/2)
	|| t < (1-tmp + tmp/2) 
	&& t > (tmp - tmp/2)){
	opacity = 1;
}

Oi = Os * opacity;
Ci = Cs * Oi;

}