{ RCSid $Id: diskcoords.cal,v 1.3 2018/12/03 19:49:11 greg Exp $ }
{
	Shirley-Chiu coordinate calculation for BRDF data types (plasdata, metdata)
}
			{ Compute square position from disk coordinates }
norm_radians(p) : if(-p - PI/4, p + 2*PI, p);
idr(idx,idy) = sqrt(idx*idx + idy*idy);
idp(idx,idy) = norm_radians(atan2(idy, idx));

idrgn(idp) = floor((idp + PI/4)/(PI/2)) + 1;

square_a(idr,idp) = select(idrgn(idp),
			idr,
			(PI/2 - idp)*idr/(PI/4),
			-idr,
			(idp - 3*PI/2)*idr/(PI/4));

square_b(idr,idp) = select(idrgn(idp),
			idp*idr/(PI/4),
			idr,
			(PI - idp)*idr/(PI/4),
			-idr);

square_x(idx,idy) = (square_a(idr(idx,idy),idp(idx,idy)) + 1)/2;
square_y(idx,idy) = (square_b(idr(idx,idy),idp(idx,idy)) + 1)/2;

inc_sqx = square_x(-Idx,-Idy);		{ Negative because of Klems reversal }
inc_sqy = square_y(-Idx,-Idy);

sqx_in(sx,sy,sz) = inc_sqx;		{ Only a function of incident ray direction }
sqy_in(sx,sy,sz) = inc_sqy;

sqx_out(sx,sy,sz) = square_x(Ldx(sx,sy,sz),Ldy(sx,sy,sz));
sqy_out(sx,sy,sz) = square_y(Ldx(sx,sy,sz),Ldy(sx,sy,sz));