o_face.c
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	#ifndef lint
	static const char RCSid[] = "$Id: o_face.c,v 2.8 2021/01/31 18:08:04 greg Exp $";
	#endif
	/*
	* o_face.c - compute ray intersection with faces.
	*/

	#include "copyright.h"

	#include "ray.h"
	#include "face.h"
	#include "rtotypes.h"


	int
	o_face( /* compute intersection with polygonal face */
	OBJREC *o,
	RAY *r
	)
	{
	double rdot; /* direction . normal */
	double t; /* distance to intersection */
	FVECT pisect; /* intersection point */
	FACE f; / face record */

	f = getface(o);

	/*
	* First, we find the distance to the plane containing the
	* face. If the plane is parallel to our ray, or the
	* previous hit was better, we return. Otherwise,
	* we determine whether in fact the ray intersects the
	* face. The ray intersects the face if the
	* point of intersection with the plane of the face
	* is inside the face.
	*/
	/* compute dist. to plane */
	rdot = -DOT(r->rdir, f->norm);
	if ((rdot <= FTINY) & (rdot >= -FTINY)) /* ray parallels plane */
	return(0);

	t = (DOT(r->rorg, f->norm) - f->offset) / rdot;

	if (rayreject(o, r, t)) /* previous hit is better? */
	return(0);
	/* compute intersection */
	VSUM(pisect, r->rorg, r->rdir, t);

	if (!inface(pisect, f)) /* ray intersects face? */
	return(0);

	r->ro = o;
	r->rot = t;
	VCOPY(r->rop, pisect);
	VCOPY(r->ron, f->norm);
	r->rod = rdot;
	r->pert[0] = r->pert[1] = r->pert[2] = 0.0;
	r->uv[0] = r->uv[1] = 0.0;
	r->rox = NULL;

	return(1); /* hit */
	}