/**
* Creates the unit normal to the polygon by taking the cross product of the first two edges.
* Call with 'nsides' set to two if the polygon is visible from both sides.
*/
void setNormal() {
// We need three points to define two edges.
if (n < 3) return;
if (normal == null) normal = new float[3];
float[] p0 = {ps[points[0]], ps[points[0] + 1], ps[points[0] + 2]};
float[] p1 = {ps[points[1]], ps[points[1] + 1], ps[points[1] + 2]};
float[] p2 = {ps[points[2]], ps[points[2] + 1], ps[points[2] + 2]};
float[] e1 = new float[3];
float[] e2 = new float[3];
Tools3d.subtract(p1, p0, e1);
Tools3d.subtract(p2, p1, e2);
Tools3d.cross(e1, e2, normal);
Tools3d.makeUnit(normal);
dirtyNormal = false;
}
/**
* Decides if this polygon is a 'back face'. A back face is a polygon that is facing away from
* the camera and therefore should not be drawn. If this polygon is double sided then it can not
* be a back face. In such a case this routine has the side effect of ensuring that the normal
* points towards the camera eye rather than away from it.
*
* @see Obj3d#draw(Graphics)
*/
boolean isBackFace(final float[] eye) {
if (dirtyNormal) {
setNormal();
}
p[0] = ps[points[0]];
p[1] = ps[points[0] + 1];
p[2] = ps[points[0] + 2];
Tools3d.subtract(p, eye, ray);
if (doubleSided) {
// flip the normal if necessary so this is NOT a backface
if (Tools3d.dot(normal, ray) >= 0) {
Tools3d.scaleBy(normal, -1);
}
return false;
} else {
return (Tools3d.dot(normal, ray) >= 0);
}
}