1. A method for gaze direction estimation, comprising: detecting at least one iris of an eye in an image; estimating a 3D eyeball center and radius, a fovea position, and an iris center; determining an estimated eye gaze directional vector between the fovea position and the iris center, based on at least the estimated 3D eyeball center and radius, fovea position, and iris center; calibrating the estimated eye gaze directional vector with respect to a known condition, by directing the eye to look into at least one camera and toward at least one calibration point; solving for t a using the following quartic equations: 
 St a 4 +Tt a 3 +Ut a 2 +Vt a +W= 0, 
 S=Y 2 , 
 T=− 2 TZ− 2( A*B ) XY, 
 U=Z 2 +2( A*B ) XY−r 2 y 2 +x 2 , 
 V 2 r 2 YZ, 
 W=−r 2 Z 2 , 
 X =( A×B×A )* P, 
 Y =( A×B×A )* B, 
 Z =( A×B×P )* B; wherein: r is a 3D eyeball radius, E is an estimate for the 3D eyeball center, E′ is an estimate for the 3D iris center, P is a calibration point having a known 3D location, m1 and m2 are iris centers represented in the image, A and B are normalized vectors between m1 and m2 and the camera, t a and t b represent lengths such that At a =E and Bt b =E′, wherein ∥At a −Bt b ∥=r and point E′ (or Bt b ) is a point of intersection between C=At a −P and B, equivalent to a point of intersection between B and a plane determined by point P and normal N=(A×B×C), such that t b = N · P N · B ; scaling A by t a and obtaining a 3D eyeball center estimate E for a given eyeball radius r; and estimating r based on a statistical calibration process.