This paper proposes a 3D light field optimization algorithm for a pixelwise directional 3D display using a diffraction grating. Design of grating optics is important in the development of 3D displays and augmented reality devices. Unlike typical 3D optics such as lenticular lenses and parallax barriers, grating optics can be manufactured for each independent pixel. This can be a significant advantage, but it can also cause difficulties because the light field arrangement can reduce the width of the gratings, yielding low luminance and high crosstalk. A light field generation algorithm that simultaneously optimizes the grating width and 3D quality is proposed in this paper. Experimental results confirm the proposed theory.