1. An image sensor for on-chip phase detection, comprising: a pixel array for capturing an image of a scene, the pixel array including: a plurality of horizontal phase-detection rows, each including a first plurality of phase-detection pixels, comprising a plurality of left-masked pixels and a plurality of right-masked pixels, for detecting horizontal change in the scene, and a plurality of vertical phase-detection columns, each including a second plurality of phase-detection pixels, comprising a plurality of top-masked pixels and a plurality of bottom-masked pixels, for detecting vertical change in the scene; wherein each of the plurality of horizontal phase-detection rows intersects each of the plurality of vertical phase-detection columns.