1. A stereoscopic image display apparatus comprising: a display panel including pixels that are arranged in a matrix pattern at intersections of data lines and gate lines; a display panel driver that increases resolution of input 3D image data and displays the 3D image data on the display panel; a patterned retarder that includes a first retarder transmitting first polarized light from odd-numbered lines of the display panel and a second retarder transmitting second polarized light from even-numbered lines of the display panel; and polarization glasses that includes a first polarization filter transmitting the first polarized light from the first retarder and a second polarization filter transmitting the second polarized light from the second retarder, wherein the display panel driver includes a resolution enhancer that calculates an average brightness of first left-eye image data included in an nth (n is a natural number) line of the 3D image data and second left-eye image data included in an n+2th line of the 3D image data to obtain left-eye average data, calculates an average brightness of first right-eye image data included in an n+1th line of the 3D image data and second right-eye image data included in an n+4th line of the 3D image data to obtain right-eye average data, and converts the brightness of the left-eye average data and the right-eye average data into grayscale values.