1. A method for recognizing markers printed on a learning material, comprising: sampling an image of the learning material; grouping the sampled image of pixels into a first image group and a second image group based on a threshold; calculating medians of the first image group and the second image group to update the threshold with a first average value of the calculated medians; repeating the above until a difference between a previous threshold and an updated threshold is equal to or smaller than a reference value; and binarizing an image captured by a camera based on the updated threshold. detecting the markers based on the binary image.