Provided are a method and a device for processing image data. According to the method, first characteristic values respectively of a plurality of regions of a first image are determined by sequentially calculating, for each of the plurality of regions, characteristic values corresponding to each of pixel values of pixels included in each of the plurality of regions while sequentially receiving the pixel values of the pixels; based on the determined first characteristic values, an illumination image related to a second image that is a following image of the first image is obtained; and, based on the obtained illumination image, the second image is modified.