Provided are an image processing apparatus and method of controlling same for detecting, in pixel units, an edge that exists prior to encoding, thereby protecting an edge that exists prior to encoding and reducing noise produced by encoding. To accomplish this, the image processing apparatus compares a threshold value generated from quantization information of each pixel when decoding is performed, and the absolute values of differences between a pixel of interest and pixel peripherals thereto, detects, in pixel units, an edge that existed prior to encoding, and corrects only noise produced by encoding.