1. A compression method of quantizing and compressing image data of an image, said method comprising the steps of: determining a method of quantization for each pixel; quantizing the image data in pixel unit according to the determined quantization method; storing the quantized image data in a same region of a memory in spite of the quantization method; and storing discrimination data for discriminating the quantization method used for quantizing the image data by corresponding to the quantized image data in the memory.