An image processing apparatus for embedding additional information in an image having a plurality of pixels and image information. A generating unit generates (a) the additional information to be embedded and (b) a bookbinding type of a medium containing the image. A holding unit holds a plurality of quantization conditions for embedding the additional information, including a quantization threshold. A selection unit segments the image into a plurality of embedding regions, and selects a condition to use in quantization based on the image information. A quantization condition control unit controls at least one of the plurality of quantization conditions for a predetermined region of the image based on the bookbinding type generated. An error diffusion processing unit distributes an error to peripheral pixels of a target pixel. A separating unit separates the embedded additional information from the image.