1. A warning system comprising: determining means for determining, prior to printing by a printing apparatus of an image based on data obtained after an image processing of image data, whether or not there is a possibility in which an undesirable printing result may be caused based on the image data; data forming means for simulating, when it is determined that there is the possibility in which the undesirable printing result may be caused, the image processing by the printing apparatus to form the data obtaied after the image processing with respect to thus determined image data; and display means for displaying, prior to printing of the image by the printing apparatus, a preview image based on the data after the image processing.