Apparatus for erasing and extracting image data from particular region of orignal document

An apparatus applicable to a digital copier, a digital printer, a printer using a mimeograph and others for specifying a desired region of an original document by use of a marking sheet, and erasing, extracting or otherwise editing a portion of the image of the original document which lies in the particular region. An image of the marking sheet read by an image scanner are converted into two-level pixel data. The pixel data are reduced in density before written in a memory, such that pixels are extracted, or thinned, one out of each predetermined number of pixels in each of the column and row address directions of the memory. One of two differnt image modes is selected: a text mode with applies MTF (modulation transfer function) processing to a text document, and a graphic mode which applies dither processing or like halftone processing to a graphic document. When the marking sheet is to be read, the text mode is set up automatically.