1. An image processing apparatus comprising: a scanner which reads a color document to provide color data; a means for obtaining lightness data and chroma data of an interest pixel from the color data obtained by said scanner; a decision means for deciding from the lightness data if the interest pixel exists at an edge and for deciding from the chroma data if an image at the interest pixel is black or not; and a first data operation means for increasing the data of black of the interest pixel by a prescribed amount when said decision means decides that the interest pixel exists at the edge and that the image at the interest pixel is black.