1. An image processing apparatus, comprising a spatial filtering unit which applies spatial filtering to input image data to generate filtered image data such that said spatial filtering provides a broader dynamic range for outputs than for inputs; and a high-resolution conversion unit which interpolates the filtered image data in a first direction by an average of values of two pixels adjacent in the first direction, in a second direction perpendicular to the first direction by an average of values of two pixels adjacent in the second direction, and in a direction diagonal to the first and second directions by an average of four pixels surrounding a pixel of interest, thereby converting the filtered image data into high-resolution image data.