1. An image forming system for observing a subject image, which has been formed on a recording medium, by superposing an optical member, which has a one-dimensional periodic structure, on the subject image, comprising: image acquisition means for acquiring an image sequence of the subject image; image synthesizing means for synthesizing a stripe image in such a manner that pixels having identical coordinates in each image of the image sequence will be arrayed as adjacent pixels; error diffusion quantization means for converting the stripe image to a multivalued image by reducing number of tones through the error diffusion method; binarization means for converting the multivalued image to a binary dot pattern through the density pattern method; and image forming means for forming the binary dot pattern on a recording medium; wherein said binarization means selects any dot pattern of a prescribed plurality of dot patterns in accordance with a pixel position, in the multivalued image, in a direction perpendicular to direction of repetition of the periodic structure of the optical member, thereby converting the multivalued image to a binary dot pattern.