Contour is an important element of an image, usually expressed by the difference in density between two areas of an image. Objects are usually recognized from observation of their contours. In some cases, emergence of a false contour is a problem in image coding and decoding. Halftoning is an essential image process for digital printing of continuous tone. Contour perception is studied experimentally under various halftoning conditions, observation distances, and illumination conditions. The results show that in the human visual system, contour perception becomes easier with a decreasing stimulus of dots, which serve as the component of a halftone image.