If you scan a photo from a newspaper or magazine, then examine the results on your computer screen, you may see a criss-cross pattern of fuzzy lines over the entire print. If you print the scanned photo, you may also see such a pattern. This happens because pictures in newspapers and magazines are printed in a "halftone" mode.

The halftone process was invented when it was desired to print black-gray-white photographs using a printing press that used only single-color black ink. In this process, the photo is divided into a pattern of small squares, then each square is replaced with a black "blob" of a size proportional to the average shade of black in the square. For example: a white square remains white; a light gray square becomes a small black blob; a dark gray square becomes a larger black blob; a black square remains black. The gray shades were called "half-tones", which explains the name of the process. Originally, the conversion was done by re-photographing the picture with a camera that contained a wire screen. then developing the picture in a high-contrast mode. Nowadays, the process can be accomplished on a computer.

If the density (squares per inch) of the scanned picture is not an exact multiple of the pixel density of the scanner, computer screen, or printer, an interference pattern occurs. They may also appear if the original photo is slightly rotated after scanning.

Colored pictures from publications may also give you interference patterns. For these, the halftone process uses filters to split the image into more than one black-gray-white photo, with each photo representing the intensities for each color. In each resulting halftone, the blob pattern is slightly offset from that of the other halftones. The picture is reproduced by printing the same paper once for each halftone, in the corresponding color. The printing press does not print one color on top of another because of the slight offset of the halftones.

You can get rid of the interference pattern by processing the picture with any photo software package that has a "blur" or "soften" selection. For example, in Paint Shop Pro, the IMAGE/SOFTEN menu selection spreads the black blobs so that the fuzzy bars magically disappear.

This article has been provided to APCUG by the author solely for publication by APCUG member groups. All other uses require the permission of the author (irving.elliott(at)att.net)