In prepress and multi-color printing, a trapping technique, or means of effecting accurate registration on press by adjusting the size of an image and the opening in which it will be inserted. A choke is a photographic enlargement of the background color in which a second image will print. This has the effect of reducing the size of the hole in which a foreghround oibject will be printed. Or, in other words, the opening is "choked."A spread is the slight photographic enlargement—or "spreading"—of the image that will print within the choked image. This combination of reducing the opening and enlarging the image creates a slight overlap when the images ultimately print, eliminating unwanted white spaces or gaps between the two images.
There are many ways of preparing spreads and chokes, and each printer or prepress house may have his/her own pet process. Essentially, the process of making a spread or a choke involves creating a sandwich of the original photographic negative (or positive), a diffusion sheet, and a variety of clear spacer sheets. When light is then passed through the original negative, it is scattered, and as a result, it will fall in a slightly larger area on the unexposed film. The spacer sheets can control the extent of the scattering, adjusting the size of the spread or choke. Controlling the length of the exposure also affects the size of the spread or choked image. In some cases, the camera operator manually shakes a diffusion sheet between the light and the films, creating randomly scattered light. This isn't always efficacious.
Spreads and chokes are also called fatties and skinnies, respectively.