ELIMINATING BANDING IN BLENDS AND GRADIENTS

“Banding” or “stepping” are terms used to describe the distinct “chunks” that are occasionally visible in a blend or gradient.

You can determine whether or not your blend will have banding by using the following series of equations:

Take the resolution of the output device (our platesetter is 2400 dpi) and divide it by the line screen (covers are 150 lpi and most text is 120 lpi) and square the result.

(2400 dpi ÷ 150 lpi)² = 256

The result (256) is the number of shades of gray available to you.
If your blend goes from 0% to 100% then you have 100% of the 256 shades available. If however the blend goes from 20% to 70% then you only have

70% - 20% = 50%
50% × 256 shades = 128 shades of gray available

The next thing you will need to know is how long your blend is. That is, will it go the full length of your book? The easiest way to make sure is to measure the length of the blend.

For example, use 9" as the length of the blend. Take the length of the blend (9") and divide it by the number of shades available to you (128).

9" ÷ 128 steps =  0.0703125"

This is the size that each step will be. In order for the blend to appear smooth to the human eye, you want each step to be 0.03" or less. Therefore, in this example, you would have noticeable banding. You have several options to reduce the size of each step and thus eliminate the banding.

You don’t have to consider switching to film instead of Computer-to-Plate to ensure that there will be no banding. If banding occurs through our CTP system, it will also occur using film. The only difference is that the film-based proof will show you the banding. A sample of high-resolution output generated in addition to a digital proof will show the same thing. If you are concerned about the appearance of your blends upon final output, ask your Project Manager for a sample of high-resolution output to be sent along with your digital proof. This sample of high-resolution output from our imagesetter will show you how your blend will appear on the printing plate.

Another strategy for dealing with blends is to create them in Photoshop and apply “Noise” to blur the transitions somewhat. Save this blend as a TIFF and then place it in your page layout software. For covers, this is a better option than relying on QuarkXPress™ to create a blend.

A sample of high-resolution output from our imagesetter will show you how your blend will appear on the printing plate. Different programs can generate different CMYK percentages for the same PANTONE colour.

An important issue to be aware of is that the Transparency feature from Illustrator, CorelDraw or InDesign is not yet supported by Adobe’s imaging model. Transparencies have to be flattened in order to image to plate or film.