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Frequently Asked Questions - Image banding


The output film or prints have visible banding.

Banding on globes is often a problem for people who are unaware of the limitations of PostScript, which does not give perfectly smooth gradations. But it is avoidable to a certain extent - that is, to an acceptable tolerance.

The problem with PostScript is that you can only get a maximum of 256 shades of each of the four process colours. In normal 4-color printing that is not generally a problem, but as soon as you involve images with a smooth gradation the limitations of PostScript become more obvious - if certain criteria are not met. These are:

  1. Input (Image) resolution.
  2. Halftone screen.
  3. Output (Imagesetter) resolution.
Image resolution does not have a direct effect on the number of shades of a colour, but for the best results the resolution (in pixels per inch) of an image should be at least twice that of the halftone screen frequency. In other words, an image to be output with a halftone screen of 150 lines per inch should have an input resolution of 300 ppi (which is what GlobeShots is - AT ACTUAL SIZE). A common mistake made by designers is to enlarge an image without changing the image resolution - in other words an image enlarged by 200% ends up with a resolution of 150 ppi, which is too coarse.

The most significant thing that affects smooth gradations is halftone screen (H/T) versus imagesetter resolution (I/S). To get the optimum number of shades, the I/S res relates directly to the H/T screen. This can be worked out as follows:

Method 1: To work out the number of shades you can get from, say, an image to be output at 150 lines per inch from an I/S with a 2,540 resolution you calculate the square of 2,540 divided by 150, giving 286 shades, which is slightly more than the PostScript maximum, and may result in some banding. For 2,540 dpi output, a H/T screen of 159 lpi would be better (255 shades), but this is an unconventional screen frequency. You use this method when you have no control over the resolution of the output device.

Method 2: To work out the I/S resolution for a H/T image with a fixed frequency you simply multiply 16 (the square root of 256) by the halftone screen. Thus a 150 lpi H/T should be output from an I/S with 2,400 dpi. This method is best used if you have no control over the printing process.

It's important to remember that individual separations of 4-color film will often show banding, but, if the above criteria are adhered to, when all four colours are printed the banding should be imperceptible.

Other factors which can affect smooth tones are UCR (undercolor removal) and GCR (grey component removal), but it is best to refer to the printer for advice on these, because each job is different.


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