The magazine of the Melbourne PC User Group Printing Shades of Grey and Smooth Fades What is Banding? Stephen Davey   smd@pagination.com.au

0ne of the great features of many graphic art software packages is their ability to create a wide variety of interesting fades, vignettes or blends, very easily. These effects were extremely time consuming and expensive to create with traditional methods.

However, often when your print job is produced, something that was meant to be a smooth fade from one colour to another, or from light to dark turns out to be a series of rather ugly steps or bands as they are known.

Why Does This Happen?

Often it is a function of the quality of the output device you are using to print the job. Low resolution laser printers are much more likely to cause banding than high resolution imagesetters. However, even some 3000 dpi (dots per inch) imagesetters can produce banding - due to the limitations of Postscript not of the imagesetters themselves.

Most images or fades contain many different colours or shades of grey.

Unfortunately black and white laser printers and imagesetters can print only black. So in order to create the illusion of different shades Postscript instructs the printer to put down evenly spaced lines of different sized dots. You can see these image dots when you magnify a printed photograph. These lines of dots are referred to as the screen ruling, or sometimes the screening. Screen ruling is measured in lines per inch (lpi).

Lines of very small image dots give the impression of light grey, while larger dots are used for darker shades.

In simple terms, Postscript creates these image dots by grouping together one or more of the smallest possible printer dots the device is capable of producing.

The size of the smallest printer dot is determined by the type of printer you are using. For example a 300 dpi laser printer puts down dots of approx 1/300 inch. A high resolution 3000 dpi imagesetter makes dots as small as 1/3000 inch.

Now let us suppose (using simple mathematics) that we want to create an image or fade using a 300 dpi printer and we want a screen ruling of 30 lpi -which is quite coarse. Newspapers typically use 75-85 lpi screen ruling.

With only 300 dots available per inch and wanting 30 printed lines per inch, means that the thickest line or image dot can be only 10 printer dots wide. Of course the thinnest possible line would be one printer dot wide.

With a grid of 10 x 10 printer dots it is possible to create 100 different sized image dots. Nine are shown below -ranging from almost white (one printer dot) to solid black (100 printer dots).

It follows that if we can print 100 different sized image dots, then we can produce 101 shades -white being the additional shade.

Knowing how many shades of grey your printer can produce is very important if you wish to achieve good results when printing graphic images. 

Let's Calculate Shades of Grey

Shades of grey possible = ( printer dpi / screen ruling (lpi) )²  + 1

Using our 300 dpi Printer:

30 Ipi screen
Shades of grey = (300/30)²+1 = 101

60 Ipi screen
Shades of grey = (300/60)²+1 = 26

100 lpi screen
Shades of grey = (300/100)²+1 = 10

Note that the shades of grey available decreases as you increase the line ruling (Ipi). This is why sometimes photos that are printed with a coarse screen look sharp and have good contrast, as opposed to the same photo printed at a finer (higher) line ruling.

So as you can see, unless we print photographs at very coarse screens, they don't look very good when printed on a 300 dpi laser printer.

To produce reasonable black and white photographs we need at least 64 shades of grey, and to produce a long, band free fade we might need many more shades.

High resolution imagesetters create much better results because they can produce many more shades, even at higher screen rulings:

With a 3000 dpi Imagesetter

100 lpi screen
Shades of grey = (3000/100)²+1 = 901

133 lpi screen
Shades of grey = (3000/133)²+1 = 510

200 lpi screen
Shades of grey = (3000/200)²+ 1 = 226

To produce good black and white photographs (requiring 64 shades of grey) for:

Newspaper (at 85 lpi) needs at least a 700 dpi printer

Glossy Brochure (at 133 Ipi) needs at least a 1100 dpi printer

Glossy Brochure (at 175 lpi) needs at least a 1400 dpi printer.

For fades and vignettes, the number of shades of grey you need depends on how long the fade is and how much the colour changes from one end to the other.

If we want to create a fade from 100% to 0% over a given distance Postscript divides the required distance by the number of shades available and creates bands of different sized dots to make up the fade.

If the printer we are using does not have many shades of grey available, then the thickness of the bands may become very noticeable. Bands wider that 0.5 mm are generally undesirable.

Let us consider a 29 cm long fade that is printed by a printer with only 65 shades of grey. The fade will have to occur over 65 bands. This means each band will be (290/65), or 4.46 mm wide and will be very noticable.

To get better blends you need to do one or more of the following:

Increase the shades of grey.
Either by reducing the screen ruling or increasing the dpi (le. use a better printer)

Reduce the Length of fade.

Increase the range of the fade.
Increase the range of the fade (eg. from 100% to 0% instead of 50% to 0%) and you will have a greater number of shades available. This reduces the width of each band and the blend will be much smoother.

So Much For the Theory

Now for the real world.

Despite the fact that (in theory) a 3000 dpi Imagesetter can produce 901 shades of grey at 100 lines per inch, in reality this may not be the case. Much depends on the RIP (Raster Image Processor) software driving the device.

Early versions of Postscript were limited to 256 shades of grey, no matter how technically superior the hardware might have been.

More recent Postscript versions and some manufacturers' special tweaking of the RIP, now support up to 4000, even 5000 shades of grey. So if you are having trouble getting good clean fades etc., determine the precise capability of the output device and if necessary adjust your design accordingly.

About the Author:
Stephen Davey smd@pagination.com.au has worked in the printing and graphics arts world for 28 years and manages his own bureau providing design and pre-press services to clients throughout Australia.

Reprinted from the June 2000 issue of PC Update, the magazine of Melbourne PC User Group, Australia

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