Banding looks very similar to stair steps across the fountain fill, and is the result of abrupt changes
between shades or colors. A common myth in the graphics industry is that smooth fountain fills
are the norm, and banding is the unusual result, however, it is usually the other way around.  

There are several issues which affect the amount of banding; the size of the object, the number of
fountain steps or bands,  the Custom Screen Frequency, the resolution of the output device, and 
the percentage of gray change from one color to another in the Fountain Fill.

An accepted formula for calculating the maximum number of bands available to the printer at the
current dpi and lpi is as follows:
      # of Bands=[(DPI/LPI)2 x (% grey change)]/100 where,
      dpi = resolution in dots-per-inch 
      lpi =  line screen frequency in lines-per-inch 
      
Example:  A 300 dpi PostScript printer usually prints using a 60 lpi screen frequency. A Fountain
Fill from black to white would produce a 100% grey change, therefore,

The number of bands=[(300/60)2 x 100]/100=(25 x 100)/100=25.

The following chart shows some of the possible values:






















Where,  %    = % gray change
                         max                                               = maximum length of object to be Fountain Filled

The human eye can see objects as small as .03 inches (1/32") in size. If our object had bands that
small, it would appear smooth to our eyes. If we had all 250 bands available to us (250 x .03),
then the object should be no longer than 7.5 inches.

Note: Since the current PostScript technology limits the maximum number of stripes to 256
(CorelDRAW  3 has a current limitation of 250), all values with an asterisk (*) are not possible.

Setting the Screen Frequency lower will give you an increased number of bands or steps, but
setting the value too low can cause the image to print too coarsely.

Once the file has been spooled to the printer, the PostScript interpreter will make changes to the
values specified:

The Adobe PostScript Language Reference Manual (the "Red Book") states: "...the best choice of
screen parameters is often dependent on specific physical properties of the output device itself
(e.g., pixel shape, overlap between pixels, and effects of electronic or mechanical noise). ...The
setscreen operator may make slight adjustments to the requested frequency and angle so as to
ensure that the patterns of enclosed pixels remain constant as the screen cells are replicated over
the entire page."

If you are printing only black and white fountain fills, the above seems straightforward. However,
if you are using color, one must also calculate the gray equivalent of the colors. The following
formula is from the Adobe PostScript Language Reference Manual:

%gray = .30 x (% red) + .59 x (% green) + .11 x (% blue)

CorelDRAW can show the RGB (Red, Green, Blue) equivalents of the colors you are using, as
explained in the following example.

Example:  If you are using Orange as your start color:
1.  Use the Uniform Fill tool to fill an object with Orange.
2.  Click the Custom Fill tool from the Fill tool flyout.
3.  Assuming you are using CMYK, Orange is a combination of 0% cyan, 60% magenta, 100%
yellow, and 0% black.
4.  Change to the RGB color method, and you will see that Orange is a combination of 100% red,
40% green, and 0% blue.
5.  Use the formula to calculate the percentage of gray: %gray = .30 x (100% R) + .59 x (40% G)
+ .11 x (0% B) This works out to be 53.6% gray.
6.  Calculate the % of gray for the destination color. With these two values, you would be able to
determine the percentage of gray change between the two colors, and apply the original formula
to decide the number of steps or bands in the fountain fill.

Experimentation will be your best guide as to what looks the best.  Run test prints using different
screen frequencies with different numbers of steps in objects of various sizes.  From these test
prints, use the values which seem to work the best to produce your artwork.

Note:  This information is meant to act as a general guide only, and should not be considered as
the definitive solution to a particular problem.