Re: Printing gradients with 2 Pantone colors

Why not just move the 2 centers of each gradients toward their respective zero points to make the gradient darker in the middle?

*On the banding problem*: this is a mathematical formula from adobe:

Creating smooth gradients for PostScript output 12/3/07 10:29 AM

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ID: tn_3668

Products affected

FreeHand

Adobe

TechNote

Creating smooth gradients for PostScript output

Issue

Sometimes when I print graduated fills from FreeHand I get large bands of

color, not smooth gradations.

Reason

Postscript actually creates gradations of color by combining small areas of

single color. In most print output these bands of color are so small that your

eye blends them together. But sometimes Postscript can cause these

bands if it does not have the correct information. However, there are steps

which you can take to reduce the bands so the gradation seems smooth.

Solutions

The simplest solution is to set File/Output Options/Maximum Color Steps to

256.

Further Information

It is possible to calculate the size of tint bands in a gradient (i.e., gradient

fill, whether radial, linear or logarithmic) to predetermine banding and

assure efficient gradients for PostScript output.

A correct understanding of the mathematics involved in creating smooth

gradients will provide the tools for producing the desired effect without

compromising quality.

A. Creating Gradations Digitally

B. Formula for Calculating Tint Bands in Gradients

C. Determining the Available Number of Tints& Band Size

D. Working with Process Colors

TechNote Details

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E. Adjusting Known Variables to Reduce Banding

F. General Tips

A. Creating Gradients Digitally

Banding is a result of producing halftones digitally. It is possible to calculate

the band widths before imaging. These calculations will aid in creating

efficient gradients with the optimum number of steps to prevent noticeable

banding and optimize imaging time.

The results of the calculation will provide:

1. How many different tints the printer can print.

2. What range of those tints the color change can use.

3. Whether the resulting tint bands are fine enough to create the illusion

of a smooth gradient.

The following information is needed to calculate tint band widths:

1. The printer resolution in dots per inch (dpi).

2. The screen ruling in lines per inch (lpi).

3. The change in color between the beginning and ending tints in the

gradient described as a percentage.

4. The physical distance between the beginning and ending tints of the

gradient described in points.

B. Formula for Calculating Tint Bands in Gradients:

([Resolution(dpi)]/[Screen Ruling(lpi)] [Squared] +

1= [a]

Example:

A printer with a dpi of 600 printing a file with a Screen Ruling set at 85 lpi

would yield this equation:

600/85= 7.0588 (round off to 7) 7 squared= 49 + 1 =

50 [a] = initial number of tints available to be printed.

[a] x [amount of tint change in gradient] =

Example:

An object in FreeHand has a gradient set that starts at 90% black and ends

at 10% black. The change in the tint is 80%, and would yield this equation:

50 x 80% (or .8) = 40

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** is the actual number of tints available in the current tint change.**

[length of the area to be filled] / ** = [c]**

Example:

The object in FreeHand is a horizontal rectangle 4 inches wide (288 points)

by 1 inch (72 points) high, and the gradient fill above is applied at an angle

of 0 degrees (the fill is horizontal over the 4 inch distance) would yield this

equation:

288 / 40= 7.2

[c] is the size in points of each band, or how visible banding will be and the

minimum width of objects used to create the gradient.

C. Determining the Available Number of Tints & Band Size

The first part of the calculation determines exactly how many different tints

the printer can produce at the screen ruling chosen. A certain tint is based

on the size of the halftone dot the printer creates, which in turn is based on

the combination of printer resolution and screen ruling chosen. The higher

the resolution of the printer, the more tints it can potentially create.

1. Divide the printer resolution by the screen ruling chosen in setting up

the PPD.

2. Square the integer value (discard the remainder or decimal), then add

1 to the result. This is the approximate number of tints the printer is

capable of producing with these two variables.

3. If the number calculated for the initial number of tints available is

higher than 256, reduce that number to 256 because the PostScript

Level 1 graphics language cannot create more than 256 different tints

per ink. (Some PostScript Level 2 devices may allow more than 256

tints per ink.)

([Resolution (dpi)]/[Screen Ruling (lpi)])[Squared]+1 = [initial number

of tints available]

For example:

If printing a job at 150 lines per inch (lpi) at 1270 dots per inch (dpi),

this formula yields roughly 72 different tints available. If the resolution

is raised to 2540 dpi, then 287 different tints are available, but since

PostScript Level 1 is only capable of 256 tints, the number 256 must

be used.

4. Calculate the percentage of available grays or the actual number of

tints available.

Express the "percent change between start and end" as a decimal

(e.g., if the gradient goes from 20% to 80%, the tint change is 60% or

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(e.g., if the gradient goes from 20% to 80%, the tint change is 60% or

.6).

[Initial number of tints available] x [amount of

tint change in gradient] = [actual number of

tints available]

Note: This step is necessary because a tint change of less than 100%

uses less than 100% of the total available range of tints (e.g., 60% of

256 tints, or 153 tints).

5. Measure the object in the direction of the gradient and convert that

length into points (there are 72 points in an inch).

6. Divide the length by the actual number of tints available to determine

the size of each tint band. This number reflects the exact width of any

banding that might occur in the gradient, and is the minimum width of

objects used to create the gradient.

[length of area to be filled]/[actual number of

tints available] = [size of each tint band]

For example: If the fill is 4 inches (or 288 points) long and has 153

tints available, then each tint band will about 1.9 points wide. If the

calculation specifies the band size at less than 1 point wide, banding

will not be visible. If between 1 and 2 points, this is adequate for most

circumstances. If over 2 points, it is up to the discretion of the

designer as to whether the quality will be suitable for the type of work

produced. In most cases, as long as there are 256 total tints

available, the human eye is less likely to perceive any banding once

the final work is printed.

When predetermining banding that may occur in process color

gradations, it is important to remember that four separate inks are

involved, each having its own degree of banding. It is best to

calculate banding for the ink that has the most change in color, and

the ink with the least change. This gives the complete spectrum of

what banding to expect. Just run the color change in each ink through

the formula above.

Heavy banding in a lighter color (i.e., yellow) will be less evident when

the final product is printed than banding in a darker color (i.e., black).

E. Adjusting Known Variables to Reduce Banding

The variables that effect banding are the four numbers needed to

predetermine banding: the device resolution, screen ruling, tint change in

the gradient, and the length of the gradient.

To adjust a gradient that is banding:

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1. Determine the variables that can be altered:

a. Are 256 tints already available from the first equation? Raising

resolution or lowering screen ruling after passing 256 tints is

pointless because PostScript Level 1 can only produce 256 tints

per ink. If the 256 limit has already been reached, adjust the

remaining variables.

b. Does the nature of the job dictate a strict requirement for screen

ruling? If it cannot be changed, adjust other variables.

c. Adjust the remaining variables, adjusting first the variables that

have the least effect on the visual composition.

2. Adjusting variables to reduce banding:

a. Raise the resolution. Raise until the formula shows the gray bands

are fine enough or until the number of initially available grays

passes 256.

b. Lower the screen ruling. Lower the screen ruling value until the

formula shows the bands are fine enough or until the number of

initially available tints passes 256

c. Increase the difference between the starting and ending tints in the

gradient.

d. Decrease the length of the gradient.

F. General Tips

Do not throw in extra gradient steps to create a smoother effect. The

number of tints produced is related to the results of this formula. Enter only

the number of steps needed to match that result. Specifying a number

significantly more than the available tints is extra information that the printer

cannot use but still has to process (RIP), making the print time considerably

longer with no increase in quality.

TechNote Details

Last Update: 07-06-2006

ID: tn_3668

Permanent Link: http://www.adobe.com/go/tn_3668

Products Affected:

FreeHand