Illustrator problem rasterizing gradients

[thomas_hes...@adobeforums.com]

When i design something using a gradient and the printer tries to rip the file i they get really pronounced line divisions in the gradient. Tried making background in Phooshop, flattening and then setting in Illustrator and have same problem. What's going on here? Is Illustrator not powerful enough to build an accurate gradient on export?

+corwin

[LenH...@adobeforums.com]

Corwin,

How smoothly a gradient prints is dependent upon the distance the gradient covers, and the difference between start and finishing values.

Printing to a PostScript Level 2 device, even with a gradient going from 100% black to 0% white, there can only be 255 levels of black. As the start and end values become closer, the number of possible levels decreases, so for a 10% to 20% black gradient, you could only have 25 different levels. If these are 'stretched' over a large area, the gradient bands will become visible.

To determine the maximum blend length based on the color change:

1. Select the Measure tool , and click the beginning point and the endpoint of the gradient.
Note the distance displayed in the Info palette on a piece of paper. This distance represents the length of the gradient or color blend.
Calculate the number of steps in the blend using this formula:
Number of steps = 256 (number of grays) X Percentage change in color

2. To figure out the percentage change in color, subtract the lower color value from the higher color value. For example, a blend between 20% black and 100% black is an 80%--or 0.8--change in color.

3. When blending process colors, use the largest change that occurs within any one color. For instance, take a blend from 20% cyan, 30% magenta, 80% yellow, and 60% black to 20% cyan, 90% magenta, 70% yellow, and 40% black. This indicates a 60% change, because the greatest change occurs in magenta--from 30% to 90%.

4. Using the number of steps calculated in step 3, see if the length of the gradient is larger than the relevant maximum length indicated in the following:

No.of Steps Max Blend Length(inches)
10 0.3
20 0.6
30 0.9
40 1.2
50 1.5
60 1.8
70 2.1
80 2.4
90 2.7
100 3.0
110 3.3
120 3.6
130 3.9
140 4.2
150 4.5
160 4.8
170 5.1
180 5.4
190 5.7
200 6.0
210 6.3
220 6.6
230 6.9
240 7.2
250 7.5
256 7.7

[thomas_hes...@adobeforums.com]

we're talking from 100% to 0% back to 100% in a matter of 6 inches...

+corwin

[John_K...@adobeforums.com]

What Len posted is only 1 part of the equations needed to determine if bands will appear in gradients.

Second part is the device resolution in relation to lpi. (on a ps2 device)

Resolution-maximum lpi

300-19
400-25
600-38
900-56
1000-56
1270-79
1446-90
1524-95
1693-106
2000-125
2400-150
2540-159

and so on.

Both must be used to accurately determine whether a gradient will have visable banding.

[LenH...@adobeforums.com]

To make John's post a little clearer, the following may help:

For any given printer dpi, as the linescreen frequency increases, the number of reproducible grey tones decreases. Here is a ready-reckoner:Screen
frequency (lpi) Output device resolution (dpi)
150 300 400 600 1200 2540
30 26 101 179 401 1601 7169
40 15 57 101 226 901 4033
50 10 37 65 145 577 2582
53 9 33 58 129 514 2298
60 7 26 45 101 401 1793
65 6 22 39 86 342 1528
85 4 13 23 51 200 894
105 3 9 16 34 132 586
120 3 7 12 26 101 449
130 2 6 10 22 86 383
150 2 5 8 17 65 288
200 2 3 5 10 37 162
220 1 3 4 8 31 134
300 1 2 3 5 17 73

Intermediate values can be obtained by using the following formula:

(dpi/lpi)^2 = No. of available tones.

This does not take into account dot gain, tone splatter, PostScript limitations etc.

If you have the LJ4 set to 300ppi and a default linescreen of 53lpi (plc mode) then you will only get 33 distinct tones, which is not enough for even shading. At 600 dpi that will increase to 129 tones which will be much better.

But for what is normally accepted as 'continuous tone' (256 tones) you would have to drop the linescreen down to less than 40 lpi which would give a very coarse screen

[John_K...@adobeforums.com]

Thank you Len for making the information more clear.

[Hersh_We...@adobeforums.com]

I read your reply about lines printing in gradients. However I'm a output service bureau and I don't want to play with my clients files. How do I get around the problem easily. On my HP Designjet 5000 printer it actually prints a black line where the gradients step up or down.

Hersh Weingarten
Brooklyn, NY
MAC X
ILUSTRATER 10