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Image Output
continued from page 1. While the issues covered here are generally beyond the DTP operator, benefit will come from an appreciation of, or respect for them.

This page looks briefly at the complex issues concerning the creation of halftone dots at various angles. To fully understand the different methods used to overcome the problems highlighted here, extensive reading must be undertaken. These pages (and site) are restricted in size and the number of images that can be used etc..

Rational, Irrational!
So far we have only looked at this topic using simple examples. A black to white greyscale image should output with the halftone dots set at an angle of 45 degrees, and 2 or 3 colour and CMYK imaging uses several different angles.

Animated halftone shapes

Through out these tutorials I have often sacrificed bitmap quality for a reduced file (byte) size, reducing download times. When creating examples that include halftone dots, exact shapes etc are impossible to achieve because only a few screen pixels are used at the standard 72ppi monitor resolution to depict the halftone dots. The jagged edges do not render accurately or consistently.

To some extent, this problem is also encountered when outputting digital halftone screens. In fact, probably the last big issue faced by the industry once fast CPUs and clever graphics software with powerful postscript were readily available, was the need to manufacture imagesetters that could output almost as well as the conventional camera and high end scanners.

Early halftone producing imagesetters created very noticeable moiré patterns. It was not so much that the angles could not be calculated accurately, but that each halftone cell would unpredictably create halftone dot shapes different to others, and although at very tiny sizes, the jagged edges upset the ideal rose pattern that we were introduced to earlier.

The animation above should highlight this sufficiently, where we can see an improved shape as the Output Laser Dot resolution increases. To get all possible grey levels appearing in our output, we must use a minimum laser dot resolution - i.e. 150# = 2,400 - but even at this seemingly high resolution the problems remain.

OK, output at 10,000 laser dots per inch and see what happens. A cut lunch and a slow camel ride around the city could be an option while we waited for the job to be RIPped and imaged. And then we would also have to have film and controllable lasers that rendered this extremely high resolution. Perhaps one day, but lets see, simply, how the problems are largely overcome.

Rational & Irrational angles The second pair of images on the left show us that like the simple 0/90 degree axis angles, 45 degrees also give us fairly good shapes (remember that the halftone dots grow in an irregular pattern where, one grey level up or down against another would show an odd bump or hole around the perimeter).

But look at the situation that arises once we move away from the 0/45/90 degree angles - mayhem and a poor rose pattern results! The faint lines describe the Halftone Cells overlayed on to the Laser Dot array and we cannot have half laser dots etc.. The 0/90 and 45 degree angles are referred to as Rational Tangent angles (the difference between the laser dot angle - 0/90 degrees and the halftone screen axis), and any variations from them are referred to as Irrational Tangent angles.

One widely used approach to help overcome the irrational angle problems is to use Super Cells where the angles are calculated according to the best average of many halftone cells and then applied accordingly. This results in some output angles requiring accuracy to 6, 7 or more decimal places, and they will change with the slightest alteration to the halftone screen ruling between jobs.

Luckily the DTP operator, while now understanding some of the complexities of their equipment and why they must confine themselves to strict disciplines and habits, does not have to calculate or adjust these values. Our job would be as useless as an ashtray on a motorbike if we had to calculate things to 8 decimal places all day! Leave well enough alone and leave it to the boffins that design this complex equipment and software. The newness (in design) and inevitably the cost of the imaging equipment is what counts and the difference in quality can be startling.

If you want to study this further, or delve into the variations between manufacturers, this introduction should prove to be a good primer.

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