The Photoshop Dither Filter package is a set of filters for Adobe Photoshop that dither grayscale images to black and white so that they can be printed on a bilevel device, such as a laser printer. It currently consists of two filters: VGDither and PNDither. When printing to a laser printer, these dithering algorithms may be superior to the halftoning algorithms that are built into Photoshop. Why?
Laser printers suffer from a problem called dot gain: isolated black pixels on the printed page are larger than they “should” be. In fact, a certain amount of dot gain is unavoidable: since the dots that a laser printer makes are round, each dot has to overlap slightly with its neighbors so that the laser printer can produce a solid black. What this means is that most laser printer output is slightly darker than it should be. In practice, most laser printers produce dots which are somewhat larger than this minimum necessary dot size, making the problem even worse.
The standard PostScript halftoning algorithm (which Photoshop calls “Halftone Screen”) compensates for this dot gain by clustering black dots together—it doesn’t matter if a dot is larger than it should be if the dot it overlaps is also black. Unfortunately, by clustering the dots, this halftoning algorithm loses some of the detail in the original image.
Another of Photoshop’s algorithms, Diffusion Dither, produces an image with much more detail than Halftone Screen, but it makes no attempt to compensate for dot gain. Since it produces many isolated black (and white) dots, the printed results are much darker than they should be. It is possible to partially compensate for this darkening by adjusting brightness and contrast before converting to a bitmap, but this adds an extra step and the results may still be unsatisfactory.
Each of the filters in this package uses a different method to attempt to compensate for dot gain. More information about them is given below.
Installing and using the filters.
Drag the VGDither and PNDither files into the “Plug-ins” folder in the folder which contains your Photoshop application. That’s it!
The next time you start Photoshop, a new “Halftone” submenu will appear in the Filter menu, and the filters you have installed will appear in this submenu.
To remove the filters (though I can’t imagine why you would want to do that : -) ), simply drag them out of the Plug-ins folder.
These filters work only on grayscale images; if you attempt to use it on RGB image, it will ask you to convert to Grayscale first. Furthermore, before printing your converted image, you should convert it to a Bitmap using the 50% Threshold method. If you don’t, what prints will not be what you expect!
VGDither
The VGDither filter dithers a grayscale image to black and white using the Velho-Gomes algorithm [1]. The Velho-Gomes algorithm attempts to steer a course between the Halftone Screen algorithm and the Diffusion Dither algorithm. It clusters dots to minimize the effect of dot gain, but does so in a way that keeps more detail than the Halftone Screen algorithm.
The “Cluster size” parameter controls how many pixels VGDither considers when painting pixels black—the larger the number, the more pixels it considers at one time, and the larger the groups of black pixels. The default value of 11 is a good compromise between preserving detail and the original tonal values. Larger values will produce grainier pictures, while smaller ones will not do as good a job of preserving the brightness and contrast. A cluster of 1 will produce results that are very similar to those produced by Diffusion Dither, but not nearly as quickly. Note that not all values will produce results that are as good as you might expect! If you try a cluster value that produces large horizontal or vertical clusters of dots or other artifacts, try a slightly different value. Through experimentation, I have found that 14 and 23 also produce good results; other good values also exist.
[1] Velho, Luiz and Jonas de Miranda Gomes, “Digital Halftoning with Space Filling Curves.” Computer Graphics, v25 n4, July 1991.
PNDither
The PNDither filter implements a modified version of the Pappas-Neuhoff algorithm [2]. The algorithm described in this paper is a version of the Floyd-Steinberg Error Diffusion algorithm that uses a physical model of a laser printer to take into account how making a dot black will actually affect the blackness of the page. PNDither modifies this algorithm by adding blue noise [3], which reduces the patterns that the Floyd-Steinberg algorithm tends to produce. What this means is that PNDither should produce results with as much detail as those produced by Photoshop’s Diffusion Dither, while doing a much better job of preserving the tones of the original image.
The “Dot diameter” parameter is used to tell the filter how much dot gain to compensate for. An ideal printer, one with the smallest dot that allows it to produce a solid black, would have a dot diameter of 1.0. The default value of 1.25 should be about right for most laser printers. If you find that the printed results are too dark, use a larger value; if too light, use a smaller one. Allowed values are in the range 1.0 to 1.414. Note that since the filter is compensating for distortions produced by the printer, it is not possible to judge on screen exactly how the printed results will look; you need to actually print them to do that.
[2] Pappas, Thrasyvoulos N. and David L. Neuhoff, “Model-Based Halftoning.” SPIE, Vol 1453 (1991).
[3] Ulichney, Robert A. “Dithering with Blue Noise.” Proceedings of the IEEE, Vol 76, No 1, January 1988.
Future work.
If I am given enough encouragement, these filters could be made to run faster and to use less memory. If you use VGDither or PNDither, let me know! (See contact information below.)
In addition, I am currently at work on filters that implement other halftoning algorithms. Some of them may produce results even better than the ones in this package.
Copyright and contact information.
The Photoshop Dither Filter package consists of the files:
README (this file)
PNDither (version 1.0b0)
VGDither (version 1.0b0)
It is Copyright 1994, David Hull. All rights reserved. However, the author grants permission to distribute copies of this package as long as they are complete and unmodified. No money may be charged except for nominal copying and distribution charges.
I can be reached for suggestions, encouragement, or bug reports at:
