Creating Custom Halftone Dots

When we think about halftone dots we're usually thinking in the traditional terms of Round, Elliptical, Square, etc., however, halftone dots don't have to be restricted to such simple shapes. For specialty projects you might consider using a custom halftone dot (click on images to enlarge).

Perhaps a "Star" dot to reflect the iconic status of the subject:
Or you could even use the subject itself as the dot shape:
How to create a custom halftone dot

A halftone screen is built using several components. The two that are needed to create a custom halftone are the "spot function" which defines the shape of the dot and the "threshold array" which determines how each dot is created.
The spot function can be any graphic, including a photographic image. It should be visually simple, made up of 256 levels of grey and fit into a square shape.
For this example we'll use the Apple logo for the spot function - making it our custom halftone dot.To convert it into a threshold array we'll use the blend tool in Adobe Illustrator:On the left is the original logo in Illustrator. Next is the logo at two sizes - the small black apple in front of the larger white logo. The third graphic is a 256 level Illustrator blend of the small black apple and large white one. The graphic is then imported into PhotoShop and cropped to minimize the amount of white in the graphic. This will become our threshold array: The Apple logo threshold array.
To apply the custom halftone dot:

1) In Photoshop, reduce the apple image to make a "dot" the size desired for the final image. For example: an image that is 75 pixels wide would make 8 dots per inch for a 600 pixel wide image. Use "Image"--> Image Size to change the entire image as required.

2) Select the apple image with the Rectangle Selection Tool. Then choose Edit--> Define Pattern. Give it a name (in this case "Apple logo") and press OK.
3) Halftone the original image by choosing Image--> Mode--> Bitmap. For "Method", choose "Custom Pattern" and then choose the "Apple logo" pattern. Then enter an Output Resolution. The amount entered will determine how small the halftone dot will be in the final image. The higher the number, the smaller the dots. Choose a resolution that is a multiple of your target output device's resolution. For example, if your output device has a resolution of 2,400 dpi, choose 2,400, 1200, 600, or 300 dpi for the bitmap.
Click OK.

Voila! Steve Jobs is now rendered with a custom halftone using the Apple logo as the dot shape.
While black and white images are the easiest to do as well as the most effective, it is possible, with a bit more experimentation to do a 4/C image. This one uses the Star dot. Click on image to enlarge:

Because you can't rotate the halftone screens for each of the process colors - the trick is to take each process color channel and rotate it to the correct angle, screen it and then rotate back so that they overlay correctly and recompose the image. Here is the step by step process: Open each channel as a new document. Rotate each channel to the correct angle: C +15º, M+75º, Y 0º, K +45º.
Convert each channel to a bitmap using the pattern/threshold array. Convert each channel back to greyscale. Rotate each one back to its original 0º state, C -15º, M-75º, Y 0º, K -45º. Finally, return each bitmapped channel into a composite CMYK image and align the channels.

FM Screening Halftone Dot Shapes/Patterns

The choice of FM halftone dot shape is important because it has an impact on plate choice, plate imaging, the aesthetics of the final presswork as well as on the lithographic performance of the press. Because there can be a wider variation of FM dot shapes than AM, this post will just illustrate several representative dot shape options. Each vendors' offerings can then be evaluated by using the information in this post as a guide.

In an FM screen each dot is formed in a halftone cell, typically based on a grid of 16 by 16 pixels. The pixels within the cell are "turned on" in pseudo random fashion in order to form the FM dot shape or tone area. The cells are then stitched together, like a mosaic, in order to form an area of dots or tone area.
On the left (enlarged) is a single halftone 16x16 pixel cell with several pixels turned on. On the right (reduced) is a tone area defined by a mosaic of sixteen individual 16x16 halftone cells.
Following are some basic FM screening dot shapes/patterns and their performance characteristics in use.

Click on the images to enlarge.

First Order FM: Dots are all the same sizeBenefits:Image has a photographic quality even when viewed under a loupe. Often used for fine art reproductions.
Issues:Grainy as well as mottled looking flat tone areas, small dots are more difficult to image consistently on plate and hold on press. Because the issues usually outweigh the benefits, this FM dot pattern is seldom used today except for specialized work.

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Second Order FM (a.k.a. Hybrid FM): Dots grow in the tone scaleSecond order FM screens are the de facto standard in today's print production. With this type of halftone screen the dots grow in size through the tone scale. Dot growth can be in one direction – perhaps forming worm-like features as in the first example (Kodak Staccato), or grow in both directions – forming more conventional looking dots as in the second example (Screen Spekta).
Benefits:Depending on the specific vendor's implementation, graininess in flat tone areas is eliminated.
Issues:Thin one or two pixel wide worm features, as in the first example, may demand higher resolution plates, and/or imaging in order to maintain consistency. Dots shapes, as in the second example, may cause a propensity for shadows to plug on press.

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Second Order directional FM: Dots grow directionally in the tone scaleDots have a strong directionality. Sometimes this dot shape in a vendor's FM offering is used for only one of the process colors in order to reduce "clumping" or secondary patterns when process colors overprint.
Benefits:Eliminates secondary patterns when process colors overprint.
Issues:Directionality of the dot shape can exaggerate directional issues, such as slur and doubling, on press.

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Nasty FM: Dots are plain uglyThis graphic is just intended to emphasize the fact that there is a great variety in FM screen patterns. Therefore one should not apply general statements such as "FM screening is grainy" – quality will vary according to each vendor's implementation.

AM Screening Halftone Dot Shapes

The choice of halftone dot shape is important because it has an impact on the aesthetics of the final presswork as well as on the lithographic performance of the press. This post describes the basic dot shape options available with conventional "AM" halftone screening. The descriptions apply to all the various vendors' offerings – though there will be some very subtle variations between their various implementations.

When dots grow in size from highlight to shadow there is a point at which the dots first touch each other. When that happens there is an effect called the “optical bump.” In a gradient blend this may show up as an artifact of a dark line in an otherwise smooth blend. Another issue is that, because paper moves through a press from printing unit to printing unit, presses are effectively directional imaging devices. This means that halftone dots shapes that are also directional, can interact with the directionality of the press and may exaggerate some issues such as slur and doubling.

In an AM screen each dot is formed in a halftone cell, typically based on a grid of 16 by 16 pixels. The pixels within the cell are "turned on" in order to form the dot shape. The cells are then stitched together, like a mosaic, in order to form an area of dots.
On the left (enlarged) is a single halftone dot within its 16x16 pixel cell. On the right (reduced) is an area of dots within their 16x16 pixel cells.
Following are the basic AM screening dot shapes and their performance characteristics in use.

Click on the images to enlarge.

Round dot: Dots are round through the tone rangeBenefits: Dot shape is the same for all screen angles and frequencies, optical bump is hidden in the shadows at the 75% tone, dot is non-directional so it is less affected by press problems. Reduces single channel moiré issues. Dot is non directional, i.e. all screen angle dots react the same to directional press issues such as slur and doubling
Issues: Not suited for film-imaged plates because the diamond shape that results at 75% and darker tones is very sensitive to dot gain and sudden loss of shadow detail. Excellent for computer-to-plate imaging because of the greater integrity of halftone imaging as well as the ease of dot gain compensation with tone reproduction curves.

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Euclidean dot (a.k.a. Transforming Round Dot): Round/Square/RoundThis is the classic dot shape that resulted from the original etched glass screens from the 19th century and is now replicated in digital form.
Benefits: Dot shape, except for 50% tint, is the same for all screen angles and frequencies, dot is non-directional so it is less affected by press problems. Shadow dots are less prone to dot gain - especially in a film workflow – than the simple Round dot.
Issues: optical bump occurs at 50% midtone tint which puts it in the most important and visible image tones.

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Elliptical dot (a.k.a. Transforming Elliptical Dot, Chain Dot): Rounded corner diamond shapeBenefits: Optical bump is moderated by being split into two – when the dots first touch at the long width at the 40% tint and then again at the short width at 60%.
Issues: dot shape varies at different screen angles which can cause single color moiré and uneven dot gain. Dot is directional, at low lpi frequencies the “chaining” of the dots as two points touch can cause lines to appear as artifacts. Directional problems on press such as slur and doubling can cause strong tone and color shifts depending on the angle of orientation of the dots relative to the angle of the paper as it travels through the press.

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Square dot: SquareHistorically used for catalog work, letterpress, and specialty work.
Benefits: Gives an impression of a "sharper" looking image on press thus reducing the need for excessive sharpening in image editing applications.
Issues: Very prone to loss of shadow detail due to very thin spaces between shadow dots.

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Line dot: LineHistorically used for newspaper work, lenticular, and specialty work.
Benefits: At lower screen rulings can have a strong graphic effect.
Issues: Directional problems on press such as slur and doubling can cause strong tone and color shifts depending on the angle of orientation of the lines relative to the angle of the paper as it travels through the press. Directional lines can introduce very visible subject moiré. Image edges can appear ragged.

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Specialty dot:
Any custom dot shape designed to meet specific requirements.
Benefits: Halftone dot optimized for specific applications.
Issues: Requires extensive knowledge, development time, and testing in prepress and pressroom in order to implement.

Here are two examples:

Pepper Dot:Uses small dots within larger conventional dots in order to reduce ink usage on press – especially for newspaper work. (Click on image to enlarge)

Novelty Dot:Uses a recognizable image or graphic to form the dot. (Click on image to enlarge)