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As I understand it, halftoning is the process of taking an image with a given color range (as defined by the colorspace) and blending the colors to form an image with a smaller range of colors.

How is this different from dithering? Or are the two essentially the same thing?

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4 Answers 4

up vote 11 down vote accepted

I disagree with John's assertion that the difference depends on how many colours you use, I believe it's possible to dither using a single colour.

Halftoning is a term used in the print industry to describe how to reproduce varying tones with significantly fewer inks. It's usually synonymous with amplitude modulation halftoning, where a fixed pattern of dots vary in size in order to create the illusion of continuous tone.

Left: two examples of monochrome dithering, firstly pattern dithering and secondly diffusion dithering. Right: two examples of colour halftoning by amplitude modulation, the first with a very large dot size, the second with a smaller dot size that is more convincing.

When colour halftoning, the grid of circles for each ink colour are set at different angles so they don't land on top of each other.

Dithering is a more general term that refers to randomisation or perturbation of colours values or positions or intensity in order to simulate more tones than are available. Essentially both terms try to achieve the same effect, to fool the eye into seeing more colours than are there, and you could argue that halftoning is a form of dithering.

Like all terms they are misused, but to me the difference is that you wouldn't usually talk about halftoning if you were working with images for display on a computer. Whereas, if you were talking about printing then you would usually use halftoning to refer to amplitude modulation (little circles), and dithering otherwise.

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I never said what the size of the palette had to be... –  John Cavan Dec 17 '10 at 11:39
    
@John the gist of your answer seemed to me to be that halftoning was monochrome whereas dithering used different colours. Maybe I misinterpreted it. How would you characterise the difference then? –  Matt Grum Dec 17 '10 at 12:31
    
That was the gist, but that doesn't mean dithering requires more than one, just that it usually does. Anyways, I was responding to you more as a joke and, to be honest, I think your answer is much more complete, I wrote my just before going to bed. –  John Cavan Dec 17 '10 at 12:41
    
sorry I didn't get the joke! –  Matt Grum Dec 17 '10 at 12:58
    
I think this answer gets the big points quite well. I'd just like to point out, though, that the wikipedia entries seem also to do a decent job of describing each in ways where the differences become apparent (to me, anyway) -- en.wikipedia.org/wiki/Halftone and en.wikipedia.org/wiki/Dither –  lindes Dec 18 '10 at 7:01

I work in the printing industry and my understanding of halftone mostly matches up with Matt Grum's.

Creating a halftone involves breaking up an image into dots (or similar shapes) all of equal ink density but varying in size. The dots are arranged in a regular grid. A black and white halftone would consist of black dots with large dots for the darkest areas of the print and very small (or no dots) for the lighter areas of the print. A full-color halftone involves dots of varying sizes in the colors of Cyan, Magenta, Yellow and Black -- with the color inks being used to optically produce other colors (Cyan + Yellow = Green). As mentioned above this is an amplitude modulation technique.

Frequency modulation in printing normally means a pseudo-random pattern of dots where the number of dots in a given area is changed to make that part of the image darker, but the tone of all the dots remain the same. You can use frequency modulation for printing color images as well.

Dithering has a specific technical definition used more in digital images instead of the printing world. According to Wikipedia -- "Dithering is a technique used in computer graphics to create the illusion of color depth in images with a limited color palette (color quantization). In a dithered image, colors not available in the palette are approximated by a diffusion of colored pixels from within the available palette. The human eye perceives the diffusion as a mixture of the colors within it (see color vision). Dithering is analogous to the halftone technique used in printing. Dithered images, particularly those with relatively few colors, can often be distinguished by a characteristic graininess, or speckled appearance." (http://en.wikipedia.org/wiki/Dither)

I suppose that dithering could use FM or AM techniques, but Wikipedia's example looks more like FM (although the grid is regular in the example).

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Reading more on Wikipedia, it looks like AM and FM screening are both types of dithering, if you look at things from a mathematical view. –  David Rouse Dec 17 '10 at 14:53

They are, to some degree, very much the same thing.

Halftone is single colour, usually black, dots which are sized and spaced appropriately to convey the tonality of an image and has been used in printing, especially newsprint, for a lot of years. Dithering is a way of combining a number of differently coloured dots to convey colour using a limited set of possible options (the palette).

So, yes, essentially the same in that the selection and placement of the dots create the impression of tone or colour, but they are different in that one is monotone and the other is not.

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Halftoning is dithering on a line-based grid. Halftone resolution is defined as 'Lines Per Inch'. It's basically dots of varying sized aligned in a x/y grid. For color printing, as stated, the angle of the grid is altered for each color to help the eye blend the colors to gether.

Dithering can be a number of things (like being a halftone) or pointillism (like Wall Street Journal portraits). In printing, the common dithering technique is 'stochastic' or 'Frequency Modulation' printing. This uses very small dots of the same size. The density of the amount of dots is what creates the shades of color.

The latter requires higher quality inks, paper and presses, but ultimately produces an image much closer to continuous tone (film) than halftone printing does. I believe some higher end ink jet printers can use the latter method.

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The advantage of conventional halftoning is that the quality of output is far less sensitive to variations in the output process than many other approaches. Portions of a picture containing only near-mid-level grays could use a finer screen than lighter or darker areas, but conventional halftoning uses the same screen for all areas. When outputting a halftone picture to a printer with greater positive or negative dot gain than expected, light or dark areas may look bad, but mid-level areas will be fine. Other dithering approaches, by contrast, would make the entire picture look bad. –  supercat Mar 26 at 18:18

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