Dithering full-colour images down to monochrome bitmaps
February 10, 2006 4:35 AM   Subscribe

Dithering full-colour images down to monochrome bitmaps?

By monochrome I mean either a black pixel or a white pixel, not greyscale. Does anyone know of any software or a tool (preferably free) that will do this? Saving as a monochrome bitmap in, for instance, MS Paint will just save each colour as black or white depending on which it is closest to and will not attempt to blend them, resulting in an unrecognisable image. This page, featuring the man with the rather extraordinary hairdo describes exactly what I am after but I’ve not had any luck in finding anything further besides technical descriptions of the mathematics behind it.

Do the predominant programs (Photoshop, Paint Shop Pro, etc.) have such a feature?

Thanks in advance!
posted by ed\26h to Computers & Internet (10 answers total) 2 users marked this as a favorite
Best answer: Just about any photo editing software can do that. It's a basic function to be able to convert from one color mode to another.

Photoshop can do that. If you are looking for a shareware solution, Irfanview should be able to do it as well.
posted by MegoSteve at 4:56 AM on February 10, 2006

Response by poster: Simple as that. Cheers buddy.
posted by ed\26h at 5:12 AM on February 10, 2006

Gimp is probably a better freeware choice if you don't have something like Photoshop. It has the power and probably will allow you more options in conversion than Irfanview.
posted by JJ86 at 5:54 AM on February 10, 2006

I second GIMP because it gives you four different options for dithering and its free and it does a whole lot more.
posted by psychobum at 6:32 AM on February 10, 2006

What they said, plus a fighting chance of finding the feature in the program you end up using: the technique it sounds like the you're looking for is halftone.
posted by mendel at 6:49 AM on February 10, 2006

One tip: you'll want to first convert the image to grayscale, then punch up the contrast to get a really good pixelisation.
posted by delmoi at 7:03 AM on February 10, 2006

mendel: Dithering is entirely different from halftoning. Halftoning involves converting the image to black and white shapes in a pattern to replicate tone; those shapes which make up the tone are made up of many pixels depending on the frequency setting you use (at least in Photoshop).

Dithering converts the photo to individual black and white square pixels. It results in a much finer dot pattern, looking more like pointillism in drawing.
posted by MegoSteve at 7:31 AM on February 10, 2006

At the risk of comment removal for a self-link, you can download a 30 day trial version of dotImage here.
In the demos is an demo app ("Atalasoft dotImage Demo") that will do what you want. Open the image(s), then do Commands->Effects->Reduce Colors.
Note - the company's product is not a paint program, it's .NET libraries for image manipulation, but the demo app is more than enough for this task.
posted by plinth at 7:44 AM on February 10, 2006

The effect pointed to in the question is called Diffusion Dither, in Photoshop. It's available in the conversion dialog from grayscale to bitmap. MegoSteve is correct, dither and halftone are entirely different effects.
posted by doctor_negative at 8:11 AM on February 10, 2006

Just to make this as easy as possible for anyone who finds this thread in the future, to do this in the Gimp:

Open image

Image menu → Mode → Indexed…

In the window that appears (“Indexed Color Conversion”), choose “Use black and white (1-bit) palette,” then hit OK.

The default dithering method is “Floyd-Steinberg (normal),” which is good in most cases, but it doesn’t hurt to try the other dithering methods as well to see what looks best. Undo is in the Edit menu and mapped to Ctrl+Z by default.
posted by hilker at 8:26 AM on February 10, 2006

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