Three-colour greyscale
April 20, 2010 4:52 AM   Subscribe

How can I create a three-colour greyscale of a digital colour photograph (i.e. a version in black, white and only ONE shade of grey)?

I've recently ventured into the field of shadow or illusion knitting (check out the Mona Lisa, she's amazing!), which allows the creation of hidden images in knitted objects using three shades: two main colours, with a third shade possible as a combination of those two. I want to be able to turn photographs into knitting patterns for use in this technique, but have so far been unable using paint or photoshop to convert a normal, digital colour photograph into a greyscale that uses only three shades (black, white and a single grey). Does anyone know how to do this?
posted by Brentusfirmus to Computers & Internet (8 answers total) 4 users marked this as a favorite
In Photoshop:
Open image
Convert image to grayscale (Image>Mode>Grayscale)

Convert grayscale image to bitmap (Image>Mode>Bitmap)
When the "Bitmap" window pops up, select "Halftone screen..." or "Diffusion Dither" under the Method section.
Press okay
posted by kisch mokusch at 5:30 AM on April 20, 2010

I think you want to Posterize, rather than dither. After converting to grayscale, choose Image>Adjustments>Posterize…, then 3 levels
posted by Etaoin Shrdlu at 5:34 AM on April 20, 2010

Also try adding a Gaussian Blur before Posterizing for a smoother transition.
posted by Etaoin Shrdlu at 5:36 AM on April 20, 2010

What I would do is a multi-stage process in something like GIMP or Photoshop.

To start with, do a Gaussian blur to get rid of fine detail (which will cause rough edges).

Then threshold the stuff you want to be white - do this by selecting by colour and varying the range of the selection until you get the right amount of white for the image; fill that selection in white. Do the same for the black. Colour everything else in grey, and if that doesn't look right, start again.

The thresholds are going to be different for each image, and you might want to vary the level of blur per image too as it kind of depends on the original. You might find it useful to do a contrast stretch before you do the thresholding - you could do this using curves or just by adjusting contrast.

All of these functions are in pretty much every graphics package - what do you have installed? If nothing, GIMP is powerful and free.

(And that's some cool knitting.)
posted by handee at 5:38 AM on April 20, 2010

If you have photoshop you can do a gradient map.

Image->adjustment->gradient map

Start with the black->white gradient and double click on that to bring up the edit window. By creating new stops you can make it 3 colours only - you'll need 6 stops in total - eg. black at 0 and 33 then grey at 33 and 66 and white at 66 and 100. From there you can adjust the levels of black/white/grey by moving the sliders around to get the right balance. You can also save the gradient as a preset so you can use it as a starting point for future conversions.
posted by missmagenta at 5:38 AM on April 20, 2010

I will also point out that the Halftone screen approach can also help create the illusion the effect:

Frequency: 8-10 lines/inch
Angle: 0 or 90 degrees
Shape: Line

Just stretch the image as desired afterwards (Image>Image Size) to fine tune the illusion (just remember to unclick "Constrain Proportions" so that you can adjust the width and height independently).
posted by kisch mokusch at 5:53 AM on April 20, 2010

(frequency above assumes a 72 pixels/inch image, and only gives a two-tone image, so may not be what you're after)
posted by kisch mokusch at 6:01 AM on April 20, 2010

If you're using the GIMP (which has a stupid name, but is free) then select Image -> Mode -> Indexed and choose 3 colours. Then Image -> Mode -> Greyscale to reduce those 3 colours to shades of grey. Or convert back to RGB and desaturate for slightly better results.
posted by alby at 8:39 AM on April 20, 2010

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