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I often want to edit or enlarge images I find in books or on postcards for things like wallpaper or coffee mug &tc. However most images in books are heavily dithered and the usual methods for removing dithering suck badly. undither (a newer version of Image) takes a different approach to the problem. It calculates the image's power spectrum, and write it out as a colour ppm file. Any dithering will generate (coloured) bright spots in the power spectrum. You can edit the power spectrum in your favourite graphics editor and draw over these bright spots in black. undither will then use this edited power spectrum the original image removeing all the energy in the image where you've drawn black on the power spectrum. undither can also do quite a good job of finding these bright spots by itself (but ony in colourful images). undither mmaps the images into core and internally deals with the image in 1024x1024 tiles, so you can process arbitary large images. undither requires the fftw library. The examples below should make it more clear.

Here is a postcard of the Pergamon altar in Berlin, I've drawn a red square arround Hecate's left hand, you'll see why shortly. Let's call this image perg.ppm.


Here's that little red square blown up, you can see the dithering the printer has used, on the right is the power spectrum which undither has made.
[james@lamia pergamon]$ undither -i perg.ppm -p ps.ppm

detail from perg.ppm ps.ppm
detail from perg.ppmps.ppm

Now you can either paint over the spots in the power spectrum yourself, or ask undither to do it for you.
[james@lamia pergamon]$ undither -i perg.ppm -a 0.01 -p akill.ppm
[james@lamia pergamon]$ undither -i perg.ppm -k akill.ppm -o automatic.ppm

or better still do it all in one step
[james@lamia pergamon]$ undither -i perg.ppm -a 0.01 -o automatic.ppm

This does a pretty good job - see below - but it can't get the monochrome spots.

akill.ppm detail from automatic.ppm
akill.ppmdetail from automatic.ppm

With a little help from the gimp I've drawn over the four white spots in akill.ppm and called it mkill.ppm, and then fed this into undther
[james@lamia pergamon]$ undither -i perg.ppm -k mkill.ppm -o manual.ppm

mkill.ppm detail from manual.ppm
mkill.ppmdetail from manual.ppm

As you can see that's removed all of the dithering.

Here's another example done entirely manually, it's part of a large japanese drawing

[james@lamia bell]$ undither -i bell.ppm -p freq.ppm

Detail from bell.ppmfreq.ppm

We load the image into our favourite paint program and draw over the bright spots and lines, then run it through undither
[james@lamia bell]$ undither -i bell.ppm -k filt.ppm -o out.ppm

filt.ppmdetail from out.ppm

Applying this filter gives us an almost perfect reconstruction of the image.

Take me to the files