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I want to manipulate a photo so that the number of colours is reduced significantly (to say 15-50 colours which I would specify the number). I would like those colours to be automatically optimised for the photo in question.

(As a bonus, it would be great if the result could be manipulated in interesting ways so that if the boundaries of the colours were traced onto paper, then filled in by good-old-fashioned paint, the result would look more natural, or interesting - anything other than pixellated basically. (For example, plain dithered pixels would be impossible to paint by hand)

I would like free (open source) software to do this. I have searched for GIMP plugins but I have not found anything. I don't think GIMP implements custom/optimal quantization out of the box.

In the past I remember seeing some android apps which did quantisation plus the bonus features, but I suspect they had resolution limitations among others - last but not least, it was only available on a phone, and PC (Windows) would definitely be preferable.

Any ideas welcome!

  • AFAIK GIMP conversion in indexed mode (Image -> Mode -> Indexed...) perform an optimization of the colors without need of plugins. You can edit the palette for the unused colors. – Paolo Gibellini Jan 26 '14 at 23:18
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As Paolo Gibellini and jsbueno said, the Gimp image mode conversion can calculate an optimized colour palette for as many colours as you want, or can use a colour palette that you specify. The docs are here:
http://docs.gimp.org/en/gimp-image-convert-indexed.html

However, the result will be as you suspected, very pixelated around areas where one colour is transitioning to another. You need a way to separate out lines and fine detail from the predominant colour regions.

The "wavelet decompose" filter is a great plug-in for doing just that. It separates the image out into multiple layers, such that the bottom layer is a blurred version of the image and each successive layer adds extra details back in using "grain merge" mode. Wavelet decompose is usually used for touch-ups and noise reduction, but I was able to put it to use for your purpose.

Here's the process I used:

  1. Apply wavelet decompose with the default settings, then (in the Layers toolbox) successively turn off the visibility of the top-most layers until you find a level of blur that show solid colour blocks but still has the important detail. That was wavelet scale 4 for me. The layers above this will be your "detail" layers and the layers below will be your "colour" layers.

  2. Make all your layers visible again, and use "merge down" to combine all your detail wavelet layers together (i.e., wavelet scale 1, 2, and 3 for me) and and all your colour layers together (i.e., wavelet scale 4 and 5 and the wavelet residual). Be careful not to merge your original image, in case you decide to start over. You'll have to manually set the mode on the merged detail layer back to "grain merge" -- it switches to "normal" when you merge.

  3. Select the merged colour layer and copy it, then paste as a new image (you need to do this as a new image, since indexed mode applies to the whole image, not just one layer). On the new image, go to Image>Mode>Indexed and pick the colour settings you want. (If you don't like the results, undo and try again with different colour settings.) When you've got a good colour-palette, copy the image, go back to the other file, and select "paste as new layer". In your layers toolbox, move the pasted layer to be underneath the details layer (which should be in "grain merge" mode).

  4. Select the details layer, and go to Color>Desaturate. Pick a desaturate mode that looks good to you (I used luminosity). By desaturating the details, you make sure that they only add gray lines, not new colours. The details layer on top of the converted colour layer should give you the colouring book effect you're going for. If you want to actually create a colouring book, you need another step:

  5. Duplicate the converted colour layer. Go to Filter>Edge-detect>Edge and find a set of options that finds the edges nicely (I couldn't tell much difference between them, and just went with the default). The result will be a layer with coloured edges on a black background. Use Color>Invert to get a white background, and then use Color>Desaturate to make it grayscale. To make all the gray lines equally dark, go to Color>Levels, and move the middle selector in "Input Levels" (i.e., the gamma correction value) to the right. The edge layer, plus optionally the details layer on top, will now be your colouring book outlines.

  6. If you want to combine the edge layer with the colour layer, use Color>Color To Alpha to make the white areas of your edge layer transparent.

Original:
JPEG of flower garden

Simple conversion to a 16-colour palette:
16-colour unoptimized flower garden

Colour-block version of the 16-colour palette:
16-colour optimized flower garden

Colouring-book style edges for 16-colour image, plus grayscale details:
Edges from 16-colour optimized flower garden

Colour-block version with edges and grayscale details on top:
16-colour optimized flower garden with black edges and details

Colour-block version with edges, no details:
16-colour optimized flower garden with black edges

I don't know what age range you're going for -- that final image is rather complex. Steps to try to create a simpler image:

  • use a more blurry version of the image for the colour indexing step,
  • use fewer colours
  • use a high-contrast colour palette instead of letting the computer pick an optimized palette (the computer optimization is intended to preserve as much detail as possible).
  • P.S. If you aren't interested in adding the fine details back in after the colour conversion, you could skip the wavelet decompose step and just use "Selective Gaussian Blur" to smooth out your colour transitions. – AmeliaBR Jan 27 '14 at 22:50
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Si, in fact, GIMP does procude that out of the box. Just go to Image > Mode > indexed and you will be prompted with a dialog where you can set the number of colors of the quantized palette, and whether to dither it or not.

Afterwards, you can further manipulate the generated colors through the Colormap dialog box (open it with Windows > Dockable dialogs > Colormap) It allows you to double-click on a color entry to change its value, for example.

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