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. Use Color>Invert to get a white background, and then use Color>Desaturate to make it grayscale. The result will be a layer with coloured edges on a black background. 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). If you want to actually create a colouring book, you need another step:ĭuplicate the converted colour layer. The details layer on top of the converted colour layer should give you the colouring book effect you're going for. By desaturating the details, you make sure that they only add gray lines, not new colours. Pick a desaturate mode that looks good to you (I used luminosity). Select the details layer, and go to Color>Desaturate. In your layers toolbox, move the pasted layer to be underneath the details layer (which should be in "grain merge" mode). (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". On the new image, go to Image>Mode>Indexed and pick the colour settings you want. 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). You'll have to manually set the mode on the merged detail layer back to "grain merge" - it switches to "normal" when you merge. Be careful not to merge your original image, in case you decide to start over. 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). The layers above this will be your "detail" layers and the layers below will be your "colour" layers. Wavelet decompose is usually used for touch-ups and noise reduction, but I was able to put it to use for your purpose.Īpply 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. 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. The "wavelet decompose" filter is a great plug-in for doing just that. You need a way to separate out lines and fine detail from the predominant colour regions. However, the result will be as you suspected, very pixelated around areas where one colour is transitioning to another. 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.Īs 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. I don't think GIMP implements custom/optimal quantization out of the box. I have searched for GIMP plugins but I have not found anything. I would like free (open source) software to do this. (For example, plain dithered pixels would be impossible to paint by hand) (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. I would like those colours to be automatically optimised for the photo in question. 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).
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