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I am trying to prepare colouring-in pictures as children's activities.

I have seen this question and attempted using this kind of technique. It works well for touching up existing line art, but isn't appropriate with arbitrary images.

I have also tried using Posterize in Gimp followed by edge detection and threshholding. I have also tried Trace Bitmap functionality in Inkscape, but I find that it that most of my attempts are producing too many tangled contours to be useful for colouring-in without a lot of time consuming cleanup.

I am wondering is there an existing filter or script that automates this process for Gimp or Inkscape or recipes to use with existing filters.

I am using Gimp and Inkscape (but I may consider Software-as-a-Service or a commercial solution if there was something that works really well.)

EDIT I have added two attempts below for clarity.

Attempt using Inkscape Trace Bitmap

Below is an example of my attempt with Inkscape using this NASA image of a tardigrade. After the image is Bitmap Traced, the Fill is removed and Stroking is added to produce the contours.

Inkscape Trace Bitmap settings

Inkscape results

Attempt using Gimp Posterize with Inkscape Trace Bitmap

This uses the same settings as above but with initial Posterizing.

enter image description here

  • Are you trying to prepare flat image sketches (like in the line art example) or photo jpegs? You mention arbitrary images, creating line art from any image will be problematic. From a pencil sketch its not too bad. – Lex Jan 12 '16 at 6:26
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    Automatic methods work ok for some types of images, but not all. It also depends on what quality you're expecting from it. Tracing it manually would give you the best possible quality with any image. Also, why not just google some pictures? – Joonas Jan 12 '16 at 7:12
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    It really depends on the type of image, what you already tried in Gimp is pretty much as good as it gets. – Patareco Jan 12 '16 at 9:41
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    As @Patareco says, it depends on the type of image. In Gimp you can also use threshold or posterize to reduce the color depth before the tracing in Inkscape (this, generally, improves the result). You can also select the subject using the free select tool or using a layer mask and delete the background, but if the starting image isn't well contrasted, it could be a long task. In my opinion there isn't a general purpose automatic way to convert an image to a cartoon or to a simple vector. For your example I suggest you to trace the image using levels of colors, the result should be better. – Paolo Gibellini Jan 12 '16 at 12:48
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    @PaoloGibellini how could I forget threshold, the easier way of having smooth painting using MSPaint software! – Patareco Jan 13 '16 at 18:37

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