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I have some design diagrams only on paper. Scanning them to bitmaps is easy, but I've had no luck getting useful vectors out of them. I've tried vectorizers in programs like gimp, and a few online services. Generally, I end up with enormous numbers of spurious vectors (from dust, dotted lines, text on the diagram, slightly variations in scanning contrast, etc).

What tools and/or techniques can I use to get a more useful vector result, that I can then modify in a normal CAD tool without spending absurd amounts of time cleaning it all up first?

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There is the capable but somewhat expensive Scan2CAD.

Otherwise, if you're happy with outlines and not centre lines, scan b&w, aggressively clean up macules, mask off text, and then vectorize in potrace, autotrace, etc. Alternatively, load the bitmap at the correct resolution into a drawing package as a raster layer and draw the lines/objects you want over it. This avoids the horrors of dotted lines.

Both ways are quite a bit of work, sadly.

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I would recommend using something like Paint.NET to "fix" the images before attempting to convert them to CAD.

To my knowledge, most of the Image-to-CAD applications are going to use the grayscale intensity of each pixel to get the Z-axis value. So, you can help this process by pre-filtering the image into grayscale and playing with the contrast until you get enough distinction between the features you want to stand out in your CAD model or print. Here's a quick example using a sample image:

Poor Contrast Poor Contrast

Better Contrast Better Contrast

Whichever program you use will generally have an easier time detecting the edges of the flower pedals in the second image the further towards the center it goes.

GIMP has greyscale and contrast tools as well.

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