Convert a line drawing from raster to vector **LINES**

Question

Here are already such questions, like:

• Convert a line drawing from raster to vector
• What is the best way to convert a series of boxes (right angles) from raster to vector?
• Converting B&W to vector line art?

... and others.

I have an B&W png image, contains a drawing like an maze, e.g. it contains only lines, no areas.

Need convert it into (any) vector format, but as LINES. Every program what i tried, e.g. VectorMagic, potrace etc. converts the lines into closed path areas.

Small part of the original PNG image - zoomed:

The result of the trace:

As you can see, the "line" is traced from its both side like any other area-like shape.

Looking for a way, how to trace lines into real lines (PATHS), best with the matching line-width. (but would be enough any line with).

The reason behind: need work with the traced lines and nodes, e.g. convert some nodes to bezier and like. Not possible (easily) with shapes.

Any idea:

• how to get vectorised lines from any tracing program.
• which tool can do this? (any OS - could use Windows, OS X, Linux..)

Answer 1

Centerline Tracing

There is a free Open Source tool AutoTrace which is able to perform a centerline trace of a line-art bitmap.

Run AutoTrace with at least the following options:

autotrace -centerline -color-count 2 -output-file output.svg -output-format SVG input.png 

We can then fine tune the strokes and add the desired stroke strength..

We may also install the graphical frontend Frontline for AutoTrace in case we are not familiar with the command line.

Linux users may be able to install autotrace provided from their default repositories in most distributions. For Windows there are precompiled packages to download form the project's page.

There are many additional parameters for AutoTrace which can be found in the application's man page or by calling it with autotrace -help.

To rather get straight lines than splines we can e.g. use the following parameters:

• -line-threshold [real]:
  If a spline does not deviate from the straight line defined by its endpoints by more than the specified number of pixels, then treat it as a straight line (default: 1).

• - line-reversion-threshold [real]:
  If a spline is closer to a straight line than this, weighted by the square of the curve length, keep it a straight line even if it is a list with curves; default is .01.

Update

In more recent versions (Inkscape 1.0+) centerline tracing using autotrace is included^* in Inkscape via Path > Trace Bitmap:

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_{^{* Reference: Inkscape 1.0 release notes}}

Answer 2

Quite honestly... grab the pen tool and manually trace the paths. It'll result in the best output.

Auto-tracing is often not the best option. When you only want paths you can stroke, auto-trace generally fails miserably.

Answer 3

Try using Interpolate Paths after your trace, as show here:

Average stroke from a fill

Answer 4

Illustrator can do center line tracing much like autotrace. Its not super useful for most things but in this case the image is highly synthetic and it might work.

Yor source image is notoriously bad so theres no real way to try this without losing quality, your originals should be better (no need to be so zoomed)

Do this:

1. In trace settings disable fills and enable strokes.

2. Object → Expand

Image 1: Object after trace and expand.

3. To further enhance the results from any tracing errors do: Object → Path → Simplify, check straight edges.

Image 2: After simplification

4. Seems to me your image is somewhat limitted in line directions, and corner locations, if thats true you could now try to quantitze the points for a even better (if not perfect) results.

Image 3: What quantizing might do to the result.

PS: given that the source that you shared is incomplete i would say my result as pretty good. While i agree that in general its not possible to rely on line tracing it would work in this case.

PPS: Your corners are overlapping, this causes auto tracers some grief You could detect and fix these with a few Morphology operators in say ImageMagick that should make tracers MUCH happier.

Answer 5

For Inkscape, there is also a centerline trace extension available on github: https://github.com/Moini/inkscape-centerline-trace (fork of https://github.com/fablabnbg/inkscape-centerline-trace)

Answer 6

I found a good freeware Tool called WinTopo, which for me did the job, i.e. centerline tracking.

The method is called "one touch vectorisation" and works well.

As I come from a GIS context - in the pro version there is an option to Georeference the raster data first - could be useful, not tested yet.

Answer 7

This is actually an answer to this question

Fitting vector curves/lines onto shapes in Illustrator

It's clearly closed by a single person who doesn't at all believe something useful can still be shown.

In the so called duplicates only the suggestion "trace it manually" is a good one. Unfortunately it's the same as draw a copy in Illustrator. It surely would be no problem for the maker of the original face drawing. He and his hands obviously know what he wants so well that he would draw a good copy on a graphics tablet in half a minute. But for others it needs well sharpened Bezier curve tool skills.

The next example shows how automatic middle-line stroke-only tracing fails in Illustrator:

In the left there's a raster image of a curve. In the right it's traced. In the crossings Illustrator couldn't guess the right path route and some unwanted twists are added.

You can make the tracing easier to Illustrator by erasing a little of the image in the crossings. Leave the most curved curve and erase pieces from the straighter branches so that there'a a well visible gap in both sides:

In the left the gaps are made in Photoshop.

In the middle there's the tracing result - expanded, ungrouped, the different sections are colored differently for clarity.

In the right the gaps are bridged. In this case I selected all and typed Ctrl+J to join the ends and nothing more.

If there's only a single open branch or three open branches coming towards the same crossing you must move one endpoint with the white arrow or extend the ends by drawing manually with the pen or line tool, because in Illustrator paths do not have three or more branch joints.