I did it like this:

• Cut the image in 1 pixel horizontal strips.
• Go over the line, adding object start-X and end-X pixel color.
• Resize it by multiplying each start-X and end-X with the factor (2 or 4, 3 looks very ugly).
• Reconstruct the lines into a new image canvas.

As a 1 pixel width item will have the same start-X and end-X value, it will retain the 1 pixel width. And things that have different start-X and end-X values, will get streched as if normal...

edit: Then, you need to rotate the image 90 degrees, and do it again to strech it in the Y axis.

However, what you want is the inset function in inkscape, not a perl program, as your source is a bad quality jpg, instead of a pixel perfect png.... (so dithered pixels get in the way of the above mentioned algorithm... it gets confused)

Can you make an example image available somewhere?

I guess this would make a 1 pixel red border around a blue rectangle.

use Graphics::Magick;
$image = Graphics::Magick->new;
$image->Set(size=>'10x10');
$image->ReadImage('xc:white');

$image->Draw(stroke=>'red', primitive=>'rectangle', points=>'3,3 7,7')
+;
$image->ColorFloodfill(geometry=>geometry, x=>4, y =>4 , fill=>'blue',
+ bordercolor=>'red');
#$image->Set('pixel[1,3]'=>'green');
$image->Write('block_normal.png');
[download]

(Just) Horizontal and vertical lines are pretty easy to deal with.

On the other hand, if the OPs images contain diagonals, it gets much harder. Unless they are all the same angle, or just a few angles.

Of course, his images might contain curves, which gets harder still.

And if they contain a mix of all three ...

And, then there is black and white; or grey scale or 8-bit dithered; or 24-bit fullcolor; or 48-bit; plus or minus alpha channels ....

Attempting a totally generic solution is a waste of effort if his images only contain horizontal and vertical lines.

Posting a solution for horizontal and vertical lines is a waste of time, if all his lines are curves.

Knowing what you're up against is the key to an efficient, effective algorithm; and a picture paints a thousand words.

---

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Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

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In the absence of evidence, opinion is indistinguishable from prejudice.

Seems that tinypic enforces a maximum size of ~1600 (hence why two of your images appeared the same size), so I've switched the links below to imgbin.

How do this & this compare to your current solution?

Tweaked algorithm defaults: Sample 1: doubled and thinned Sample 2:doubled & thinned

Like in the example, you will need to take it slow, and not select the whole picture at once, but work each element separately for better effect, although selecting all and insetting 3 times gave the left tree a nice thin line, and the branches looked quite good.

If you are not satisfied with the conversion, use GIMP or another program to depekle (remove the jpeg artifacts) before converting. Also, convert to, say, 8 grayscale colors before you do that, it will save you time. Remember: For a good quality, you need to spend time. Heh... graphics answer on a Perl forum... sorry for that.