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I can get good trace results in Inkscape converting a bitmap to a vector image - except for these persistent thin outline artifacts. They resemble a topographic map and pretty much ruin the trace.
There are no strokes on these paths, just fills. So there shouldn't be any visible outlines.
In case it helps, here are my trace settings:
Such artifacts appear in Illustrator traces as well. But specifically I'm seeking a solution for preventing or removing them in Inkscape SVGs.
The problem is basically in the way Inkscape is deciding to split your image into color regions: instead of simply having the lighter colors overlap the darker ones (or vice versa), you're ending up with two adjacent color areas that both overlap a third color.
Since the edges don't line up perfectly (due to both tracing inaccuracies, and also some fundamental limitations of anti-aliased vector image rendering), some of that third color ends up showing through the gap, creating the "edge" you're seeing.
Alas, I don't know of any good way to fix this issue. You can get rid of the background colors by unchecking the "stack scans" box, but then you get transparent gaps between all paths, which is generally even worse. Sometimes, it may be possible to obtain acceptable results by tracing the image with "stack scans" unchecked, and then applying a small outset (say, 0.5 px) to all the resulting paths to fill in the gaps, but don't expect this to preserve fine details very well.
For your specific example image, one possible solution might be to trace it in grayscale (i.e. selecting "Grays" instead of "Colors") and then re-coloring the traced image afterwards; the grayscale tracing mode doesn't seem to suffer from this issue (which I'd really consider a bug, or at least a design flaw, in Inkscape's color bitmap tracing). Of course, this only works well for images that are essentially monochrome, or otherwise have simple and easily recreatable coloring.
The artifacts you experience come from the supposed-to-be invisible strokes at the outer rim of the stacked objects created for each color chosen at tracing.
Artifacts may become worse in case we had applied a Gaussian blur to the bitmap before tracing. This is done by the smooth option in the Inkscape trace dialog. Therefore it was a good idea to not check this option.
Reproduction of the line artifacts
Now to get rid of the artifact is a bit cumbersome. We have to create a stroke to each of the 256 color objects we had created and set the stroke color identical to its fill color.
Ungroup the selected stacked objects (Shift + Ctrl + G)
Select an object (path mode helps to see we had selected the right one):
Open Object > Fill and Stroke dialog ShiftCtrlF
Copy fill color on the Fill tab
Open stroke paint tab and make a Flat color stroke of 1 px width.
Paste the fill color to the stroke color:
Repeat 2. to 6. for each of the objects in our trace.
One single line only was removed
We may find a script or external plugin which allows to define the stroke color matching the fill color for each object of an SVG but my preliminary search on this sadly was negative.
To also remove the still visible color step between neighboring objects we may add some blur effect to the objects but this will lead to a tremendous increase in rendering time of the SVG.
I'm using Inkscape 0.91 r13725 on Debian 8
Go to: Filters => Blur => Cross Blur
I left the Brightness at 0, Fading at 0
GUESSING that the gaps were 1 pixel I set Horizontal blur to 0.50 I set Vertical blur to 0.50
and there was noticable improvment, but not perfect. Hopefully this helps towards the perfection you seek. The image was too large for my underpowered laptop to experiment much.
The artifacts in fact come from the tracing process and the sorted colour order of the traced layers and the “stacked” method of showing layers. This method relies on stacking a layer, adding the paths of the next layer, stacking again etc. that way the colours don’t have gaps between them.
Eg if the colours are sorted by brightness then an image with a gradient from red to blue will have artifacts if the relative brightness is similar
If the colours are sorted by hue then the gradient will have a smooth apprearance.
If the image has a gradient from light to dark the. The colours would need to be sorted by brightness for this to look smooth.
I don’t know the sorting method for colours in Inkscape it is a possibly sorted by hue which means some artifacts will show on your image.
I only know this because I am working on developing an image tracing app and am coming up against similar issues
