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I have this cross which I've traced from a bitmap image:

traced cross

When selecting the actual path that makes up the detail, I see the following nodes:

With nodes shown

I am trying to obtain a similar cross whose lines are thicker, so that it will look better when scaled down as a white cross on a dark background. How do I increase the stroke thickness?

I understand that every line or curve visible in the image is in fact composed of many paths, because the paths formed by the nodes surround the strokes, which are filled in black, rather than the path between the nodes being the actual stroke. This is what is causing my issue in finding a "thickening" algorithm, and I'm wondering whether it even exists.

I am fine with people pointing me to other software, so long as it's open-source.

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If you autotraced it, then there will be no strokes, only paths with fills. One possible method is to use an Offset LPE effect. Don't add too much though, as it will fill in. Then you can change the fill to white, and place it over a black filled square.

enter image description here

Another possibility is to simply add a black stroke, but then you'd need to set both the fill and stroke to white to invert it. The result is more or less identical

enter image description here

There are other approaches too. For example here I made a copy of the cross, Break Apart the paths, and Unite all as one solid cross shape, then I added a thick white stroke to the solid cross shape, placed the actual cross on top of that, and added a thicker black stroke to it. Finally a black square underneath. The result is not actually reversed out, and only has a thick white outline.

enter image description here

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Considering that you are starting with a raster graphic anyway, you could also do the thickening on the raster graphic. You can do that with a standard operation, erode makes black areas bigger, dilate makes white areas bigger. In Gimp they are available in Filters -> Generic.

In general those operations would take a parameter called the kernel which determines how much and in what exact way the black/white areas are shrunken/expanded, but in Gimp it seems you don't have that choice. You can just repeat the operation as many times as you want until you like the result.

If you spent a lot of time tracing manually / cleaning up the trace result and the original is not a good clean base, then you can also export the vector graphic that you already have to a high res raster graphic and then auto-trace it after the raster erode/dilate operation with negligible loss of quality.

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