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Making illustrations, how to change line width?


I have recently tried to make illustrations for math and physics problems and examples with DSM. I have mostly planar surfaces and occasionally there's also a cylinder, sphere or other simple 3D form which can be got easily by pulling, revolving, blending. and using the combining tools.

I have used Adobe Illustrator and Inkscape, but I have learned that a 3D scene comes out much faster with a 3D program. DSM is a good one due its intuitive user interface. Some other low cost 3D programs are 100% useless because there's no transparency or all lines and curves are always shown on the top, there's no way to let them be partially behind some opaque items.

Even a screenshot is mostly sharp enough for my purposes. It keeps the perspective, which vanishes in attempts to export the result as a vector image. The perspective is a huge boost for the visualizing power and the 2nd major reason to try this.  I can add some annotations with sharp lines and texts easily in a 2D drawing program. The next image presents well how complex images I need (it's an example, in math and physics it's nonsense).


The problem:  All lines have the same width. They are as thin as the rendering allows. I want to make some lines or curves a little thicker, maybe the red and green ones in this case. The curves are splines.  So: How to change the width of a selected line or curve?

I have already tried the following workarounds;

A) make a copy and move it a little. This works if the curve is on a plane and the moving direction is selected well, but the result is poor if the curve is non-planar. Make it surface by blending the copies or simply by extruding the curve; the result is a surface which can be made solid by pulling some thickness to it. But it looks odd if there are the automatic edge curves ON. There's no way to suppress them only for a single object.

B) make it solid by sweeping a small circle along the curve. This works as a single sweep only with smooth enough curves. Too high curvature and corners make this  geometrically impossible. Making it in pieces needs a huge amount of work. And even in those cases where a single sweep succeeds the effort is big when compared to drawing a curve and changing a single number like one does in 2D drawing programs.

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