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What about freehand line drawing with POVray? Here is my quickest'n'dirtiest solution!

Start the sodipodi package (it's Gnome stuff), select freehand writing, draw a curve; sodipodi "vectorizes" it, and then you can save all to his SVG format file (it's actually contained in an XML uncompressed file).

The freehand-drawn curve is saved as a curve with control points; the POVray spline functions seem to behave quite well taking out that curve: ignoring the control points gives out a decent draft, and with some trickery I can import it as a sphere_sweep object in KPovModeler.

Since I cannot import POV file, I had to write a little program that scans the beginning of the "d" node of the first "path" object found. It then outputs a compressed XML format suitable for loading in KPovModeller (it just seems a .kpm file containing a simple object), moving the coordinates to x and z axes (nice when one works on the upside view). The only little drawback, also correctable with a rotate instruction, is the orientation of sodipodi's coordinates.

I know that my dirty hack is only a "five minutes trick", but it made me able to immediately draft a number of curves and then "sweep" them into my kpovmodeler scene. Since the process involves fetching and placing some data between XML containers, I hope that someone will write a decent "import curves from SVG file" (working out the control points, maybe "bezierating" them) or even a decent generic freehand tool for KPovModeler...

Well, I wrote (using my mouse!) "Povray freehand demo" (well, it surely will be nicer using a pen+tablet, but I only have a mouse and a trackpad here). Then I imported the single curves in the KPovModeler scene, changed a few minor details, added glass-like media, a single light point and photons effect, and here it is the final image! (note that it required nine hours to render on a Celeron/1000).

I think that with minimum (or even none) effort, you will be able to get more from other vector-drawing programs like sodipodi (anyways, I mentioned here only free software: sodipodi, kpovmodeler, povray, you don't have to pay to use them).

Some italian notes...

Insomma, m'è venuta la mania di disegnare a mano libera e poi di importare in KPovModeler per farli tracciare da POVray... l'esperimento è riuscito abbastanza bene, anche se è assai grezzo. Ho usato sodipodi (della collezione di Gnome) per tracciare la curva, ed ho scritto un programmino in C per recuperarla come oggetto di KPovModeler. Il risultato finale, di poco più di mezz'ora di lavoro totale (tra redazione del programma in C e smanettamenti con KPovModeler) è l'immagine qui sotto (che ha richiesto nove ore circa di rendering su un Celeron/1000, a causa degli effetti "vetro", "fotoni" e "radiosity" di POVray).

Some extra notes

I don't like to use a changing radius for the sweep_sphere, although in some (rare) cases it may be useful. A fixed radius on a cubic_spline sweep_sphere will have the real freehand feeling, as shown on the image below. Some weird effect can be obtained changing to Bezier-type spline. I think that this solution is useless with linear splines.

With KPovModeler you can still draw your own splines, but if you want to build some freehand-style drawing like the one below, you will have a rather hard day (the image below contains five sphere_sweep objects, from 9 to 45 points each).

Click here to download the ugly C code that translates a sodipodi curve to a kpovmodeler sphere_sweep object. If you want to experiment, just remember that the starting and ending points will be drawn in the sodipodi canvas, but will be invisible in the sphere_sweep object.

Click here to download the nice KPovModeler scene file that KPovModeler can render through POVray 3.5 or later.

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