# Painting with motion tracking? 'Grow hair' along track, dynamic paint, bevel curve.

Dear community,
so after a few years I’m back to blender for a media project I’m launching and, just to say, how the program has grown is AWESOME!

I got one problem I can’t find a solution. The idea is that I’m in front of the camera, and paint into the air with a nice, clear, very professionally looking line

So, for example, in this test I’m in front of the cam and the saccharine in my hand is tracked.

Now I’d like the line to be a nice, rounded line.

One way to do it is to make a UV sphere follow the track, add a plane as a canvas and use dynamic paint. The disadvantage of this is that to make it very precise, the paintmap needs to have a very high resolution and many substeps. Nevertheless, if I don’t find another way, that’ll be how I do it.

Another maybe something with particle systems. In the image a 1-vertex object emits particles. One obvious way to get to a line is to simply increase particles until they connect But is there a less messy way to ‘connect’ these particles? Like make a hair grow along the animated path of an object?

My preferred option would be if there were an option to convert the motion tracking into a curve! Because then I could simply animate the bevel of the curve – which since 2.71 with CND’s Bevel Factor Mapping isn’t a problem any more That would be by far the most efficient and neat solution I think. So is there a way to convert all positions of a motion tracking track into a curve?

Thanks very much for your attention.

Regards, Marc

a quick thing I just tried…

1. track your object from video
2. solve > geometry > link empty to track
3. track object panel > calculate Motion Path
4. run this little script to get a curve from the motionpath
5. bevel the curve -Map Taper option can give nice shapes-
6. animate bevel End factor, set interpolation to linear

seems to work but may be smarter ways to do it -or a script could take care of all steps-

@liero: i’m very sorry i never saw this message! thanks a lot! i’ll use that in the future!