I have always been a big fan of Frei Otto. His work has been revolutionary on many levels but the most fascinating area for me are his early experiments. One of this experiments is the famous “optimized path system” where he used wet wool threads forming a network of minimal pathways. This technique works in 2D and in 3D and due to its self-organizing characteristics it was and still is heavily used for various form-finding experiments in many different areas. Quite interesting for obvious reasons in this regards is edge bundling which is used mainly in scientific visualization. Conceptually it is very similar to Otto´s wool threads experiments and is used for avoiding visual clutter by bundling together edges in large graphs. Out of interest and because I love visually appealing diagrams I tried to implement something like this in houdini.It´s a well researched area and so you can find various different solutions to the same problem but aber some digging I found the two papers which I finally implemented in Houdini:
C. Hurter, O. Ersoy, A. Telea: Graph Bundling by Kernel Density Estimation
D. Holten, J. Wijk: Force-Directed Edge Bundling for Graph Visualization
Both algorithms worked quite nicely and even if they are conceptually very different I got more or less similar result. Though, personally I prefer kernel density based bundling since I like the brilliant simplicity of the idea and overall it seemed to produce better results. At least in my test (what doesn´t mean anything, tbh.). Anyway, after all I implemented a third homegrown version which is somewhat a combination of the other two. Basically it is a spring based model which is guided by a density function. The advantage of this setup is that it is quite flexible and art-directable.
After rendering with Mantra and adding some grain, the bundled edges got a somewhat hand-drawn character.