Houdini Resources

Updated on June 26  2024


Vellum Grains

Deborah R. Fowler


Legacy and Turn back Time

Created July 13  2021
Posted July 26  2021 - took awhile to render locally - coming soon (glass renders, also pentagon renders, thin film glass shader screensnap)

keywords: vellum grains, stopped, point wrangles in dops

We hope that what we do may have an impact, both in the present and perhaps for longer. Vellum grains were the topic of the day and I wanted to explore a classic hourglass but with a twist. Like most projects I undertake there is an intersection of learning as well as what is on my mind at the time. Having time to reflect and the ability to express thoughts in my art are important. Summer is an excellent time to learn, but also to remember those who have influenced me over the years. The limit of time and the inevitability of that time coming to an end are intricately weaved into our thoughts, memories and dreams. Time is precious.

Below are some of the technical problem solving that occurred during this project. Model of me by Texel Scan at SIGGRAPH Asia 2016.



Also on youtube at https://www.youtube.com/watch?v=CsKSPZ7lUWM

The three major lessons for this project were:

Vellum grains are cool in that they allow particles to pile up. However they do seem to be a bit harder to control than pops themselves (my opinion). Houdini offers vellum grains (piled points), pop grains (before we only had the interact node but pop grains work on a position-based solver unlike the interact node that pushes them apart) which use a position-based dynamics approach. In looking into this topic there is also a stand-alone grains package called Storm offered by EffectiveTDs.com (I had heard of Thinking Particles but this one was new to me - no opinion as I have not tried it personally).

For the visuals, "Legacy" was created using the standard attribute transfer of color to create points (see particle disintegration) fed into a vellum grain and emission type always. (COMING SOON) For the visual "Turn back Time" I used an approach similar to the one found here, however it needed to be modified as my particles were headed downward, and the particles would stop once started if they were in the bounding box range. Great for creating a particle skirt but not what I wanted. The trick of using the geometry wrangle in the dop with a sop solver instead of in sops to turn particles to be active was key. Thanks to Resilient Pictures Company for that tip instead of using emission always. As noted the emission type "always" behavior can be a bit crazy, but I used that to my advantage in the first visual as there are no rules for how a character inside of an hourglass will behave.

I set all points to stopped (which if you look at the documentation they use 0, 1, 2, 3). Fully stopped is 3. This is nearly identical to the method using attribute transfer, except now a group is used so that we can set the activation in dops.

Like with my bullet example of geometry wrangles, you will need to use a multisolver in dops or the information will be ignored



I am continuing to explore vellum and will post any new insights here.