Water Swords FX/Animation with Houdini FLIP and Maya

Software packages work best when you play to the strengths of each one. Houdini works well in giving realistic effects with more flexibility, and for animation I like to use Maya. That is why for my third Visual Effects 2 project I decided to use both, and learned a lot about going from one program to another, which can be a powerful workflow technique. I decided to recreate a move from the Pokemon Greninja which are his water swords.

There were a lot of challenges that came with this idea, similar to my last project, the concept is more based in fantasy then reality, so finding real life reference was a little tricky. I decided instead to look for reference of real swords and how they react to slicing through objects like water bottles and how the water trails along with the sword. This is all in addition to the in game reference of course.

The next step is in Maya, and that's animating the model. The model I got here and it came textured and rigged so I was able to get straight into the animation process. Though for this project the effect was the main focus, not the animation, being mainly an animator, it was very important to me that I include fundamental aspects like the slight anticipation for the charge up and follow through in the scarf (which is actually his tongue). These animation basics were visible in the game footage and I feel would detract from the effect if they were not there.

Animation is possible in Houdini but that is not the main focus of the program, thus it's a little less intuitive. Because of this though, I got to learn about a really cool feature of Maya and Houdini I didn't previously know about, and that is the Alembic file type (.abc). These files allow you to store all sorts of information including the model, its animation, and its UV data (and they export quickly too, about as fast as a playblast). Exporting as an Alembic can be found in the animation menu set under Pipeline Cache.

Once you have the file, it's as easy as going into the geo node in Houdini and importing it like you would an OBJ file. One thing to watch out for is when you export make sure you check the options you want to store, like the UV data, because to save on file size it wont do everything by default, resulting in the UVs not working in Houdini.

This was all for the character animation though, the sword animation was a bit different. I tried using the same process by animating the swords in Maya using parent constrains and exporting Alembics, but they wouldn't take the effect in Houdini when it came to adding the water. Instead I hand animated them in Houdini so there was no potential for them to goof up.

The effects I used in Houdini were the FLIP Tank and Emit Particle Fluid. FLIP is a great way to handle water in CG but as I kind of learned the hard way it is super expensive when it comes to sim time, so you may need to take some steps to ease the strain on the computer. One thing I did was size down the model by a lot. You can see in the Maya picture above that the model is much larger than the grid, I brought this way down in Houdini. A larger scale is better for making more realistic effects, but it's all about striking a balance between look and efficiency.

Now that the swords were emitting particles, I had to change a few things to have them emit correctly. Initially the water would just fall down while it emitted from the swords. To fix this I turned down the gravity, but now there was no trail on the swords, they moved like normal blue swords. Adding more substeps was the solution to that issue because now the motion would be assessed multiple times a frame. But of course that brought up the new problem of there now being a trail of particles that would not disappear. This was one of harder issues to troubleshoot because it not in the fliptank, flipsolver, or gravity nodes. This fix was found in the source node in the form of a Life Expectancy checkbox, which when checked lets you adjust after how many seconds the particles died.

The sound I added in After Effects and I got them from here (this pack includes all of Greninja's sounds, so go wild if you decide to try something yourself.)

Though I accomplished quite a bit with this project, I didn't get to do everything I had wanted. I had another move animated but wasn't able to give the effect in time, this was one of Greninja's taunts where he throws his hands up and shoots little water spouts from is palms, similar to a water fountain.

I learned a lot from this project about workflow optimization and Houdini FLIP specific features, and from the others throughout the month. Hopefully you might have also gotten something from my experiences, and that my Visual Effects projects kept you at least somewhat entertained. It was good fun (even when things wouldn't always work) and I look forward to learning more every step of the way.

I'll be putting up a compositing project soon that'll wrap up this Smash Bros theme I've been rolling with, so stay tuned for that.