Mitchell D. Harper
Motion FX Artist
SIMULATION OF THE MIXING OF INK AND WATER IN HOUDINI
Ink drop references...
First Thoughts...
After watching several reference videos, I started to put together a simple Pyro smoke effect. This gave me similar (but not exact) motion as the original videos. I am pretty sure that I am going to need to create several Pyro simulations to mimic the different colors of ink mixing in water. In the example to the right I used a sphere connected to a Mountain node to add some interest in the smoke sim.
Just for the heck of it I went ahead and avected a few million particles to the Pyro sim. As stated earlier, I started with a simple Pyro sim and then added a simple particle simulation. I then added a "popadvectbyvolumes" node to the pop sim. This (loosely) makes the particles follow the motion of the Pyro sim.
Pyro R&D... The Pyro simulation is the foundation of most modern cloud tank effects (there are other ways to achieve it, which I would like to explore). The Pyro sim is going to make or break an Ink Drop effect.
Here is a detailed version of the Pyro tree I ended up using.
Single Drops
For a single drip, this is the overall look I am trying to achieve. Unfortunately, Houdini crashed while rendering this and I have very little information about the settings. Thank the stars for backup files ;) This drip looks more like a constant spray of ink. I had the Diffusion setting way too low causing the ink not to dissolve.
Continuous Spray
Particles R&D...
For this project, I will be comparing Pyro based simulations against particle based renderings. A common practice is to advect the particles by the Pyro volume. The example to the left is 5 million particles advected to a continus stream Pyro simulation.
The screenshot below details my node tree and advection settings to achieve the look in the example above. I found that setting the Advection type to "Update Velocity" and the Velocity blend to 1 gives a pretty nice result.
Basics - The emitter
When starting any particle or Pyro sim, you need something for your particles and/or smoke to come out of. Solid geometry works ok, but to really make things go crazy you want some movement on the emitter. In this example we created some basic geometry, roughed it up with a mountain node, and converted it to a volume with points scattered i in it.
Ink Surface R&D... A common method of doing fluid effects with particles is using motion blur on the particles. Below is an example of this method. This method creates a light and airy feel, however, that is not really what I am looking for on this project.
As an experiment, somebody suggested that I tried using a particle to fluid surface node. This is really fun, I like slime! ;)
Render Tests...

Continuous Stream - Pyro

Continuous Stream - Particle

Single Drop - Pyro

Single Drop - Particles

Final Render