In the creation of the Viscous Fluid project, I decided to
create a chocolate fountain. I chose chocolate, as it is a relatively thick
flowing liquid.
I gathered my reference and began working to match it from
the very beginning.
The goal was to create a thick flowing fluid that would
match the speed, reflectivity, and consistency as what is seen in the photo
above. I decided it would be fun to attempt dipping something into the
chocolate as the strawberry is being put in the chocolate above. For time’s sake,
I chose to go with a marshmallow.
I began modeling in Maya to create the chocolate fountain,
and decided to give it an extra tier to add to it a little bit.
I created the fluid emitter and decided first to work on
correcting the collisions. Despite how complex creating the actual physical
fluid is, getting the collisions to work correctly has been one of the more
difficult and important parts of Houdini projects in my experience.
In the FlipFluid Object in the Autodop Network, I set the
Collisions Volume Offset to 0 and checked “Use Point Velocity for Collisions.”
On the Chocolate Fountain’s Shape itself inside of the
Autodop Network, I turned off “Use Volume Based Collision Detection” in the RBD
Solver, and set the Geometry Representation Bullet Data to “Concave.” With the
Collision Padding and Angular Threshold set to 1 and the Linear Threshold set
to 0.8, I found that I got very effective Collisions with the object.
One of the biggest struggles of the project was perhaps the
very next step, which was making the liquid emit out of the top of the fountain
as though it was being pushed upwards.
After about an hour of finagling and struggling to get it to
emit correctly, I finally found the exact perfect placement of the emitter in
the top of the fountain.
Set at this height above the Fountain Object, the emitter
would emit into the little bowl at the top, the bowl would fill with particles
relatively evenly, and they would spill over the edges. If the emitter was set
even a fraction of a number higher, they would splash out of the top. If the
emitter was any lower, the particles trying to push their way up out of the
bowl would fight with the particles falling downward, pressure would seem to
build, and the particles would shoot up into the air like a vertical upward
fountain.
This exact positioning caused a nearly perfect emission, and
this is what I stuck with for the remainder of the project.
After making the fluid Viscous, I turned the Velocity up to
the recommended 1000 to get a good amount of thickness in the liquid. When
playing back the simulations, I felt it was still too watery, so I turned the
Viscocity up to 1200. This gave me, what I felt, was a very good result.
I set the Particle Separation to 0.1, and both the Particle
Radius Scale and Grid Scale to a value of 2. This gave me a very good
flow. I did not lower the Particle
Seperation because I did not want to increase my render times and simulations,
as they were already becoming quite slow. It was giving me a decent look, so I
felt good about it.
The next problem I was running into was that when the
chocolate was falling from one tier to the other, it was bouncing off of the
next tier rather than just landing on it and staying. It wasn’t bouncing a ton,
but just enough to make it bypass the next tier after the chocolate fell again.
This did not match my reference at all.
You can see in the picture below that the liquid is falling
off of the second tier too quickly, and is bypassing the third tier. I needed
to find a way to keep that from happening.
The next thing I did to solve that problem was readjust the
Density.
I realized that turning down the Density made the liquid
fall down straighter and more inward rather than outward, and turning up the
value of the liquid did the exact opposite. I found that a Density value of 500
helped out a lot, and decided to keep it there.
Changing the Density helped, but my particles were still not
reacting to hitting the surface quite the way I wanted them to. To further fix
this, I put a value of 0 in the Bounce Attribute on the Fountain’s geometry.
After changing the Viscosity Scale several times and doing
some testing, I found that a Viscosity Scale set at a value of 2 seemed to be
the most effective.
The liquid was continuing to bounce just a little bit and
wasn’t quite falling the same way the chocolate was, so on the Flip Fluid
Object, I set the Bounce to 0, Bounce Forward to 0.9, and Friction to 0.3.
I also wanted to cause the liquid to have only a sleight
amount of stickiness without turning on the “Stick on Collision,” so I set
Surface Extrapolation to 0.7 and that gave the liquid a perfect amount of
stickiness and roll on the fountain’s surface.
To get the Chocolatey look that I wanted, I set the Diffuse
color to a mid-level brown, set the Diffuse Intensity to 0.105, and the
Roughness to 0.257. I set the Specular Intensity in Reflections to 0.756, with
a skin-tone pink as the specular reflection color. The Refraction Intensity was
set to 0.01, and the Refraction Minimum set to 0.24. With the Opacity turned up
to 1, I got a very good chocolate look upon rendering.
I set the camera up and animated it moving in a circle
around the fountain, so you get almost a 360 degree view throughout the
rendered final.
I don’t know what my problem was, but for some unknown
reason, my files would not Cache properly, therefore I never got a cache that
went beyond a couple hundred frames, (and my simulation lasted 500 frames.) So
I could not scrub through the scene properly and see what it would look like
beginning to end.
Because the chocolate falls perfectly and thickly off the
first tier within the first couple hundred frames, I assumed that it would work
correctly going down the other tiers.
For other unexplained reasons, Houdini would not allow me to
do a Flipbook/Playblast at ALL. I have been able to do them in the past, but
this time I was hitting strange errors no matter what I would change in the
settings. This was a problem I tried tirelessly to resolve, but could not.
Therefore, I had no clue how it would look all the way
through.
This affected me negatively in the final due to how clumpy
and goopy it looks near the bottom of the fountain. At the top of the fountain,
(the part which I was able to visualize), the chocolate falls in perfect
sheets, but not quite so much so near the bottom.
Overall, the render turned out okay, and the whole thing was alright. But I wish I the computing power to render things more quickly and to be able to visualize things correctly before rendering. I did not have this luxury, therefore did not know what the end of the clip and last half of the simulation would look like.
I placed a Distant Light and an Environment light into the
scene and put simple materials on all other objects in the scene.
The render took about 17 hours, and by then I knew I did not
have the time to correct things, but I know that if I was to return to the
project, I may turn up the emission rate, set the Viscocity slightly lower, and
turn down the Particle Separation Rate to a lower number.