Lost Place is a German-based film that tells the story of four high school students who stumbled upon a top secret Cold War military facility hidden deep within the Palatinate Forest of Southwestern Germany.

Robert Zeltsch is a freelance 3D artist and experienced Houdini user who was brought into this project as a VFX Supervisor through the German studio MovieBrats Films and Animation. His challenge was to generate the VFX shots needed for this film using a small, primarily compositing oriented team and with a limited available budget and technical resources such as a substantial render farm.

For the purpose of this feature Robert has produced a set of four breakdown videos that explain in great detail the Houdini tools and techniques he employed in order generate key VFX shots for the film, including a set of circular volumetric clouds and an elaborate piece of lab equipment outfitted with a Tesla coil.


Ring-Shaped Clouds

Robert's team began by generating a source volume simulation using a custom velocity field, used to move the ring outward along with an “up-vector field” that created curls in the clouds. In doing this, they calculated the cross product of the current velocity and the up-vector which is added to the velocity field for each simulation step.

The rendering of the cloud volumes was then completed in comp by exporting the simulated volumes out of Houdini and then into a custom real-time volume manipulation and rendering pipeline.

“A few days before finishing this particular shot, we decided to add an additional cloud layer showing a more ‘fluffy’ style to the rings” says Robert. “Because Houdini 12.5 was released just at the same time, I directly used the new OpenVDB based cloud tools to create an additional set of rings. I animated the noise and – since time was short – rendered them directly in OpenGL”.


Generating & Animating Lightning Bolts for the Tesla Coil

The lightning bolt rig that Robert created here consists of four key elements; the core, strings, hair elements, and a bounding tube.

  • The core was directly based on a guide curve, but was resampled and modified using customized noise generated via VopSop’s. Following this, a circular cross section was sweeped and skinned along the guide curve.
  • The strings were duplicates of the re-sampled core curve, but modified with different types of noise and amplitudes and assigned attributes allowing them to work with a hair/fur shader.
  • The hair elements were created using the re-sampled core curve as well, however in this instance the curve was the origin geometry of a simple fur object. It created the hair along the curve and also assigned a shader, which was adjusted to fit the desired look.
  • The bounding tube was an object that encompassed all of the various layers associated with the lightning. It received its own shader used to return its position in world space. This pass was later used to modify and create elements in compositing, such as the particles visible when a lightning bolt dissolves.

There are two distinct types of lightning bolts that can be seen moving across and emitting from the Tesla coil:

  • Firstly, there are ‘crawling bolts’ that move across the Tesla coil that were created using creeping points along a skinned mesh placed along a copy of the source curves, similar to the copy of the actual fin geometry that makes up the model of the coil. The idea behind this was to have two set of points moving in the opposite direction and polarity of each other. This was later used for connecting a custom hair setup between the points that were within a close proximity to each other. This particular setup made it possible to have thousands of tiny additional electrical discharges running along the coil.
  • Then there are the ‘secondary bolts’ that exhibit a different behavior where the source points ‘stick’ to points on the fin geometry from which they are emitted. The endpoint of the lightning bolt does not follow the normal/orientation of the fins, but instead remains at its point of origin creating a more realistic behavior.

All the layers of animation are essentially attributes assigned to a circular set of points. Later, using the stamp function in a copy SOP, they are controlling the individual fins. They were rendered using mantra’s default surface shader while including exports of additional deep raster planes. The additional bolts were rendered using a hair shader.


Constructing the Tesla Coil

The coil object consisted of 182 individual metallic fins that were controlled using two layers of a sinusoidal oscillation to position them, an additional layer was added to rotate them. Having been set up procedurally, these fins could then be animated and transformed into very complex shapes that were designed to express the “mood” of the temperamental coil. Additional animation such as scene specific behaviors were done via Channel Operators (CHOP’s).

In modeling the coil, two circular rings were created to form a single fin element. This allowed the radius of the coil to be interactively adjusted based on the source object, additional proxy geometry was created along with the final mesh. The single fin was then copied to the points that contain all the rotation and position attributes.


Rendering and Manipulating Volume Fields in Compositing

With Houdini as a core component in his pipeline for this project, Robert and his trim-sized team were able to quickly construct an intricate Tesla coil design, apply VFX to it, produce the highly realistic volumetric clouds, and turn out their final shots using minimal resources. Houdini and its Mantra renderer proved to be an advantageous tool primarily due to its procedural approach - achieving the complexity of the elements they desired without the need for a much larger budget.


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