The Bone Wave Breakdown

Initially, from concept to Niagara asset, the “Bone Wave” looked very challenging. The idea was having a forward-moving pile of bones that would kill everything in its way.

It would be a one-off ability, triggered in sync with a summoning animation. Defining the overall shape of the ability was the first step. A wedge or "pizza" shaped area of effect seemed the best one. I like to use a curve color atlas for keeping color consistency among different particle systems and materials. The concept art is a great place to sample important color values, like shadows, highlights and mids.

The first test was basically a burst of mesh sphere particles with collisions and roll control activated. I really wanted this ability to look like a physical system with proper behaviour.

Next, I replaced the spheres with skeleton parts. Making the parts was easy, using a simple cell fracture tool in Blender or Houdini with a free marketplace skeleton asset.

These meshes were then exported to .fbx and ready to be used as mesh particles in Niagara. I used mesh array in a particle mesh renderer with random selection to add variety to the particle's look.

The overall setup is a bunch of particles emitted in an arch shape that act as lead particles for other elements in the effect. As I wanted to have all this working as a GPU simulation, instead of location events, I used the “Spawn particles from another Emitter” module, which uses a particle attribute reader that can sample many properties from the parent particles. In this case, the most important attribute is the location of these parent particles.

Another important element of the effect is what we called “the wave”; the magical part of the effect, the energy, that summons the bones. Initial thoughts were using bespoke geometry with panning textures materials. I went for tube ribbons instead, which is a very useful option in the ribbon renderer. I used partition particles to geometrically sort out a rate of particle bursts within a semicircular shape. It's important in this setup to export the ribbon ID, making the ribbon geometry not tangled and look coherent, so that each burst is a ribbon.

For the look of the wave tube ribbons, I created a very simple panning material, which was reused in other effects in the reel. It's advisable to try and use dithered masked materials for performance and look. The panning is smudged in a way that makes it look like a motion blur (this may vary depending on your antialias setup for your project). Vertex offset animation adds complexity, breaking the simple shapes of the ribbons that need to have good enough mesh tessellation (adjustable in the ribbon renderer).

Here is a breakdown of the particle setup. I like using simple sprites to first tackle the general behavior, then I add the ribbon renderer with the material described above.

Next, I added some extra ribbons for the "souls". It was a similar material setup as the wave, using a particle attribute reader again for doing location trails. There is a point attractor force with a centre of attraction in the simulation position, so all the “souls” travel to the player's hands at the end of the ability.

I added a particle with a light renderer with a greenish tone, selected from the default color curve used in these effects. This light helps integrate the various elements and also it grounds the effect in the environment. If not, everything can look like it’s floating in another layer of the render. The particle renderer inside Niagara is very efficient. The lights don't cast shadows, and used carefully, can greatly improve the look of any effect.

Usually once all the elements are there in some form, there is a lot of back and forth with art direction when doing the final color and integration tweaks, to make sure all the elements are readable and well integrated with the environment. Lots of feedback notes are passed around, paintovers, value tests, until we are confident enough about the look of the effect.

This is the final look of the three most important elements of this effect, the bones, the wave and the souls:

After art direction, optimization is a very important aspect of the VFX design. There are many factors to look at. Using masked materials here was a good way to achieve a better render performance. Having lots of particles collide with the ground must be done carefully and in this case, using distance fields collisions helps enormously. GPU simulations are preferred over CPU in most cases.

Complexity view can give you an idea about how optimized the render of this effect is, along with some other render statistics available. The particle updates must be sustainable and the memory consumption should not go above budgets. For this effect, the most expensive parts are the translucent ribbons and dust particles, but given how far from the camera is, the total amount of pixels rendered is not too much. It would be worse if these effects were seen closer at fullscreen.

Overall, we are very happy with the end result. The effect has a magical and ethereal look along the more physical bone details that closely matched what we had in mind when creating the concept for it.