PIPELINE TECHNIQUES: Create a lightning explosion in Houdini
Shape a pyro simulation and master RBD simulations
This tutorial shows you my basic workflow in creating the lightning explosion. i will mostly be talking about the idea and my thought process, showing how i lay out and plan the whole shot before starting a project. This is absolutely key in order to finish the shot on time, try to keep it simple and layer the complexity later if time allows. i will also go through some tips on shading and shaping the pyro simulation, as well as the creation of the lightning. hopefully this tutorial will give you a better understanding of creating a VFX shot from start to end.
Prepare Geo and Animation Planning ahead before starting a project is crucial. in this case, i know i’m creating a naval mine exploding in the air, therefore the mine has to be hollow on the inside for me to fill in fuel and temperature. i started the model in Maya, and made sure that i modelled each panel of the mine to create an interesting look. Fracturing in houdini will give me a voronoi look, and that’s not what i’m going for. i also made sure that each panel i model for the shell are not intersecting. next, nailing down the camera angle and motion is important so we can see and simulate our explosion at a fixed angle for a better composition.
Create RBD simulation i wanted to create some interesting force field effects right before the mine explodes. Therefore, i created two forces: one with softer strength just to push the parts of the mine outwards, and the other stronger one that triggers later on in the frame when the explosion detonates. The RBD wasn’t the main point of this project, so i animated the time scale to get the time freeze effect. Then, by introducing gravity and bringing the time scale back to normal and with a stronger force, i was able to achieve the desired look.
Create the lightning After nailing down the RBD simulation, i’ve proceeded to creating the lightning. The idea is to have lighting around the outer shell of the mine and a stronger lightning core creating a force field around the bomb right before it detonates. i’ve scattered the same amount of points on both the rods and outer shell, and paired them up with an Add sop which connects them together. Then, i defined an attribute to trigger the lightning in the frames it activates, as well as defining a lifespan for each lightning. After that, i used cd to blast away non-active lightning, and the lightning turn on at a random frame range defined with a fit node. To add detail to the lightning, i’ve
created and blended high-frequency noise for the big electric waves and another lower-frequency noise for the details. Finally, to mesh each lightning, i’ve defined a width attribute based on the uvs of each line to get a tapering thickness at the tip of the lightning.
Add the lightning core i’ve also made another stronger force field-like lightning around the mine right before it explodes. To achieve that, i’ve used a similar technique for the lightning on the outer shell by scattering points at the core where the explosion will happen and an outer sphere that i’ve created. i’ve also used a Mountain sop on the outer sphere to avoid a smooth, round silhouette that the lightning hits. The lightning striking out to the outer core doesn’t really give me the force field effect i’m going for, so i’ve made another layer of lightning along the outer sphere that we created. i scattered some points on the surface of the outer sphere and connected them by using Add sop – just skip every nth point. With that, we get a random connection of lines along the surface. To get a nice arc, we need to add a third point in-between the straight lines. We can do this by first resampling the lines to get a third point in the middle, and then use neighbour count node to find the middle points by filtering only points with more than one neighbour. Then, take those points and displace them along the normals of the sphere. Finally, convert it into a bezier curve to get a nice, smooth arc and then we can add the lightning detail we desire.
Pyro simulation source An interesting-looking pyro simulation always comes down to the pyro source. i want to create an interesting-looking pattern, so i’ve used a simple sphere as the emission source for my particles. To layer the complexity even further, i’ve used a noise on the sphere to get a randomised emission source. Also, i’ve emitted a layer of thin smoke first, and when the inner core – which is filled with temperature and fuel – ignites, it will push out the smoke. Lastly, i’ve emitted some particles shooting outwards to get some fiery streaks shooting out from the core.
Pyro simulation shape The shape of the explosion is crucial to catch the viewer’s eye. using the micro-solvers, we can achieve very interesting shapes by layering them one by one. First, we want to start with turbulence, as it’s the main force that drives an interesting shape. isolate the turbulence by turning the other disturbance forces off. After we get the big shape that we want, we then create another turbulence force with a higher frequency for the smaller details. For explosions, we want the turbulence to be strong at the beginning and fade off afterwards. One thing i like to do is to keyframe the timescale; making the simulation twice as fast for the first few frames from detonation gives the simulation more explosive look.
Pyro shading As for shading the pyro, adding dark areas within the fire ramp, as well as using a sharper or spiky graph for the fire graph, will give you nice hot layers within the fire. it will also give the fire a really nice texture and also a darker area within the pyro, adding another layer of realism. On the other hand, adding the white values above 1.0 adds the blown-out hot spots at the very core of the explosion. To achieve a pyroclastic smoke shape, i like to use a gradient graph to get the round plume-like smoke shapes of an explosion. shading the pyro can be time-consuming, but always try to get a wide range of values within your fire so you can use it to get the desired look in compositing. it’s much faster to tweak the values when compositing than getting the desired explosion look in the render itself.
Create cloudscape and lighting having the camera angle nailed at the very beginning helps with designing the cloudscape. i only created six different cumulus cloud shapes – you might think that it is far too few for a cloud scape, but by rotating and even merging them together, we can get a totally different-looking cloud. Then, i placed them around the scene for a nice composition, making sure that i had a clear foreground, midground and background. Then i’ve lit the scene and made sure that the clouds do not look too flat.
Dynamic clouds and scene interaction
Interactions among elements in a shot is important to avoid having a shot looking like it is from a different world. As for this shot, the mine needed some interaction with the surroundings. But to make the whole cloudscape dynamic is way too expensive and heavy to simulate. Therefore, by strategically placing only one cloud in the area where the mine flies through and simulate, that one cloud gives us the interaction we need. To further blend all the different elements together, having the light from the explosion affecting the clouds is crucial. These simple additions tie all the elements together, and thus gives us a more complete shot.