Now that the mushroom cloud is animated with displacement maps and is looking fairly decent it'd be good to add a bit of a fireball look to it. Even if it is a bit stylized it helps create the illusion that the mushroom cloud is made out of smoke and flames, so I think it's a worthwhile addition. I won't go too deeply into the theory behind the flame material as my material is just a derivative of the one from the fireball texture tutorial and is covered there.

The Materials

So we need to start of with a base material for the mushroom cloud, what the cloud looks like minus the flame. A fairly simple material will do, just make it nice and diffused. I messed around with the Oren Nayor diffuse shader override for fun and it turned out okay. This material is really subjective and also depends on the what mushroom cloud is made of. The traditional radioactive dust and water vapour or is it going to be gravy ala Chicken Run? It's all up to you..

Anything that looks cloudy

 

The Fireball

I lied before about the the fireball material not being the base material, it actually is. It's all fire material, extinguished to the 'base cloud' material with animated material effectors. I tried it the other way around initially using the fireball material on the material effect but it was just too clunky. The fireball material itself turned out to be incredibly simple, much to my supprise. All it is is a perlin combiner (how good is the perlin combiner!! It does everything!) with the top attribute a nice shade of orange with the ambience cranked up to 100%, along the 'override base attribute' and 'glowing' boxes ticked under the glow options. The bottom material is pure black and ambience once again at 100%. Don't forget to put the octaves up to 5 under the perlin options before you slap it onto your mushroom cloud to start playing with the scale options. You can turn off the displacement mapping for all your test renders as it will just increase the render times. I found that even if the displacement was at 0% it would still initiate the displacement mapping part of the renderer a lot of the time so I suggest changing it to a color map at 0% instead. This is another one of those 'use global axis' situations - the fireball material should look like it's rolling so the cloud should move through the texture - so tick this box under the material's general properties after its been dragged onto the mushroom cloud surface object. Another thing to tweak while you're here is the glow options in the choreographies properties panel, this will obviously affect the look of your fireball material.

Its just a simple Perlin combiner	With a bit of glow

 

Animating the Fire

Create two material effectors, one will be for the stem of the mushroom cloud pushing up, the other will be for the crown of the cloud pushing down. Drag'n'drop the 'base cloud' material onto both the effectors and drag'n'drop both of the effectors into the choreography. The shape and size of the effectors doesn't really matter but it's best to make them just big enough to affect the mushroom cloud and no other objects in close proximity. Animation time! The stem effector starts below the mushroom cloud and starts moving up just after the cloud goes off, quickly reaching up to where the stem meets the crown of the mushroom cloud and following it up. The crown effector comes into play later in the animation, about where the next layer in the atmosphere would cool the flame into a vapour.

Two Material Effectors, one above, one below	Extinguish the fire through the animation	It looks something like this

 

After you've got the timings right for the effectors you'll see that it still doesn't look quite right if you render with shadows. While the flame part looks good and has the glow thing going on, shadows are still cast onto the stem by the crown. Even if shadows were never going to be in your final render it will still possibly look wrong because we expect the fire part of the crown to light up the stem a little bit - after all flames to cast some light. I had a lot of trouble over this issue and the solution I came up with isn't the most elegant but it is effective. Basically I constrained a ring of diffuse only spotlights around the stem aiming up just under the crown, and to get the falloff right I used animated maps. It's a bit awkward to set up initially but it produced nicer results and was much faster to render than anything else I could cook up.

Faking the Fire Glow

The lights are simply a series of diffuse only spotlights set in a circle around the stem of the mushroom cloud. If you intend to view the mushroom cloud from only one angle you can get rid of the back half of the lights. Evenly space the spotlights around the mushroom cloud aiming towards the centre then 'translate to' constrain all the lights to a single light with the compensate mode on. Make sure all these lights are instances of the same light, that way you can make alterations to the single light in the project workspace tree that will be uniform across all the lights in the choreography. Next 'translate to' constrain the master control spotlight (the light all others are constrained to) to one of the 'rise' nulls with compensate mode on. Which one depends on your set up, but somewhere halfway up the stem. You'll have to animate the Y-Offset to make sure the lights are at the right height at the right time, and while you could just as easily animate it without constraints it just guarantees the lights stay with the cloud which makes things easier if you want to reposition the mushroom cloud in your scene later on. You want all the lights to be aimed upwards so they cover about the top half of the stem.

This is what the light array looks like from the front..	..from above..	..and from a 3/4 view

 

Animated Scrims

All these lights will get their color and intensity from rotoscopes, so what you need to do is go into your image editor and create a image that's about the same color as the top attribute in your fireball perlin material and apply it as a rotoscope to all the lights, do a test render, then go back to your image editor and make the color lighter or darker to adjust the light's brightness. Once the color and brightness are correct we'll need to work on the falloff, and to make things interesting the falloff needs to be animated so that the lights turn off completely as the fireball material becomes cloud material. To do this we need to set up another choreography similar to the setup used to generate the displacement maps. Create a new choreography with an orthographic camera looking down and with no lights, then create a new single patch square to hold a decal. Create a new material with same color as the light rotoscope image and put the ambience up to 100%. Now go back to your image editor and make a black to white gradient and apply this as a transparency map to the single patch square you just created. The camera should render a square image starting with the flame color falling off to black at the bottom. Render off a frame and swap it in as the rotoscope on your lights to test how it looks. Adjust the surface under the camera accordingly to alter the falloff. To get the lights to fade off as the fire extinguished you can animate the surface under camera moving along the Z-axis out of frame (make sure your camera background color is black). All the timing here is dependant on timing of the material effectors putting the fire out, so look at how you've animated them to get timing right from them. Render off the animation, import it back in and swap it in to the light rotoscopes. Now you can render off the mushroom cloud animation, by now it should look quite slick!

The transparency map is just a gradient	A standard texture generating setup	The scrim rendered through the camera

 

Download the Project file for Part 3

Part 4: Smoke Rings ->

Back to Part 2

 

Copyright © 2001 by Ilya Anisimoff. All rights reserved.