..::bunsen burner flame::..

Note from the author: The new Deconstructing the Elements book (estimated release: June / July 2004) also contains detailed flame tutorials which are similar to this paper, but go into a lot more detail (at great length - approx 15 pages!), resulting in a much more realistic effect

These types of questions tend to surface quite regularly. Fire is one of the most complex things to emulate in CG as it’s one of the most common things we see in our day to day lives, next to water, smoke and human motion. As we see these things very regularly it’s very difficult to convince an audience that a CG version is real because they can easily spot any irregularities in motion, colour or form. We therefore, as always, need to grab as much reference material as possible to observe the object’s properties.

From this reference material (and recalling old physics and chemistry lectures!) it is relatively straightforward to create a convincing effect, provided no other forces are exerted on the flame; for example wind or motion (ie moving the burner around so the fire trails around). Therefore, a simple cylinder can suffice for the main shape of the flame.

Next we need to generate the rough shape of the flame itself. By observing our reference material, we notice that the flame bulges out somewhat after the emitted gas has been ignited before coming back together to a peak before forking off at random at the top of the flame. This bulge can be created using a simple Taper modifier, or an FFD cage should you wish. Basically, use anything available just to pull the cylinder’s sides out a little and to bring the top in to almost a point.

Because each environment isn’t completely draught-proof, the flame’s motion will be slightly irregular, but only at the top. This can be simulated using a Volume Select modifier, with soft-selection enabled and just the top quarter of the cylinder’s vertices selected so any other modifiers added after this selection will have their influence reduced according to the selection falloff. The next modifier added to generate the irregular motion will be a Noise modifier with animated Gizmo.

The reason the Gizmo is animated and not the Noise’s phase is because we want the noise motion to travel up the cylinder. Therefore the gizmo’s position is animated along the Z-axis.

Even now we should have a convincing motion. But something is missing. The flame seems to be stationary, and if you observe a real flame being emitted from a Bunsen Burner, you will notice that it bobs irregularly; it’s height grows and shrinks and the flame seems to pulse somewhat. The simplest way to simulate this is by animating the height setting in the cylinder’s base parameters. This can be easily keyframed, but a more convincing effect can be created by using a Noise controller to control the cylinder’s height. This noise controller will have to be set to greater than 0, and will also have to be mixed with another controller (say a TCB Float controller) to raise the initial height value before noise is added, else the flame will be too erratic and will therefore look unconvincing.

The final thing to do as far as the geometry is concerned is to smooth out the flame. Even though the flame’s motion is erratic, it’s shape is quite smooth; almost liquid in appearance. Therefore any rough edges caused by the Noise modifier need to be smoothed out by using a simple Meshsmooth modifier.

Next we need to tackle the materials. The colour of the flame, self-illumination and opacity can be set up by using simple Gradient Ramp maps within a standard material with an Oren-Nayar Blinn shader. To add extra intensity to the material, additive transparency is used, and the material is set to two sided. To create an outer glow to the flame material, we can overlay a yellow gradient on top of the self-illuminated gradient map, and mask it out using another gradient map so the falloff is only opaque in the middle of the flame; the top and bottom parts of the flame do not have this intense falloff.

Because no additional UVW modifiers have been used, the scale of the UVW map is the exact side of the cylinder even when it’s size if being continuously changed due to the Noise controller, therefore the gradients will shrink and grow to fit the cylinder no matter how large or small it gets. Finally, coloured attenuated lights are attached using attachment controllers to the flame so they stick to polygons as the object is deforming to illuminate the scene effectively. The end result is a quite convincing flickering yellow flame.

	Open up the bunsen_burner_start.max scene that can be found on the cover cd. Create a cylinder primitive with a Radius of 20, Height of 350, 15 Height Segments and 16 Sides. Label this cylinder “Flame”. Position it above the Bunsen Burner as illustrated.
	Add a Taper Modifier to the Flame’s stack. Set the Amount to -0.8 and the Curve to 4. Add a Volume Select Modifier. Set the Stack Selection Level to Vertex. Move the Volume Select’s Gizmo ½ way up the cylinder so the top half of the cylinder’s vertices are selected. Enable Soft Selection and amend the falloff setting to get a falloff intensity as shown.
	Add a Noise modifier to the stack. Set the Scale to 165, turn on Fractal and set the X and Y Strength to 80. Move the Noise Gizmo above the Flame geometry. Turn on animate and at frame 200 reposition the gizmo approx 4500 units above the flame geometry as shown. Amend the Noise Gizmo’s two new Z keyframes to create a linear motion with no velocity ramping as shown.
	Add a Meshsmooth to the stack and set the number of Iterations to 1. The Flame material has already been pre-created for you, so simply assign this to the Flame geometry. Open up the Function Curve Editor and navigate to the Height controller of the cylinder. Assign a Float List controller to the Height Controller. In the resulting first Available slot, add a Noise Float controller and in the next resulting Available slot add a TCB Float controller.
	In the Noise Float’s properties, set the Frequency to 0.2, Strength to 300, turn on “>0” and set the Roughness to 0.1 . Select the TCB Float controller and draw keys from frame 0 to 200. Delete all keys from frames 1 to 199 inclusive and set both keys properties to a value of 200.
	Create an Omni light with a Multiplier of 10, Inverse Square decay, Far attenuation to 1000, Shadow map Bias to 0.1, Size to 256 and Sample Range to 35. Change light’s position controller to an Attachment controller. Click on it’s Attach To: Choose button and choose the Flame geometry. Click on the Set position button and position the light at the top of the flame by clicking on a face at the top of the flame.
	Additional post glows and motion blur have been added to the flame to blur any harsh edges and to emphasise the brightness of the “flame”.
	Zip file to accompany.

..::tips::..

Clone the light 5 times and reposition the clones down and around the Flame using their Attachment controllers. Change all the lights colours so they form a gradient vertically down from orange to yellow matching the diffuse gradient in the Flame’s material. Finally exclude all lights from illuminating and casting shadows on the Flame geometry and turn off Receive Shadows and Cast Shadows in the Flame geometry’s object properties.

Reduce the sample range by 5 for each light going vertically down the Flame geometry. This will give the impression that the light is more intense and not as diffused as the light that is being emitted further up the flame.

Should you want to introduce more than one Bunsen burner to the scene, simply select the lights and geometry and clone them. This will result in both flame’s animation being identical, so amend the seed values in the copy’s Noise modifier and Noise controller to create a different flame shape. To prevent the Flame lights from illuminating the new Flame geometry you will have to exclude this geometry from the all (including the original) Flame lights as well.

As always there is a very good plugin solution if you can stump up the cash. Phoenix from Chaos Group is an Atmospheric-based plugin which creates realistic fire materials, lighting and dynamics. For more information on Phoenix visit www.chaosgroup.com

Initially published: 3D World magazine, Issue 40, July 2003.

Copyright © Pete Draper, July 2003. Reproduction without permission prohibited.

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