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Coffs Bedroom
by VRgrafix 2003

Creating an animated Gas Flame

By Frank “The Rookie” Dodd

Thanks to Bernie den Hertog for the gas stove reference image

 At first glance creating a gas flame seams like a simple job to create a simple object, essentially it is a smokeless burning cone that glows and flickers about. However to create a convincing animated gas flame required some problem solving as we will now see. There are five steps to this job: -

  1. Gathering References
  2. Modeling
  3. Creating Materials
  4. Texturing Models
  5. Animating the model

Gathering References

References images are essential to creating an accurate model in a short amount of time, they can either be photographs of the subject or sketches you have made by hand. In this case I started by using a web search engine to scour sites for images of gas flames (both AltaVista and Google offer good image search engines) and Bernie also helped me out with digital photographs of his gas stove.

Fig 1: Gas Stove Reference Image

By studying the flames it could be seen that the flame starts brightly at the bottom usually a cyan blue, which quickly fades into a deeper blue before fading to black. The edges of the flame had a slightly diffuse blue glow, but still had a well-defined edge. I used this description to sketch up some gas flames until I was happy with the result.

Fig 2: Gas Flame Sketch


A gas is usually modeled with a volumetric object or with particles to achieve a diffuse shape that changes as it moves within the air. However as we have seen a natural gas flame emanating as a jet has a fairly constant shape and a visible edge so it could be created as a model. The flame has a conical shape and looks good with a slight bulge at its base, it is also symmetrical so it is a good candidate for construction by rotating a NURBS curve about and axis.

Fig 3: Constructing the model

 Creating Materials

There was now a nicely shaped gray flame shaped blob, and a material needed to be created that defined how light interacted with the object before reflecting into the camera. For a detailed insight into this process please refer to David Coombes articles explaining ray tracing. Many materials can be created by using existing materials as a template or by using the built-in wizards. This material was created from scratch though by selecting the menu option New VSL Material.

Fig 4: Creating a new VSL material

First the colour of the  material was defined, as we said the flame is cyan at the bottom and as we go upward it turns a rich blue a simple way to define this is with a VSL curve object. Colour is a property of the surface and so a surface property shader was added followed by a curve object.

Fig 5: Adding shaders

The colour changes as it goes up the material so the curve input needed to be the Y position of the Mapping Co-ordinates, the higher up the map it goes the further along the curve it moves. When it is low down the curve the RGB levels are set to a nice cyan and as it goes up the curve the RGB levels become more blue, the gradient view is very useful when defining colour.

Fig 6: Defining the colour curve

Another property we noticed was that the flame faded away at the top. This is another property of the surface that can be easily define with a curve. Again the fade changes as it goes up the material so the curve input was again the Y position of the Mapping Co-ordinates. As for the shape of the curve when it is low down the flame is very visible but as it gets to the peak it fades to nothing.

Fig 7: Defining the fade curve

The flame has a blue glow all around it so some fixed glow was added to our material with a simple constant value, a nice rich blue. Another constant value was used to set the size of the glow. The settings of all of these parameters will probably need to be tweaked specifically for a specific application.

Fig 8: Defining the glow

Finally the geometry of the model was still illuminated by the lighting in the scene as can be seen at the left of figure 9, only a gas flame produces its own lighting and illuminates itself. This effect can be created by copying the colour channel into the illumination channel.

Fig 9: Self illumination

Texturing Models

Finally a Geometric model had been produced and a gas like material created. Now all that was needed was to bring the two together to produce a gas flame. The material was very simply applied as a default mapping, the mapping following the contours of the geometric object so that the fading and colour was applied from the base of the object working its way upward. 

Animating the model

Although scripting was not the only way to solve the animation problem it was the most flexible. If you don't feel comfortable with scripting a similar effect could have been achieved by creating a choreograph on the object that animates the objects scale and then simply create a rapidly changing curve, this choreograph could even have been saved and reloaded onto other objects.  However a little investment of  time in creating a script can make life much easier when you need to re use and subtly change the animation later. 

Gas flames generally tend to flicker, quickly and randomly changing the size by small amounts this can be easily simulated by scaling the object by a small random amount on each frame. The size of Gas flames can usually be controlled too for example by the knob on a Gas stove.

The best way to approach most animation problems is often in the guise of a Real3D style animation method. An animation method is an animated level running a script that uses the objects within it like parameters for defining the animation effect. Each method is separate and unique in how it works and in this instance any objects dropped into our Gas Flames method are treated like separate flames and animated as such, individually and separately. A slider associated with the method level will also be used to scale all of the objects inside it to create the effect of the Knob on the Gas Stove

Fig 10: Example Gas Flame Method


I always start by creating an example structure like figure 10 before I create a script so I know what I will be working with. Here you can see in the Example the Gas flame method and the flames stacked inside it. Once we are happy with the structure we can do a little planning of what needs to be done to create the effect. A quick way to approach this is with a flowchart, you can use a program like FlowCharter or even Microsoft Word's diagrams to scribble down jobs that need to be done. 

Fig 11: The main things we need to do

At this stage you can quickly move the tasks around and order them, inserting new items as you find new jobs that need to be done. Each time thinking through logically and examining exactly what effect each step in the process will have on the animation.

Fig 12: We introduce the idea of the Gas Knob control

Working through the procedure it seams to have defined all of the steps needed to control the animation. We get the gas knob setting go through all of the objects scale them randomly and then scale them again based on the setting of our gas knob. Once we are happy we can tidy our diagram up and make the flow between the steps more clear. Also we should clearly define what each step is doing. Before it was not clear how much of a random amount the object was to be scaled now it is.

Fig 13: Tidying up the diagram to make its progress more clear

The easiest way to create a Realsoft Script from this is to use Building Blocks translating these steps is fairly straightforward. First you can use comments to recreate the diagram inside the Building Blocks tool. Then you can enter blocks beneath the comments to carry out the tasks that they specify. For more information on using building blocks please refer to the tools manual.

Fig 14: The bare bones program, a framework to work within

Fig 15: Fleshing out the comments into complete working blocks

The Final Result

The final result of model, material and animation worked very well, especially for smaller images. More detail would need to be added to the flame if it was to be rendered at higher resolutions. This all works as a nice base for an animation and with a few tweaks it can be useful in many situations from a gas stove to a jets after burners.

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Download the Gas Flame Method

Page updated on Monday, 24 March, 2003 . For feedback / model submissions or articles - please email us.
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