s h a d e r  w r i t i n g:     c a n d l e   f l a m e   

The purpose of this project was to write a surface shader so that, when used in conjunction with the previously written (water) displacement shader, it would give a simple object the appearance of being an animated flame. As with the previous assignments, the emphasise was on the vital need to carefully observe and document a natural phenomena before creating a digital version of it.


The Results With a Color Shader

Displacement shader animation test


The noise is parented to a flameShape coordinate sytem that is moved vertically 50 meters/second. What I don't like about this is that displacement is referenced to the cicular structure of the cylinder and thus becomes radially symmetrical, creating an expansion/contraction of the apparent diameter of the cylinder instead of the whole body of the flame swaying back and forth.

Surface Shader
animation test #1


This uses the same noise coordinate system manipulator in Maya to animate the central opacity, which is based on the sin function and thus dependant on the angle the camera is to the flame object and/or its rotation to make the opacity reduction appear to be centered inside the flame near the base. Color and Luminousity use the spline function. I'd like to impliment an opacity edge feathering effect based on camera incident (facing) angle, but am not able to get the effect desired.


Surface Shader
animation test #2


HA! Was able to apply that edge opacity code after all to the surface shader and tweeked it for better color gradations. I also tweeked on the displacment shader to get a better shape. For this test I slowed the noise coordinate manipulator down to 35 meters/second and adjusted displacement amplitudes for a more subtle effect. I still want the flame tip to displace as a whole, but nature of the beast prevents it. See Surface Shader for details and a RIB to render a still. Displacement shader is below.


The Initial Displacement Shader
Take a flat 1 meter square plane...

deform it with this
Displacement Shader...


wrap it into a cylinder...

and add a little bit of large scale noise and voila! a candle flame, I think.


The Rotoscope



The Research

excerpts from Google chache of:
www.santesson.com/christ/ljus.htm (site no longer running)
image from: http://quest.arc.nasa.gov/space/teachers/microgravity/image/84.gif





Mats Ahlberg


Just above and around a burning wick there is a dark cone in a flame topped by a luminous yellow region. At the bottom of the flame there is a light blue zone. The temperature in the dark cone near the wick is fairly low, 600 oC, and rises to about 1,200 oC in the centre of the yellow region. The highest temperature, 1,400 oC, is found off the centre on the edge of the yellow portion of the flame.

Vaporised hydrocarbon molecules are decomposed stepwise by the heat, in the dark cone near the wick, loosing mainly C2H4 and CH2 radicals. The light blue zone at the bottom of a flame is a reaction zone. The blue light is primarily due to the band emissions from two excited molecules, C2 and CH, produced in their excited states by the chemical reactions creating them.


The reaction zone continues upwards around the yellow zone of the flame. Here radicals from the decomposed hydrocarbons react with oxygen from the air to form CO2 and water in a complex, not fully understood way. Because they are always separated by a layer of combustion products, there is no direct contact between undecomposed fuel and oxygen.


The most interesting part of a flame is the luminous yellow zone responsible for most of the light emitted by a candle. This zone is also called the carbon zone, because it consists of carbonaceous soot particles. These are formed, at the top of the dark cone, from decomposed hydrocarbons which are rich in carbon because they have a relatively low hydrogen-to-carbon ratio.

The primary soot particles range in size from 10 to 200 nm and eventually cluster into chain aggregates. They are heated to incandescence by the hot gases and by the heat radiated from the reaction zone. The full visible spectrum is emitted, but emission in the yellow region is the most intense. As the particles rise through the yellow zone they are consumed by reaction with water and carbon dioxide to yield CO.


excerpt from: http://www.bartleby.com/30/7.html

Scientific Papers. The Harvard Classics. 1909–14

The Chemical History of a Candle
Lecture IMichael Faraday


There is a current formed, which draws the flame out... You may see this by taking a lighted candle and putting it in the sun so as to get its shadow thrown on a piece of paper... You observe the shadow of the candle and of the wick; then there is a darkish part, as represented in the diagram, and then a part which is more distinct. Curiously enough, however, what we see in the shadow as the darkest part of the flame is, in reality, the brightest part; and here you see streaming upward the ascending current of hot air, as shown by Hooker, which draws out the flame, supplies it with air, and cools the sides of the cup of melted fuel.







excerpts from:

What is fire? A solid, liquid or gas?


When you light a candle, the flame has a definite shape and distinct regions are found within the flame. The first thing that happens is that the solid wax is melted, the liquid wax travels up the wick and is boiled off, becoming vaporised. The clear section at the base of the flame consists entirely of vaporised wax. Above the vaporised wax, some oxygen is present and the wax starts to react with the oxygen and burn. Wax is a hydrocarbon; in other words it is made up of hydrogen and carbon. The reactions that take place are many and complicated but there are two major processes: the oxygen reacts with the hydrogen to make water and with the carbon to produce carbon dioxide. The formation of carbon dioxide is actually a two step reaction; the first step forms carbon monoxide and the second step involves another oxygen joining to form carbon dioxide. This second step releases violet blue light as part of the reaction and it is this process that produces the blue colour of the flame


Just outside of the blue flame there is an oxygen-depleted area where the carbon atoms join together to form carbon particles. Now we get to the yellow flame. The carbon and hydrogen react with the more plentiful oxygen in this area as before. The heat from these reactions causes the carbon particles to get so hot that they glow. This is called incandescence and it is a good indication of the temperature of the flame, in this case the yellow candle flame is about 1200°C. The blue part of the flame is actually hotter than the incandescent yellow part but individual atoms don't incandesce, this only happens after the carbon has formed sizeable particles in the oxygen-depleted region. As you travel up the yellow flame, it cools until the temperature is too low to provide enough activation energy for the reaction between carbon and oxygen. If not all of the hydrogen and carbon has reacted by this stage, it will escape the flame as soot.


excerpts from:


United States Microgravity Lab (USML)-1
Experiment Results


The flame standoff is the distance between the flame and the wick. On earth it is typically 1-2 mm; in microgravity it is 5-7 mm.

The microgravity candle flame differs from a normal gravity flame in size, shape, color and flame structure. The microgravity flame has a larger flame standoff than that of a normal gravity candle flame (at the base). The width of the flame-standoff implies a weaker heat feedbackfrom the flame and a smaller wax burning rate.


The nearly spherical nature of the microgravity flame suggests that the flame is providing heat to the wick. This is unlike normal gravity where only a portion of the vaporized fuel reacts in the vicinity of the wick; the rest of the fuel vapor is swept downstream by buoyant convection and reacts in the plume region. Thus, the flame structure of these two flames is different. In normal gravity, the gas-phase structure of the candle flame resembles that of a downward propagating diffusion flame over a thin solid. Models of the later system show that the most intense reaction zone (highest reaction or heat release rate per unit volume) is close to the bottom of the flame near the wick. This region serves to stabilize the rest of the flame and provides the largest heat feedback to the wick for fuel vaporization.