Breath Vapor Simulation in Maya

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Once I hit the Edit button, I can click on the Runtime radio button in the middle of the expression editor window (see Figure 5) to switch to the runtime expressions so that I can make the particles grow as time passes.

For this, I will create an expression that reads the current radius of the particle, and will, as each frame goes by, increase itself by 0.01. [an error occurred while processing this directive] The runtime expression reads: Anim_particleShape1.radiusPP=Anim_particleShape1.radiusPP+.01;

Figure 7.
The reason I have a semicolon at the end of all my expressions, other than being just plain cool, is a matter of MEL syntax. MEL, as you may know, is Maya Embedded Language, the programming code, so to speak, on which Maya is built. All expressions in Maya use MEL commands and syntax. So, for each MEL command to be distinguished from the next, a semicolon is placed after the command. Even if you don’t have a second expression in the Expression Editor, it’s still a good idea to end your expression with a semicolon. And the really hot chicks (or guys) dig it.

Onto the next attribute for the particles, and that is the Opacity. I want these particles to fade in and fade out, so I must attach a ramp to the OpacityPP attribute that is a gradient from black to gray and back to black again.

I click the Opacity button under the Add Dynamic Attributes section and choose Add Per Particle. OpacityPP now shows up in my Attribute Editor. I right click on it and choose Create Ramp.

Figure 8.
I create my ramp as in Figure 7 above. This will make the particles invisible at birth, make them fade in, and then fade out before their death (which I hear is quite nasty and painful).

Now since these are software rendered particles, this OpacityPP ramp will not affect the final render. This is a quick thing I do so I can see the particles fade in and out in the preview playblasts and in the perspective windows when I scrub the animation as I work. It is for animation feedback, not for the final render. It is a short step I find works well for me, and makes me feel pretty. The actual fading in and out will occure through the shader assigned to these particles.

Ok, now that we have half a bottle of whiskey in us, we’re almost done with the animation. All we need to do now, before we render the first pass, is to assign a cloud shader to these particles and actually give it the opacity talked about above.

In the Multilister (I prefer it over the Hypershade for these general things), I choose Edit>Create and pick Particle Cloud. I adjust the newly created Particle Cloud to give it a gray color, a density of 0.744, and a slight incandescence. Figure 8.

Figure 9.
The fade in and out of the breath I need to control through this shader’s Transparency attribute. To that end I attach a ramp to it by clicking the Map button to the right of the attribute in the editor and choose a ramp texture. I have adjusted my ramp as in Figure 9 to get the best fade in and out and general opacity settings for this simulation. Keep in mind, a colder climate will allow you to see more of the breath than a less cold one, so you can make that adjustment here in the shader’s Transparency along with the emitter’s Rate.

I will then run a few test renders and tweak all the shader settings and ramps to get the breath to look right. Once I do that, I’ll render it out without the background and take it into a composite program like After Effects and mix it into the back plates, fine tuning the transparencies and color. And that should do it. A bottle of whiskey and about an hour later, you’ve got cold breath. Well, she does anyway.


Dariush Derakhshani is an Animator at Sight Effects in Venice, CA He describes himself thusly: "30. Nicely bald. Fear of commitment. Mostly malaisey. Enlarged nasal turbinates. Taste for irony and digestive biscuits with chocolate. Has no idea how he got here, no idea of where there is to go. Animator. Teacher. A few awards and a couple degrees. Happy little man at Sight Effects in Venice, CA. Flat feet." He can be reached at dariush@digitallab.com.
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