Particle systems are often employed in 3D graphics for simulating non-solid matter such as smoke, steam, and fire. For such purposes, it's typical to use the particle system's built-in "facing" particles, which are rectangles onto which you map color and transparency gradients to simulate amorphous substances. This works fine when the effect is seen from a distance. But in professional applications when you need very realistic results, or the effect is seen close up, the particle system is often augmented with volumetric software, which replaces each particle's geometry with a computed volume, such as a puff of smoke. Volumetrics let you apply fractal noise to add complexity to the particles' appearance, and color gradients to simulate bursts of flame, etc. Each particle's volume melds seamlessly with those of its neighbors to create an effect that can look uncannily like the real thing.

One of the foremost volumetric packages is AfterBurn, which works as a plug-in to Discreet's 3ds max software. The new version 3 has just been released by Sitni Sati, the Croatia-based developer, and is available from the developer and vendors such as Turbo Squid as part of the Discreet Certified 3ds max Plug-in program. It's compatible not only with max's legacy particle systems (PArray et al) and third-party products such as Particle Studio, but with Discreet's spanking new Particle Flow system as well.

AfterBurn functions in max as an Atmosphere effect; you add it in the Rendering Environment dialog, and then specify the particle system or systems it's to affect. You can also designate light sources in the scene that the software takes into account when rendering the effect.

If you render at this point, the default settings surround each particle with a small, cloud-like puff. You can then load a preset--the software comes with 10--but more likely you'll want to create a custom effect. You do so using six rollouts: Manager; Illumination/Shading; Color; Particle Shape/Animation; Noise Shape; and Noise Animation.

The Manager is the control center where you designate particle systems and light sources to be used by AfterBurn. You can use max's Select By Name dialog when picking source particles and light sources, but it would be nice to have a built-in By List (or By Object Name) option. The Manager also lets you save and load presets, and preview the effects both in a separate window and in the viewports--the viewport preview uses low-poly geometry to approximate the volume shapes. Other useful tools here let you scale all numeric parameters in the system, enable and disable animation flow curves (about which more shortly), and reset the system to default values.

Also in the Manager are global settings such as Falloff, which determines the opacity or solidity of the volumetric effect, and Step Size, which determines the complexity of the final result. A smaller step size can increase realism at the cost of rendering times. Perhaps the most important Manager setting is the Rendering Type setting. The default is Raymarcher, which gives the best results for volumetrics, but can be slow. Also available are Octane, which is much faster than Raymarcher but gives lower-quality results, and Hypersolids, which gives particles a solid, blobby look suitable for liquids like water and mercury.

The Illumination/Shading Parameters rollout lets you toggle familiar functions such as whether effects should cast or receive shadows, if they can shadow themselves, and if shadows can be colored. Numeric shadow-related parameters include Samples, Falloff, Bias, and Opacity. This last setting, and many others in AfterBurn, can be animated between two values using AfterBurn's new, improved Animation Flow Curves (AFC) feature.

Clicking the AFC button opens a graph editor that lets you set control points to determine how to animate between the start and end values, much like max's Track View editor. In previous versions of AfterBurn, each control point could be linear or corner; in the latter, the curve straightens out as it approaches the control point. New in AfterBurn 3 is the ability also to set vertices to Bezier, which have handles that you can move to determine the slope of the curves entering and exiting each point. It's a limited Bezier implementation: You can change the handle size, but this doesn't affect the curve.

Other Illumination/Shading parameters include shading type, which you leave off for volumetrics, or set to Lambert or Phong for blobby particles. Also for the latter you can set reflection and refraction characteristics, new in this version of the software.

Next is the Color Parameters rollout, which makes good use of AfterBurn's powerful implementation of gradients. Gradients can be crucial in volumetric effects. In AfterBurn, you can use gradients in a variety of ways. For example, in a stream of smoke, you can set a gradient of up to three colors that goes from the center of the stream to the outside or varies by density. Each of the three colors can change over time among any number of keys using an intuitive editor. Transitions between keys can be linear, or use tension/continuity/bias settings (new in version 3), as well as ease-in and ease-out values. Alternatively, you can also set a color field to use only the first color of the gradient, but it remembers the gradient should you wish to switch back. The color swatches in the rollouts show the actual gradient to which they're set, and you can use a right-click menu to copy, paste, load, and save gradients.

The Particle Shape/Animation controls let you set the shape of each particle to that of a sphere, box, cylinder, or metaball. In each case, you can set and animate the dimensions and regularity, or uniformity, as well as rotation. You can also specify whether particles should align themselves with their direction of travel, and have them elongate as they travel faster. Each of these offers AFC controls, plus the new Interpolation Controllers (IC) option. Heretofore in AfterBurn, animation of parameter values was based only on particle age. But now you can alternatively base the animation on particle velocity, distance from the particle emitter or an arbitrary object, or use a mathematical expression to vary the effect. If you're wondering whether to upgrade, this powerful feature alone makes it well worth the $95 cost (free if you purchased AfterBurn 2.x between January 1 and March 23, 2003).

Last but not least are two rollouts related to the all-important noise settings. You can set the noise type to Turbulence, Fractal, Rocky, Smoke, or Spots. The documentation doesn't describe these, but you can see what they look like in a special preview window. Noise Gain and Bias determine the overall contrast and brightness of the noise pattern, and two additional noise types--fBm Fractal and fBm Turbulence--provide access to Blur and Detail settings as well. You animate the noise settings with AFCs and ICs, and also set percentage values for random variation. And when using Hypersolids, you can specify whether the noise works as a bump map or a displacement map. Other animatable noise parameters include Density, Size, and Levels, which determines the amount of detail in the noise pattern. The Threshold settings control the grayscale range within the patterns, and the Phase control lets you change the noise pattern over time.

That's mostly it for AfterBurn proper, but the software also comes with a host of ancillary tools. The AfterBurn Daemons include Wind, which forces noise to move in a certain direction; Swirl, which applies a spiral effect to the pattern; Void, which pokes holes in the volumetric effects; Explode, which gives you a bunch of settings for blowing stuff up; VMap, which lets you use any 3ds max-native map for a noise effect; and a tool that lets you use other Daemons' effects to alter a 3ds max map on any object, not just a particle system. There's also Sticky Particles, which let particles move along a mesh surface; fog that give more sophisticated results than max's native fog; and a glow render effect that you apply in Video Post. And there's an enhanced version of 3ds max's Fire Effect atmospheric (formerly known as Combustion), with additional noise settings and the ability to assign material IDs to the inner and outer colors. All in all, a most impressive package.

Conclusion

With a system as powerful and complex as AfterBurn, the learning curve can be steep, even with the best of documentation. The included online manual is very good, and includes eight tutorials, but these only scratch the surface. The software also comes with a number of samples, but no descriptions, so you've got to dig to find out how they work. Fortunately, Sitni Sati provides an informational Web site for its customers, which is where I found a helpful document by Scott Godfrey entitled An Insight Into Volumetrics. As always with complex software, other users can be the best way to learn.

Volumetrics is one of the most fascinating aspects of 3D graphics. In most cases, when you're working in 3D, you're using mesh objects with well-defined boundaries, but with volumetrics, you're depicting how light passes through a very complex object, not just how it bounces off the object. As a result, it typically takes much longer than usual to render an image containing volumetrics, even with the claimed up-to-40 percent rendering-speed increase in version 3. The interdependence of the various parameters in the volumetric system, especially in one so complex as AfterBurn, means that you've got to experiment a lot to get the results you have in mind. So it helps to have a very fast system, and if you've got one with multiple processors, that's even better.

The bottom line: AfterBurn is an amazing piece of software. It's not terribly easy to come to grips with, and to fully exploit its capabilities you'll need to spend a lot of time experimenting with it. The results are fully worth the effort, though; AfterBurn-rendered images and animations look just fantastic. If you want to play with one of the most powerful tools available for digital creativity, the $495 price of admission (plus a copy of 3ds max, of course) is one of the best bargains around.

http://www.afterworks.com/ABfirst.htm

A limited demo version of AfterBurn is available from Turbosquid.com