Next Limit RealFlow 3
by Scott Willman

INTRODUCTION

Someone solving a set of equations that were deemed completely unsolvable for more than one hundred years by the world’s top aeronautical scientists is absolutely incredible to me. Now the amazing people at Next Limit are putting that amazing power in your desktop workstation. Every so often new software comes around that reminds us just how far forward our technological efforts have brought us. With RealFlow 3, we can accomplish advanced fluid simulations that are completely manageable and look incredibly realistic.

Version 3 brings us a whole goodie bag of new features but concentrates largely on improving the general stability and speed of the application’s fluid solver. Many will agree that this new version provides a much more enjoyable experience than previous versions. In fact, along with The Lord of the Rings, you can see RealFlow 3 in action on the new Blue Sky all CG film “Robots”.

WHAT IS REALFLOW?

Before we get into the new feature set of Version 3, we should briefly discuss what RealFlow is and is not for the new users. Next Limit’s RealFlow is standalone simulation software for calculating the movements of real world dynamic objects, including liquids. RealFlow provides the artist with an elaborate set of tools that cover fluid simulation (liquid, and gas) w/mesh generator, rigid body dynamics with constraints, elastics, daemons to control fluid behavior, and waves and buoyancy (previously known as RealWave). You can (and should) import your own geometry or scene into RealFlow to set up your fluid simulation. After the simulation and tweaking is complete, export the particles or mesh from RealFlow into the major 3d application of your choice to light and render. For supported applications, see the conclusion of this article.

The RealFlow 3 Interface

Against popular belief, RealFlow is not a plug-in. Plug-ins are required (and freely downloadable from nextlimit.com) to import and export the simulations and geometry to and from your 3d app, but it is a stand-alone application. RealFlow does not offer any kind of lighting or rendering capability (nor should it). You are able to affect textures by transporting UV coordinates with the particle flow allowing you to drag textures creating all kinds of rich texture effects. You are able to generate UV coordinates for mesh and create what’s known as “Wet Texture Maps”. But I’ll discuss that more later on. These abilities serve the purpose of creating very rich and realistic texture effects, but not texturing in the traditional sense. Later on in the “Put It to Work” section of this review I will show you how it fit into my workflow in a test for a feature film project.

NEW FEATURES

One of the biggest issues with new software types is learning the ropes. Admittedly, previous versions 1 and 2 had decent but incomplete reference accompanying the software. Spending time in the Next Limit community forums revealed that this was a major concern over the majority of users. Version 3 boasts a new help system and it is certainly a vast improvement. Everything seems to be covered pretty well. Although, as I dug into the manual deeper I began to find that most of the content was surface level information. It mostly contained basic information about the attributes and abilities of the tools. More specifically, it would have been good to know things like the basic unit of measurement that is used (its meters by the way). One of the most important steps in getting fluids to look the way you’d expect is to obtain proper scale. I suppose one could have guessed seeing as how Next Limit is from Spain, but it could have been revealed to the user early on. All in all, it is a nice improvement, but could contain a little more insider information and tips ‘n tricks.

The New Help System

In this release Next Limit dove in and rewrote the fluid solver for the software. This is the section of the program that crunches all of the numbers and runs all of the equations to give us that beautiful fluid motion we crave. The rebuild does favor a speed increase of what I noticed to be about 20-30% during simulation. The program has been reworked to better handle multi-processor power users and larger memory allocation. This could possibly be where they draw some of their “up to twice as fast” statement on their website. Coupled with their new (and very exciting) command line features, such as being able to run a command line simulation on a separate machine while you continue to work on your workstation, I could see how this could add to the idea of significant speed improvements. There is a noticeable improvement in the accuracy of the simulation, further setting the standards for more accurate fluid simulations once again.

The new curve and expression editors give users a lot more freedom while key framing attributes. Giving attributes the ability to “react” to conditions based on expressions is a huge bonus, especially in effects work. Those key features can also be used in the fully integrated Real Wave feature set. Thus allowing you to do things such as emit particles from the surface of a Real Wave surface based on the surface’s attributes. Real Wave gives you the ability to generate waves and buoyancy effects based around geometry or particles.

Curve Editor

One new very cool innovation is the ability for your fluids to generate what they call “Wet Texture Maps”. When a particle hits a surface it can “paint” a texture on the surface of an object. Therefore, it can leave a trail or “wet” texture onto a surface. The image comes out in the form of a grayscale texture, white revealing the wet spots, black showing the untouched areas. I found this feature to work really well and generate very convincing effects. The user can also specify how long the texture stays white after it’s painted. They’ve made it quite pleasant to create convincing wet effects for surrounding geometry.

PUT IT TO WORK: Scene Setup

During some preliminary work for an upcoming project we were exploring the use of cg water for a couple shots. One of the sequences required a series of fierce rapids that had to follow a specific path in a 3d environment. We chose RealFlow as our guinea pig. I’ll roughly describe my workflow and let you know how the application stood up to the rigors of production.

I was provided a model of a river bed minus the water (duh). The geometry was very dense and elaborate and I knew that this would give RealFlow a serious headache and definitely take over my weekend. So I spent some time in Maya reducing the poly count of the mesh to where it was manageable but not changing the general topology much. It is also important to note that the simulator requires all imported geometry to be triangulated before it gets there. After exporting the geometry as an .obj file it was ready to be imported into RealFlow. If the scene contained anything more than unanimated geometry I would have used Next Limit’s SD format (Scene Data) which also exports deformations. Imported geometry seems to be one of the major things that can make RealFlow unpredictable. It seems that different types of geometry or different configurations, such as baked key frames on a deforming model, can cause the system to either not recognize the geometry, not hold the animation, or the worst of all, crash. However this was a pretty simple model so I didn’t have any of those issues this time around.

The first thing you need to do when you import your geometry and begin to set up your scene is to get a sense of scale. You can adjust the scale multiplier in the upper toolbar to adjust up or down. I should say that having the exact scene scale for a river and filling it with a realistic amount of particles would be next to impossible. However, balancing the resolution (how many particles are emitted) and the scale become the major challenge for any project. The more particles you have, the finer the detail will be when it comes time to mesh.

PUT IT TO WORK: Emitters

Deciding on the type of emitter was the next step. RealFlow provides you with many different types of emitters including: Circle, Square, Sphere, Linear, Triangle, Spline, Cylinder, Bitmap, and many more. I first tried an emitter called an Object Emitter. This will allow you to emit fluid from the faces or the vertices of your imported geometry. It was soon apparent that a square emitter was going to be the best choice to emit for this scene. Emitting particles from the face just shot them straight up or into another surface. So I connected the square emitter to the geometry in the Scene Tree window so that they would interact. Everything great so far…

PUT IT TO WORK: Daemons

Daemons are basically the heart of all that is creative inside of RealFlow. Daemons achieve similar effects to what other packages might call forces or fields. They can also achieve things that may only be possible through expressions or not possible at all in other programs. The destructor class of daemon is what I should address first. These daemons kill particles based on a set of rules. For instance, it is always a good idea to enclose your scene with a Volume daemon. This kills the particle if it leaves the defined volume. The further particles stray from the flock, the more system resources they consume and this can greatly slow down your scene. There are many other destructor daemons provided to help you manage your particles. The only other daemon that this scene called for was the classic Gravity daemon. This does exactly what you would think it does, pulls the particles down and keeps the particles on the surface. It also represents the other type of daemons, the type that moves the particles or causes them to take on a specific shape. Opening up the scene tree again, I connected the two new daemons to the emitter so that the emitter would be affected by their forces.

PUT IT TO WORK: Tweaking, Meshing, and Exporting

One big tip: tune the motion of your fluids at a very low resolution. When you increase the resolution to a respectable level your simulation will slow down exponentially, but the motion should remain very consistent and it will allow you quicker gratification to your changes. It’s not uncommon for the simulation to take longer than your final render.

The Rapids So Far…

The next step is to begin creating the mesh for the particle sequence. Generating the mesh is as easy as going to the meshes toolset, creating a new mesh, and attaching it to the particles. From there it’s a matter of adjusting the relationship between the resolution of the particles, the radius of the individual mesh spheres and the polygon size of the faces that it creates. This is the area that can get you the fine detail your after but at the cost of longer simulations and mesh creation time. This is a good time to mention the Mesh Dispatcher that comes with RealFlow. Unfortunately simulations can not be processed by a network, but generating the mesh sure can. The Mesh Dispatcher is very simple to use, easy to set up, and can save you lots of time. It is highly recommended.

From here it’s time for the final simulation. I can now unlock the timeline and decide what information I want to output from the simulator. The two most important options are of course the mesh and the particles. They are exported as a single file for each frame and for each type of item exported. Be prepared for large file sizes taking up lots of disk space. It’s not uncommon for a series of particle cache files to take up multiple gigabytes of a hard drive.

PUT IT TO WORK: Importing to Your 3d App

With the proper plug-ins downloaded and installed it is easy to import the simulation back into Maya. I imported it into the scene containing the geometry that I initially started with and it lined up perfectly. The beauty to importing the particles is that you can use any particle render type you normally would, except that now you have this wonderful fluid movement and some extremely powerful new per-particle attributes. This makes it relatively easy to make conditional shaders that react differently based on speed, density, pressure, etc. I could have also imported the mesh sequence and textured and rendered it as though it were any normal geometry. The picture below shows just how convincing this approach can be. You can also go to nextlimit.com and visit their gallery to see what others have created.

CONCLUSION

So it appears that not all implementations of the Navier-Stokes equations are equal. In my research RealFlow clearly delivers the most realistic simulations of any professional fluid simulator on the market today. It offers a very robust tool set not only for fluids but rigid bodies and elastics as well. All of this it accomplishes with a very user friendly interface and (improving) help system. The nature of fluid simulation is indeed one of the most processor intensive computations a computer can perform. This being said, it is doubtful that anytime in the near future will this be accomplished quickly, but I thank Next Limit for trying. And to address the stability issue, it does crash from time to time. Considering the huge datasets that it handles and processor intensive calculations, I’m really not that surprised. Not that I dismiss any fault for product instability, but I would sure rather be able to take the program to its knees rather than have limits imposed just to improve the stability.

The Next Limit support team is fantastic. All of my emails have been responded to on the same day and were very informative. If you don’t want to go that route or just want to see what others are doing, the online community at nextlimit.com is a great resource as well. And the support team frequents that also.

As for the price, I feel it is very fair. For 00 I can get a full production license and one command line license for a separate machine. Different configurations of full license and command line licenses are available as well on their website. But get this, for all the students reading this, the educational package is only ! I can’t remember a time when I’ve seen any software of this caliber under a hundred dollars for any version. There is also a fully working trial of RealFlow 3 available for download. The trial does have a popup splash screen that interrupts your simulations every twenty minutes or so. All in all, RealFlow has been an extremely exciting and powerful addition to my arsenal. Although it’s a little rough around the edges yet, if you want beautifully simulated liquids, RealFlow is where it’s at.

Please visit nextlimit.com for additional product information, pricing, and system requirements. Any comments about this article, please feel free to contact me.

Now supports: OS
Windows
Mac OSX
Linux

Supports: 3d apps:
Alias Maya
3d Studio Max
Lightwave
Cinema 4d
Softimage|XSI
Houdini