Step-by-Step Tutorial for VFX Artists: Creating Water Simulations

In another excellent tutorial from ErikDoesVFX, he'll walk you through how to create water simulations using Houdini, one of the most powerful VFX tools available today. Inspired by a breakdown of various time-based simulations, he’ll explore how water animation has evolved, why Houdini is the go-to software for water effects, and how to create different quality levels of water simulations based on time constraints. By the end, you’ll have a clear understanding of how to create basic to advanced water simulations.

Table of Contents

1. Introduction to Water Simulations
2. How Water Animation Was Done Traditionally
3. The History of Computer-Generated Water
4. Overview of Modern Water Simulation Software
5. 1-Minute Water Simulation in Houdini
6. How CG Water Simulations Work
7. 1-Hour Water Simulation: A Sludge Scene
8. 100-Hour Water Simulation: Whale Splash Scene
9. Optimizing Houdini for VFX Water Simulations
10. Conclusion

1. Introduction to Water Simulations

Water is one of the most complex elements to simulate in computer-generated imagery (CGI) due to its fluidity, translucency, and interaction with light. This tutorial will cover the techniques and technologies used to create realistic water simulations for VFX, focusing on Houdini.

Water simulations can range from simple, low-particle-count scenes to high-resolution, intricate simulations that take hours or even days to compute. We’ll cover how to approach simulations with varying levels of complexity, from quick one-minute setups to full-fledged scenes that take upwards of 100 hours to render.

2. How Water Animation Was Done Traditionally

Before computer-generated water was possible, animators had to hand-draw water effects frame by frame. They studied real-life water references and often used clever techniques to mimic water's natural movement. For example, some animations involved moving a rippled glass pane over an image to create the illusion of water.

While these techniques were ingenious for their time, they couldn't capture the full complexity of water, leading to the need for more advanced, computer-driven methods.

3. The History of Computer-Generated Water

The first major leap in water simulation for movies occurred in 1998 with a film that pioneered computer-generated water. Though the results were rudimentary, they laid the groundwork for more refined simulations. By 2004, water simulations had advanced to the point where entire cities could be destroyed by realistic floods on screen.

Now, modern tools allow artists to generate water simulations with just a few clicks, achieving results that would have been unimaginable decades ago.

4. Overview of Modern Water Simulation Software

Several software packages dominate the water simulation landscape today:

• RealFlow: Known for its ease of use and realistic fluid dynamics.
• Maya's Bifrost: A powerful fluid simulation system for larger-scale projects.
• Blender (Flip Fluids plugin): A more accessible option with great results for small studios or individual artists.
• Houdini: Arguably the most advanced and flexible tool for water simulations, offering a node-based system for complete control over every aspect of the effect.

While all these tools are capable, Houdini stands out due to its flexibility and customization options, allowing you to build complex simulations from scratch.

5. 1-Minute Water Simulation in Houdini

Step 1: Set Up a Basic Scene

1. Create a Water Container: This acts as the boundary for your water.
2. Add a Water Source: Define where the water enters the scene.
3. Set Gravity: Ensure that the water will fall naturally by adding a gravity force.
4. Apply a Basic Water Shader: Add a blue water material to give the simulation a more realistic appearance.

Step 2: Run the Simulation

Since you only have a minute, the simulation will be very low resolution with a coarse particle count. This will result in thick and blocky water but will still resemble fluid motion.

Result:

You’ll end up with a basic water simulation that, while crude, demonstrates how quickly you can set up a fluid effect in modern software.

6. How CG Water Simulations Work

Water simulations in CGI are based on fluid dynamics principles. The two main components are:

1. Particles: These represent the mass and volume of the fluid.
2. Smoke Simulation (Voxels): This part handles the pressure calculations that give water its fluid look.

By combining particles and smoke (voxel-based simulation), you get a realistic water effect. The particles ensure mass conservation, while the voxels handle the fluid's motion and appearance.

7. 1-Hour Water Simulation: A Sludge Scene

For this simulation, we’ll replicate a scene inspired by The Hunger Games, where black sludge floods an area.

Step 1: Set Up the Scene

1. Track the Shot: Film a real plate (footage) and track it in 3D.
2. Create Geometry: Model basic 3D geometry that matches the plate so the water can interact with surfaces.
3. Add a Water Source: Use a sphere as the water emitter.
4. Add Viscosity: To simulate the sludge, increase the fluid's thickness.
5. Create a Wet Map: Mark areas where the sludge should stain the ground.

Step 2: Run the Simulation

With approximately an hour, you can simulate the sludge hitting a wall and splashing back. While the resolution won’t be high, you’ll get a good representation of viscous fluid motion.

Result:

The sludge simulation will have enough detail to resemble a gooey, thick fluid interacting with the environment.

8. 100-Hour Water Simulation: Whale Splash Scene

Inspired by Avatar: The Way of Water, we’ll create a high-resolution simulation of a whale jumping out of the ocean and making a massive splash.

Step 1: Animation

1. Model a Whale: Either model or purchase a whale model online.
2. Animate the Whale: Using real-world whale footage, animate the whale breaching the surface of the water.

Step 2: Water Simulation

1. Set Up the Ocean: Create a water surface using Houdini's ocean tools.
2. Simulate the Splash: Focus on capturing the intricate details of the water interacting with the whale’s body.
3. Simulate White Water: Add secondary effects like bubbles, foam, and spray to enhance the realism.

Step 3: Rendering

Render the high-resolution simulation, which might take up to 100 hours due to the complexity and number of particles involved.

Result:

You’ll achieve a stunning, realistic shot of a whale breaching water, with detailed splashes and foam.

9. Optimizing Houdini for Water Simulations

Why Houdini is the Best for Water Simulations:

• Node-Based Workflow: Houdini’s node system allows for incredible flexibility, letting you tweak and adjust simulations at any stage.
• Customization: You can script your own tools and effects using VEX, Houdini’s scripting language.
• Scalability: You can start with low-resolution simulations and gradually increase the detail as needed, ensuring that you don’t waste time on early iterations.

Tips for Speeding Up Simulations:

• Use Low-Resolution Proxies: Start with low voxel counts and fewer particles to iterate faster.
• Optimize for GPU: Consider using GPU-accelerated simulations for faster results, especially for real-time simulations like those offered by LiquidGen.

10. Conclusion

Water simulations in VFX have come a long way, from hand-drawn animations to complex fluid simulations that require hundreds of hours to compute. By using Houdini, you can create everything from basic, quick simulations to intricate, high-quality effects that push the boundaries of realism.

Whether you’re working on a tight deadline or spending weeks perfecting a shot, understanding how to control and optimize your fluid simulations is key to producing professional-level results.