Let There be Clouds! Fast, Realistic Cloud-Rendering in Microsoft Flight Simulator 2004: A Century of Flight

You're standing on rolling hills beneath a brilliant blue sky. You look up and see huge spherical white blobs suspended a few thousand feet in the air. What's wrong with this picture? Perhaps you could use a better cloud-rendering system.

In videogames that simulate outdoor reality, realistic clouds can be one of the most compelling aspects of the scene. Clouds can also set the mood - dark thunderheads for an ominous scene, light puffy clouds for a happy mood. Michelangelo spent years perfecting the heavens on the ceiling of the Sistine Chapel, but we need to render realistic clouds in milliseconds. Fortunately, we have more advanced tools to work with. This article describes the cloud modeling and rendering system that ships with Microsoft Flight Simulator 2004: A Century of Flight.

Clouds in the real world consist of many types, such as altocumulus, stratus, and cumulonimbus, and cloud coverages ranging from a few sparse clouds to a dense, overcast sky. Our cloud system models this range of cloud types and coverages. Microsoft Flight Simulator allows users to download real-world weather and see current weather conditions reflected in the game graphics, which means we need to generate compelling visuals to match any scenario that could occur in the real world.

The interactive nature of games necessitates that clouds must look realistic whether the camera is far away, next to the cloud, or traveling through the cloud. Another requirement is that we need to render at high framerates. Microsoft Flight Simulator supports a wide range of machines, from the latest PCs to those dating back several years, and the performance must scale to this spectrum of machines.

	Microsoft Flight Simulator 2004: A Century of Flight

Clouds need to be shaded appropriately to emulate both sunlight and light reflected from the sky, especially for games such as Microsoft Flight Simulator, which take place over the course of day, spanning dawn, midday, dusk, and night. We model the dynamic aspect of clouds by introducing a method to form and dissipate them over time.

Previous Work

Over the past 20 years, graphics researchers have modeled clouds in many ways, including cellular automata, voxels, and metaballs. They also modeled cloud animation via fluid dynamics. There are two reasons that these research techniques have not been widely adopted by games. The first is performance. Many of these systems produced screenshots that were gorgeous but required multiple seconds to render. The second is lack of artistic control. Imagine that you create a cloud by running a set of fluid dynamics equations. You examine the results and decide you would like a wispier top on the cloud. You must then iterate through cycles of adjusting variables such as air humidity and temperature, and recomputing the equations, which can require hours and still may not produce the visual effect you had in mind.

Realistic results in cloud shading have been achieved by simulating the scattering of light by particles as it passes through the cloud, known as anisotropic scattering. This produces accurate self-shadowing and interesting effects such as the halo when the cloud lies between the camera and the sun. We created a simple shading model for our system, forgoing these effects in exchange for fewer computations and higher artistic control.

Many flight simulation games have featured clouds, recent examples being Flight Simulator 2002, IL-2 Sturmovik, and Combat Flight Simulator III. A common approach is to paint clouds onto the skybox texture, which has minimal performance overhead, but such clouds look two-dimensional and never get closer as the camera moves toward them. A better solution is to draw each cloud as a single facing sprite. This solution looks realistic from a stationary camera but produces anomalies as the camera rotates around it. A few recent games use clusters of textured particles, similar to our system. Some use unique textures for every cloud, which has a high video memory cost as the number of clouds in the scene increases. Other systems use small blurry textures, which results in clouds that look volumetric but lack definition. All of these systems also lack the ability to form and dissipate clouds.

Our system was inspired after hearing a GDC talk by Mark Harris, who developed Skyworks, a real-time system that created volumetric clouds from sprites. Harris dynamically generated an impostor for every cloud and achieved speeds of 1 to 500 frames per second. He also modeled the fluid motion behind cloud animation. The limitation of his system is that it cannot render large clouds, such as cumulonimbus, or dense scenes of overcast clouds, due to the prohibitively high video memory cost of generating large impostors. Our system is able to address this limitation. In addition, we tackle the problem of scaling to multiple cloud types.

Cloud Modeling

Given that we want immediate visual feedback and full control over the final result, how can we design the artistic pipeline for modeling clouds? We model each cloud as five to 400 alpha-blended textured sprites. The sprites face the camera during rendering and together comprise a three-dimensional volume. We render them back-to-front based on distance to the camera.

We wrote a plug-in for 3DS Max that creates cloud sprites based on a 3D model composed of boxes. The artist denotes a cloud shape by creating and placing a series of boxes, using default 3DS Max functionality. The artist can create any number of boxes of any size and can choose to overlap the boxes.

	Figure 1. A custom tool within 3DS Max allows the artist to set properties and generate cloud sprites.

The plug-in UI contains an edit field to specify the number of sprites to generate. To create denser clouds, the artist would set a number which is proportionately higher than the size of boxes in the model. Wispier clouds would be created by setting a lower number. There are generally 20 to 200 boxes for each 16-square-kilometer section of clouds, and the number of sprites per box can vary between 1 to 100, depending on the density. The UI also allows the artist to specify a range for the width and height of each sprite, and choose between categories (such as stratus and solid cumulus) that determine the textures that will be automatically placed by the tool onto the sprites. Figure 1 shows a screenshot of the tool UI.

The artist presses a button in the plug-in UI to generate the cloud sprites. The plug-in creates a list of randomly placed sprite centers, then traverses the list and eliminates any sprite whose 3D distance to another sprite is less than a threshold value (the "cull distance"). This process reduces overdraw in the final rendering and also eliminates redundant sprites created from overlapping boxes. We have found that a cull radius of 1/3 of the sprite height works well for typical clouds, and 1/5 to 1/6 of the sprite height yields dense clouds. Figure 2 shows screenshots of a cloud model made of boxes and its corresponding sprites.

The plug-in creates an initial model of sprites, and the artist can now edit them within 3DS Max. Having achieved the desired visual look, the artist uses a custom-written exporter to create a binary file containing the sprite center locations, rotations, width, and height, along with texture and shading information. These files are loaded during game execution and rendered.

Textures

To create a dozen distinct cloud types, we mix and match 16 32-bit textures for both color and alpha (see Figure 3). The flat-bottomed texture in the upper right-hand corner is used to create flat bottoms in cumulus clouds. The three foggy textures in the top row are used heavily in stratus clouds and have a subtle bluish-gray tinge. The six puffy textures in the bottom two rows give interesting nuances to cumulus clouds, and the remaining six are wispy sprites that are used across all cloud types.

	Figure 2. Artists use boxes to build the shapes of clouds (left), and then let a custom tool in 3DS Max populate the boxes with sprites (right).

By creating interesting features inside the textures that resemble eddies and wisps, we are able to create more realistic looking clouds with fewer sprites. We place all 16 textures on a single 512x512 texture sheet, which spared the cost of switching textures between drawing calls to the video card. We automatically generate mip-map levels for this texture from 512x512 down to 32x32. To create more variations from these 16 textures, the artist specifies a minimum and maximum range of rotation for each sprite. When the binary file is loaded into the game, the sprite is given a random rotation within the range.

In-Cloud Experience

We would like a seamless in-cloud experience that looks consistent with the cloud's appearance as viewed from the outside, which does not often come with the commonly used technique of playing a canned in-cloud animation. In our system, as the camera passes through a sprite, it immediately disappears from view. We encountered a problem because the sprites rotated to face the camera, and during the in-cloud experience, the camera was so close to the sprite center that small movements in the camera position caused large rotations of the cloud sprite. This resulted in a "parting of the Red Sea" effect as sprites moved out of the way of the oncoming camera.

We locked the facing angle of the sprite when the camera came within half of the sprite radius. This removed the Red Sea effect but caused sprites to be seen edge-on if they locked and the camera then pivoted around them. Our solution was to detect the angle between the sprite's locked orientation and the vector to the camera, and to adjust the transparency of the sprite. The negative side effect is that the section of the cloud near the camera appears less opaque.

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