Real-Time Glow

Glows and halos of light appear everywhere in the world, and they provide powerful visual cues about brightness and atmosphere. In viewing computer graphics, film, and print, the intensity of light reaching the eye is limited, so the only way to distinguish intense sources of light is by their surrounding glow and halos (Nakamae et al. 1990). These glows reproduce the visual effects of intense light and fool the observer into perceiving very bright sources. Even a subtle glow around an object gives the perception that it is brighter than an object with no glow. In everyday life, these glows and halos are caused by light scattering in the atmosphere or within our eyes (Spencer 1995). With modern graphics hardware, the effects can be reproduced with a few simple rendering operations. This allows us to fill real-time rendered scenes with bright, interesting objects that appear more realistic or more fantastic, and it is an elegant means to overcome the traditionally low-dynamic-range, flat look of real-time graphics. Several games are now using various techniques to produce glows and halos of light.

Among these are Splinter Cell, Project Gotham Racing, Wreckless (Kawase 2003), and Halo 2. Another notable and widespread use of glow can be seen in Pixar's film Finding Nemo, where glows convey the murkiness of seawater and help to set the mood for various scenes. This chapter focuses on a particular technique developed for the recently released Tron 2.0 game, produced by Buena Vista Interactive and developed by Monolith Productions. The technique was designed to produce large-area glows over the entire screen, to be easily authored and controlled for a large set of game assets, and to be fast enough for a first-person shooter game running at more than 60 frames per second. The results are shown in Figures 1 and 2. Here, the effect conveys the vibrancy and electronic power of the Tron 2.0 computer universe, though the technique can also be applied to create other effects, including depth of field, light scattering, edge detection, and image processing.

	Figure 1. A Tron 2.0 Cityscape With and Without Glow. Top: A scene with the glow effect. Bottom: The same scene without glow. Courtesy of Buena Vista Interactive/Disney Productions.

	Figure 2. A Tron 2.0 Character With and Without Glow. Top: Glow can enhance the importance of a character. Bottom: The character without glow is less captivating. Courtesy of Buena Vista Interactive/Disney Productions.

Overview of the Technique

There are several approaches to creating glow in a scene. For small point-like objects, a smooth, "glowy" texture can be applied to billboard geometry that follows the objects around the screen. In Tron 2.0, this is used for the Bit character. For large sources of glow or complex glowing shapes, it is best to post-process a 2D rendering of the scene in order to create the glow. This chapter focuses on the post-processing approach, whose steps are outlined in Figure 3.

First, the parts of a scene or model that glow have to be designated by some means that will allow them to be isolated and processed separately from the nonglowing parts. The scene is rendered normally, as it would be with no glow, but it is also rendered using the glow source information to create a texture map that is black everywhere except where the glow sources can be seen. An example of this rendered texture map is shown in Figure 3b. This rendered texture map can be used as an ordinary texture in later rendering. It is applied to simple geometry that causes it to be sampled many times at each pixel in a two-step image convolution operation, which blurs the glow source points out into the soft, broad-area glow pattern. Finally, the soft glow is applied on top of the ordinary rendering using additive alpha blending. In this way, the sources of glow are spread out into convincing auras of glow using hardware rendering and texture mapping.

	Figure 21-3. Rendering Steps for Creating Real-Time Glow. (a) The scene is rendered normally. (b) A rendering of glow sources is blurred to create (c) a glow texture, which is added to the ordinary scene to produce (d) the final glow effect.

Each of these steps can be done efficiently and quickly on a broad range of graphics hardware. The technique is best suited to hardware that supports Microsoft's Direct3D 8 Vertex and Pixel Shaders 1.1 or later, but a convenient fallback exists for hardware that supports only fixed-function Direct3D 7 rendering. For the older Direct3D 7–era hardware, which has lower fill-rate and texturing performance, the resolution of the render-target textures can be reduced to improve performance while not sacrificing much in terms of image quality. Because the blurred glow texture typically contains only low-frequency features, its resolution can be reduced with little loss in quality. In fact, as explained later, reducing the resolution of the texture render targets is a good way to create larger glows at no additional performance cost.

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