Bernhard Spanlang
University College London
Using panoramic HDR images as light sources in global illumination is becoming more popular in computer graphics. This is often referred to as rendering with natural light. The Radiance lighting simulation system was the first and still is one of a few renderers that can use HDR images as light sources to illuminate geometry. Unfortunately rendering such an image can still take several minutes and therefore this can't be applied in interactive simulations. For view independent (Lambertian) surfaces, people in the computer games industry have used global illumination methods to precompute the effects of light interreflections such as shadows and colour bleeding. These precomputations can be stored in the graphic hardware's texture memory. Such textures that contain lighting and shading information of a scene are often called Lightmaps. Although, when rendering, Radiance creates an ambient cache, that is similar to the concept of Lightmaps, there is no automatic way to store this information in texture maps for rendering with graphics hardware.
In our approach we start from geometry stored in a file format for interactive simulation such as VRML. Geometry and material (diffuse colour and texture) information is converted to the Radiance format. During file conversion a Radiance view file is also generated that describes an orthogonal camera in front of every flat surface (the camera position and direction can be computed from the surface normal and position). Additionally a HDR light probe can be added to the generated Radiance scene for natural lighting. Radiance is then executed to render the whole scene with the computed viewing parameters. This can be done rather efficiently since Radiance can build an ambient cache to avoid recomputation of view independent interreflections (these are computationally very expensive but since they are view independent they only need to be computed once for a scene). Also parallel techniques usually applied for walkthrough animations are exploited for the creation of Lightmaps. After rendering all surfaces the images are composed to a single texture loadable into graphics hardware. Quality and memory requirements mainly depend on the way this composition is performed. A benefit of using Radiance to compute Lightmaps is that Radiance tools can be applied in a post process for lighting analysis visualised interactively.