506 C HAP TER 11. IMAGE-BASED LIGHTING to the angle of reflection: rays near the outer edge of a sphere's image have an angle of reflection toward the camera that nears 90º, and thus their angle of incidence also nears 90º. Thus, the ray's angle of incidence relative to the camera nears 180º, meaning that the rays originate from nearly the opposite side of the sphere relative to the camera. Each image series of the sphere was converted into an HDR image using the HDR image assembly algorithm in Debevec and Malik [62] (see Chapter 4), and the images were saved in RADIANCE's native HDR image format (see Chapter 3). The algorithm derived the response curve of the video camera and produced HDR images where the pixel values were proportional to the light values reflected by the mirrored sphere. The total dynamic range of the scene was approximately 5000:1, measuring from the dark shadows beneath the bushes to the bright blue of the sky and the thin white clouds lit from behind by the sun. As another measure of the range, the brightest pixel values in the sky and cloud regions were some 150 times the average level of light in the scene. The two views of the sphere were com- bined (using techniques presented in Section 11.2.1) and mapped into the angular map space (described in Section 11.3.2) to become the light probe image seen in Figure 11.2(b). 11.1.2 MOD EL T HE G EOMET RY AND REFLECT ANC E OF T HE SCENE The RNL scene's spheres, stands, and pedestal were modeled using RADIANCE's standard scene primitives and generators. Each sphere was given a different material property with different colors of glass, metal, and plastic. The pedestal itself was texture mapped with a polished marble texture. The scene specification files are included on the companion DVD-ROM as rnl_scene.rad and gensup.sh . 11.1.3 MAP T HE LIG HT PROBE T O AN EMISSIVE SURFACE SURROUNDING THE SCENE In IBL, the scene is surrounded (either conceptually or literally) by a surface onto which the light probe image is mapped. In the simplest case, this surface is an infi- nite sphere. The RNL animation used a large but finite inward-pointing cube, posi- tioned so that the bottom of the pedestal sat centered on the bottom of the cube