A Method for Rendering Wavelength-dependent Phenomena Using Spectral Image-based Lighting

Traditionally, RGB rendering that calculates the intensity of light in only three components has been often used for generating photorealistic images in global illumination environment, but the method cannot render wavelength-dependent phenomena accurately. On the other hand, Spectral rendering generates photorealistic images in various kinds of scenes including wavelength-dependent phenomena such as interference and fluorescence. However, the method is computationally expensive compared to RGB rendering especially in global illumination environment, because the spectral intensity of light over the range of the visible light should be calculated each time light rays collide with scene objects in the raytracing process. To reduce the computational cost of the spectral rendering, we introduce Image-based Lighting (IBL) where target objects are rendered without a number of iterations of the ray bouncing with scene objects by using a light probe image as an ambient light. We extend the IBL into a spectral IBL in order to combine IBL with spectral rendering, that is, the spectral image that includes the spectral intensity of ultraviolet in addition to visible light is used for the light probe image, and the spectral intensity of light is calculated to render the target objects. The proposed method is able to render wavelength-dependent phenomena realistically in shorter time, because the number of intersections between rays and objects is much smaller than that of the case without IBL. We have implemented the proposed method to a PBRT renderer and rendered scenes including the effects of fluorescence to demonstrate the usefulness of the proposed method.