Physically Meaningful Rendering using Tristimulus Colours

• Published in: Computer graphics forum Vol. 34; no. 4; pp. 31 - 40
• Main Authors: Meng, Johannes, Simon, Florian, Hanika, Johannes, Dachsbacher, Carsten
• Format: Journal Article
• Language: English
• Published: Oxford Blackwell Publishing Ltd 01.07.2015
• Subjects: and texture; Categories and Subject Descriptors (according to ACM CCS); Color; Colour; Computer graphics; Computer Graphics [I.3.7]: Three-Dimensional Graphics and Realism-Color; Computer Graphics [I.3.7]: Three‐Dimensional Graphics and Realism—Color, shading, shadowing, and texture; Energy conservation; Lighting; Mathematical analysis; Rendering; shading; shadowing; Spectra; Studies; Texture; Transport; Wavelengths; United States > US
• ISSN: 0167-7055; 1467-8659
• DOI: 10.1111/cgf.12676

In photorealistic image synthesis the radiative transfer equation is often not solved by simulating every wavelength of light, but instead by computing tristimulus transport, for instance using sRGB primaries as a basis. This choice is convenient, because input texture data is usually stored in RGB colour spaces. However, there are problems with this approach which are often overlooked or ignored. By comparing to spectral reference renderings, we show how rendering in tristimulus colour spaces introduces colour shifts in indirect light, violation of energy conservation, and unexpected behaviour in participating media. Furthermore, we introduce a fast method to compute spectra from almost any given XYZ input colour. It creates spectra that match the input colour precisely. Additionally, like in natural reflectance spectra, their energy is smoothly distributed over wide wavelength bands. This method is both useful to upsample RGB input data when spectral transport is used and as an intermediate step for corrected tristimulus‐based transport. Finally, we show how energy conservation can be enforced in RGB by mapping colours to valid reflectances.