Image-based lighting (IBL) is a rendering technique that illuminates objects or scenes by capturing an omni-directional representation of real-world light information, usually in the form of images. High Dynamic Rang (HDR) images are often employed for higher fidelity. To capture the omni-directional light source, each lit surface point has to do a lighting integral covering the upper hemisphere of it. If we approximate the lighting integral by uniformly sampling the upper hemisphere of each lit point, the required number of samples for achieving plausible quality would be too high for real-time applications.
Many methods have been proposed in reducing the sampling costs of IBL. Here are some of them:
1) Prefiltering the light source images so we get different coarse levels of them. When doing lighting, coarser images could be sampled to approximate glossy reflection.
2) Represent the image source images in bases that could capture the light source information with a few coefficients, as well as allow lighting integrals to be computed by simple operations, such as dot-products Spherical harmonics and Haar wavelets are two such bases.
3) Importance sampling. For glossy surfaces, only light coming from a certain band of reflective directions contribute to the final energy the viewer perceives. One way to do importance sampling is to construct probability density functions (PDFs) based on material BRDFs, then generate sample rays randomly according to the PDFs. Reflective directions that contribute more to the final energy the viewer perceives would be sampled more densely.
- Jan Kautz and Michael D. McCool. 2000. Approximation of Glossy Reflection with Prefiltered Environment Maps. In Graphics Interface.
- Ravi Ramamoorthi and Pat Hanrahan. 2001. An efficient representation for irradiance environment maps. In Proceedings of the 28th annual conference on Computer graphics and interactive techniques (SIGGRAPH ’01).
- Ravi Ramamoorthi and Pat Hanrahan. 2002. Frequency space environment map rendering. InProceedings of the 29th annual conference on Computer graphics and interactive techniques(SIGGRAPH ’02).
- Ren Ng, Ravi Ramamoorthi, and Pat Hanrahan. 2004. Triple product wavelet integrals for all-frequency relighting. In ACM SIGGRAPH 2004 Papers (SIGGRAPH ’04).
- Mark Colbert et al. GPU-Based Importance Sampling. Chapter 20, GPU Gems 3.