RCPG: Real-Time Product Path Guiding Using Radiance Cascades

We present Radiance Cascade Path Guiding (RCPG), a real-time product path guiding method that improves upon previous state-of-the-art real-time path guiding methods both in quality and performance. We derive bounds for radiance intervals such that the direction of incoming light is not misrepresented by radiance cascades. This allows us to take advantage of the hierarchical nature of radiance cascades in a sampling process that takes the product of the incoming radiance and the material’s BSDF into account. It is also possible to compute the probability distribution function of RCPG sampling for any given direction, making it readily compatible with other sampling methods, such as multiple importance sampling (MIS) and unbiased ReSTIR.

As a product sampling scheme, RCPG can use a single sampling method for producing scattering directions, as opposed to needing separate importance sampling schemes for incident radiance and the BSDF. Compared to prior state-of-the-art real-time path guiding methods utilizing multi-sample MIS, this allows RCPG to generally trace twice as many paths per pixel within a real-time frame rate. Because the data structure is defined in world space, it can be used to generate samples starting from off-screen path vertices, which is necessary for effective multi-bounce path guiding.