Global Illumination Shadow Layers

Abstract

Computer graphics artists often resort to compositing to rework light effects in a synthetic image without requiring a new render. Shadows are primary subjects of artistic manipulation as they carry important stylistic information while our perception is tolerant with their editing. In this paper we formalize the notion of global shadow, generalizing direct shadow found in previous work to a global illumination context. We define an object's shadow layer as the difference between two altered renders of the scene. A shadow layer contains the radiance lost on the camera film because of a given object. We translate this definition in the theoretical framework of Monte‐Carlo integration, obtaining a concise expression of the shadow layer. Building on it, we propose a path tracing algorithm that renders both the original image and any number of shadow layers in a single pass: the user may choose to separate shadows on a per‐object and per‐light basis, enabling intuitive and decoupled edits.

Shadow Layers for Participating Media

“…Appendix A: Equivalent formulation for a single solid object We focus on a special case of the path integral formulation for shadow (Section 5) where the scene contains solid surfaces, and the set of casters is a single object: C = {c}. This setup is identical to Desrichard et al [DVP19], and we demonstrate the equivalence with our work. Omitting the dependence on the sensor position j and light path x, their path integral for the shadow layer of C is…”

“…While these methods could be applied to extract approximate shadows in renders, we propose to reuse information from the lighting simulation for an exact result. [DVP19] contain the radiance lost in the scene due to occlusions, estimated by a path tracing algorithm that stems from a path integral formulation of shadow. This approach has two main limitations.…”

“…Global illumination shadow layers [DVP19] contain the radiance lost in the scene due to occlusions, estimated by a path tracing algorithm that stems from a path integral formulation of shadow. This approach has two main limitations.…”

“…We generate Figures 3(e) and 3(f) following Desrichard et al . [DVP19]: We compute a third render where the object of interest is made completely absorbing, which allows to further split the subtraction $$J- I$$ into gains (scattered layer) and losses (shadow layer), respectively. The shadow layer $$S$$ can then be added to the main image $$I$$ to remove any shadow created by the object, as seen in Figure 3(c).…”

“…Indirect shadows from secondary light sources, such as a photography reflector, are not picked up. Desrichard et al [DVP19] propose a definition of shadow layers, an AOV containing the direct and indirect light loss generated by a given object on the rest of the scene. Such layers are useful inputs for the post-processing stage, as they effectively isolate the shadow component in the image.…”