Global Illumination Shadow Layers

François Desrichard, David Vanderhaeghe, Mathias Paulin.
Computer Graphics Forum, Wiley, 2019, 38(4).

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.


(a) Our approach computes shadow layers under global illumination, taking into account direct and indirect shadows for each light / object pair. Compositing tasks involving shadows such as (b) shape smoothing, (c) color grading, and (d) removal are easily achieved.

Acknowledgment: This work benefited from the support of project CaLiTrOp ANR-16-CE33-0026 of the French National Research Agency (ANR).
The authors want to thank Nicolas Mellado, Loïc Barthe and the anonymous reviewers for their valuable comments, and the artists and technical experts who motivated this work: Élodie Fraysse, Benjamin Legros, Cyril Corvazier, and Philippe Llerena.
The Cornell Box was adapted to pbrt by Benedikt Bitterli; the Dragon comes from the Stanford 3D Scanning Repository.
Moana Island Scene made available by Walt Disney Animation Studios.

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