Constraints and Priors for Inverse Rendering from Limited Observations

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Inverse Rendering deals with recovering the underlying intrinsic components of an image, i.e. geometry, reflectance, illumination and the camera with which the image was captured. Inferring these intrinsic components of an image is a fundamental problem in Computer Vision. Solving Inverse Rendering unlocks a host of real world applications in Augmented and Virtual Reality, Robotics, Computational Photography, and gaming. Researchers have made significant progress in solving Inverse Rendering from a large number of images of an object or a scene under relatively constrained settings. However, most real life applications rely on a single or a small number of images captured in an unconstrained environment. Thus in this thesis, we explore Inverse Rendering under limited observations from unconstrained images.

We consider two different approaches for solving Inverse Rendering under limited observations. First, we consider learning data-driven priors that can be used for Inverse Rendering from a single image. Our goal is to jointly learn all intrinsic components of an image, such that we can recombine them and train on unlabeled real data using self-supervised reconstruction loss. A key component that enables self-supervision is a differentiable rendering module that can combine the intrinsic components to accurately regenerate the image. We show how such a self-supervised reconstruction loss can be used for Inverse Rendering of faces. While this is relatively straightforward for faces, complex appearance effects (e.g. inter-reflections, cast-shadows, and near-field lighting) present in a scene can’t be captured with a differentiable rendering module. Thus we also propose a deep CNN based differentiable rendering module (Residual Appearance Renderer) that can capture these complex appearance effects and enable self-supervised learning. Another contribution is a novel Inverse Rendering architecture, SfSNet, that performs Inverse Rendering for faces and scenes.

Second, we consider enforcing low-rank multi-view constraints in an optimization framework to enable Inverse Rendering from a few images. To this end, we propose a novel multi-view rank constraint that connects all cameras capturing all the images in a scene and is enforced to ensure accurate camera recovery. We also jointly enforce a low-rank constraint and remove ambiguity to perform accurate Uncalibrated Photometric Stereo from a few images. In these problems, we formulate a constrained low-rank optimization problem in the presence of noisy estimates and missing data. Our proposed optimization framework can handle this non-convex optimization using Alternate Direction Method of Multipliers (ADMM). Given a few images, enforcing low-rank constraints significantly improves Inverse Rendering.