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Tutorial at IEEE ISMAR 2023

A Beginner's Guide to Neural Rendering
WORK IN PROGRESS

Updated 20 October 2023 1145am

You may have heard of NeRFs (Neural Radiance Fields), or neural rendering more generally. Neural rendering brings together deep learning and computer graphics in order to generate extremely compelling 3D content from a set of 2D images. In this full-day tutorial, we'll start by learning the core principles of neural networks, deep learning, and volume rendering in order to prepare ourselves to scale NeRF Mountain. Later in the day, we’ll dissect the original NeRF paper in detail, explore extensions to the method and advancements in neural rendering, and see a lot of cool examples. We’ll close with a forward-thinking discussion on the opportunities and challenges associated with the use of neural rendering in MR. By the end of this tutorial, you should have a solid grasp of the NeRF method and the underlying technologies, including neural networks and deep learning.

nerf_fern.gif

2020
NeRF: Representing Scenes as Neural Radiance Fields for View Synthesis

[paper] [demo site] [github]
[overview video] [100LoC video] [explainer video]

This is the paper that started the NeRF revolution.

2020
Fourier Features Let Networks Learn
High Frequency Functions in Low Dimensional Domains

[paper] [demo site] [github]
[overview video] [100LoC video]

In general, MLP networks (such as those employed in the NeRF paper) struggle to learn high-frequency functions. This paper demonstrates that a simple Fourier feature mapping of the input before passing it to the MLP network yields substantially better results.

fourierfeatures.png

2021
NeRF in the Wild: Neural Radiance Fields for Unconstrained Photo Collections

[paper] [demo site] [explainer video]

This work extends NeRF in order to better handle unconstrained input photo collections (such as those you might find online) by decoupling appearance data - lighting, environmental conditions, camera intrinsics, postprocessing, etc. -  from geometry.

2021
Mip-NeRF: A Multiscale Representation
for Anti-Aliasing Neural Radiance Fields

[paper] [demo site] [github] [overview video]

Base NeRF works really well on data that is really well-structured - data where all of the viewing positions are at approximately the same distance from the model. But base NeRF has no concept of scale or aliasing, so it struggles with changing resolutions or scales.

 

By simply changing the feature mapping - which effectively creates an image pyramid, or mipmap, hence the name - we can achieve markedly superior results.

mip-nerf_square.png
d-nerf.png

2021
D-NeRF:
Neural Radiance Fields for Dynamic Scenes

[paper] [demo site] [github] [overview video]

D-NeRF (short for Dynamic-NeRF) extends NeRF to be able to handle animation sequences involving deformable objects. Naïvely adding the time index as a 6th parameter to the NeRF neural network does not yield satisfactory results, as it fails to exploit temporal redundancies in the input stream.

D-NeRF solves this problem by training two NNs instead of one: The first NN maps each point of the scene at time t back to that point in a canonical space - for convenience, the location of that point at t = 0. The second NN is a conventional NeRF network that computes the color and opacity of the transformed point from the desired viewing angle.

2021
pixelNeRF: Neural Radiance Fields from One or Few Images

[paper] [demo site] [github] [overview video]

Both pixelNeRF and the subsequent reference, RegNeRF, address the data-hungry nature of base NeRF. Base-NeRF produces very compelling results when tens of input images are available; pixelNeRF and RegNeRF present different strategies for reducing this number to a few, or even just one.

pixelNeRF, like D-NeRF, uses 2 NNs to accomplish this goal. pixelNeRF pretrains a convolutional NN (CNN) on ImageNet data, which is used as a "prior" for the conventional NeRF network. When the desired viewing position/direction is similar to the/a input view, the input view is weighted more heavily; when it is substantially different, the pretrained prior is weighted more heavily.

pixelNeRF.png
reg-nerf.png

2021
RegNeRF:
Regularizing Neural Radiance Fields for View Synthesis from Sparse Inputs

[paper] [demo site] [github] [overview video]

RegNeRF takes as its starting point Mip-NeRF and then, similar to pixelNeRF, attempts to produce compelling results for small sets of input images, in this case as few as 3.

To accomplish this, it generates additional input viewpoints that better sample the pose space, and modifies the standard NeRF cost/loss function to regularize the geometry and color of patches observed from those viewpoints.

2021
KiloNeRF: Speeding up Neural Radiance Fields with Thousands of Tiny MLPs

[paper] [demo site] [github]
[overview video] [100LoC video]

NeRFs are visually impressive, but out of the box, they are slow. Rendering a NeRF involves making millions of queries on a deep MLP NN, each of which consists of ~1M floating point operations (FLOPs). The key insight of KiloNeRF is to replace the single (big) MLP with many (tiny) MLPs.

The scene is subdivided into a coarse voxel grid, each of which corresponds to one of the small MLPs. These small MLPs are then trained using teacher-student distillation of a base NeRF model to maintain visual quality.

kilonerf_teaser_2x2.jpeg

2022
Mip-NeRF 360:
Unbounded
Anti-Aliased Neural Radiance Fields

[paper] [demo site] [github] [overview video]

Mip-NeRF 360, as the name suggests, is an extension of Mip-NeRF with optimizations that enable it to perform well on a particular type of dataset: unbounded (in depth) images taken from a 360° orbit of an object or objects of interest. It employs non-linear scene parameterization (the 2D mip-NeRF Gaussians are mapped into a 3D "ball"), online distillation (a simpler "coarse" network), and a novel distortion-based regularizer (which addresses "floaters" and "background collapse") to do so.

2022
TensoRF:
Tensorial Radiance Fields

[paper] [demo site] [github] [overview video]

TensoRF is perhaps the method that is "most different" from base NeRF of those on this page, certainly of those addressed so far. While NeRF-based methods model radiance fields purely as the weights and biases of an MLP NN, TensoRF explicitly models a radiance field as a 4D tensor: a voxel grid (3D) with per-voxel multi-channel features. They then perform various decompositions of this 4D tensor into matrix and vector components.

This new representation significantly improves reconstruction (training) time and delivers visually and quantitatively superior results to base NeRF, while maintaining a very small memory footprint.

tensorf2.png

2022
Block-NeRF:
Scalable Large Scene Neural View Synthesis

[paper] [demo site] [overview video]

Block-NeRF builds on techniques from NeRF in the Wild and Mip-NeRF to generate coherent city-scale radiance fields from millions of input images.

To accomplish this, it trains many individual NNs - in this work, roughly one per city block - using a combination of NeRF-W and mip-NeRF techniques and composites them by a simple interpolation weighted on distance from the viewpoint.

2022
Ref-NeRF: Structured View-Dependent
Appearance for Neural Radiance Fields

[paper] [demo site] [github] [overview video

Still to come.

rawnerf.png

2022

NeRF in the Dark:
High Dynamic Range View Synthesis from Noisy Raw Images

 

[paper] [demo site] [github] [overview video]

Base NeRF works well on clean (i.e., not noisy), low dynamic range, sRGB images. RawNeRF demonstrates the utility of the NeRF pipeline applied to noisy unprocessed camera output, which can preserve more of the scene's dynamic range.

It turns out that the NeRF pipeline is extremely robust to zero-mean camera noise,  enabling robust denoising and manipulation of focus, exposure, and tonemapping in the resulting images, in addition to the novel view synthesis most associated with NeRF.

2022
Instant Neural Graphics Primitives with a Multiresolution Hash Encoding

[paper] [demo site] [github] [overview video]

Still to come.

2023
Zip-NeRF: Anti-Aliased Grid-Based Neural Radiance Fields
 

[paper] [demo site] [overview video]

Still to come.

2023
RUST: Latent Neural Scene Representations from Unposed Imagery

[paper] [demo site]

Still to come.

2023
MobileNeRF: Exploiting the Polygon Rasterization Pipeline for Efficient Neural Field Rendering on Mobile Architectures

[paper] [demo site] [github] [overview video]

Still to come.

2023
K-Planes:
Explicit Radiance Fields in Space, Time, and Appearance

2023
Bayes' Rays: Uncertainty Quantification for Neural Radiance Fields

[paper] [demo site] [github]

Still to come.

2023
Plenoxels: Radiance Fields without Neural Networks

[paper] [demo site] [github]
[overview video] [100LoC video] [explainer video]

Still to come.

2023
3D Gaussian Splatting
for Real-Time Radiance Field Rende
ring

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