Not bad HTC! Looks like everyone is trying to create its own VR helmet. Face tracking and hand movements look impressive. However, manipulation part is still not comfortable. I don't want to hold those sticks all the time😐

https://tttttt.me/ai_newz/344

​​Nice infographics about the amounts of data uploaded and consumed everyday. Although it was created in 2019. Now the numbers has doubled at least IMO.

Full resolution

​​How to easily edit and compose images like in Photoshop using GANs?
MIT

🎯Task:
Given an incomplete image or a collage of images, generate a realistic image from it.

🔑Method:
This paper presents a simple approach – given a fixed pretrained generator (e.g., StyleGAN), they train a regressor network to predict
the latent code from an input image. To teach the regressor to predict the latent code for images w/ missing pixels they mask random patches during training.
Now, given an input collage, the regressor projects it into a reasonable location of the latent space, which then the generator maps onto the
image manifold. Such an approach enables more localized editing of individual image parts compared to direct editing in the latent space

📚Interesting findings:
- Even though our regressor is never trained on unrealistic and incoherent collages, it projects the given image into a reasonable latent code.
- Authors show that the representation of the generator is already compositional in the latent code. Meaning that altering the part of the input image, will result in a change of the regressed latent code in the corresponding location.

➕Pros:
- As input, we need only a single example of approximately how we want the generated image to look (can be a collage of different images).
- Requires only one forward pass of the regressor and generator -> fast, unlike iterative optimization approaches that can require up to a minute to reconstruct an image. https://arxiv.org/abs/1911.11544
- Does not require any labeled attributes.

💬Applications
- Image inpainting.
- Example-based image editing (incoherent collage -> to realistic image).

#paper_explained #cv

📝 Paper: Using latent space regression to analyze and leverage compositionality in GANs
🌐 Project page
⚒ Code
📓 Colab

🔥New video on my YouTube channel!🔥
I have created a detailed video explanation of the paper "NeX: Real-time View Synthesis with Neural Basis Expansion"

🎯 Task
Given a set of photos (10-60 photos) of the scene, learn some 3D representation of the scene which would allow rendering the scene from novel camera poses.

❓ How?
The proposed approach uses a modification of Multiplane Image (MPI), where it models view-dependent effects by parameterizing each pixel as a linear combination of basis functions learned by a neural network. The pixel representation (i.e., the coordinates in the set of bases defined by the basis functions) depends on the pixel coordinates (x,y,z), but not on the viewing angle. In contrast, basis functions depend only on the viewing angle and are the same for every pixel if the angle is fixed. Such angle and coordinates decoupling allows for caching all pixel representations which results in a 100x speedup of novel scene rendering (60FPS!). Moreover, the proposed scene parametrization allows the rendering of specular objects (non-Lambertian) with complex view-dependent effects.

✏️ Detailed approach summary
Multiplane image is a 3D scene representation that consists of a collection of D planar images, each with dimension H × W × 4 where the last dimension contains RGB values and alpha transparency values. These planes are scaled and placed equidistantly either in the depth space (for bounded close-up objects) or inverse depth space (for scenes that extend out to infinity) along a reference viewing frustum.

One main limitation of MPI is that it can only model diffuse or Lambertian surfaces, whose colors appear constant regardless of the viewing angle. In real-world scenes, many objects are non-Lambertian such as a ceramic plate, a glass table, or a metal wrench.

Regressing the color directly from the viewing angle v (and the pixel location [x,y,z]) with a neural network F(x, y, z, v), as is done in NERF, is very inefficient for real-time rendering as it requires to recompute every voxel in the volume for every new camera pose.

The key idea of the NEX method is to approximate this function F(x, y, z, v) with a linear combination of learnable basis functions {H_n(v): R^2 → R^{3x3}}.

To summarize, the modified MPI contains the following parameters per pixel: α, k0, k1 , . . . , k_N. These parameters are predicted by neural network f(x, y, z) for every pixel.

Another set of parameters -- global basis matrices H1(v) , H2(v), . . . , H_N(v) which are shared across all pixels but depend on the viewing angle v. The columns of H_n(v) are basis vectors of some color space different from RGB space. These basis matrices are predicted by another neural network g(v) = [H1(v) , H2(v), . . . , H_N(v)].

The motivation for using the second network is to ensure that the prediction of the basis functions is independent of the voxel coordinates. This allows to precompute and cache the output of f(x, y, z) for all coordinates. Therefore a novel view can be synthesized by just a single forward pass of network g(v), because f() does not depend on v and we don't need to recompute it.

Comparing with NeRF, the proposed MPI can be thought of as a discretized sampling of an implicit radiance field function which is decoupled on view-dependent basis functions H_n(v) and view-independent parameters α and k_n, n=1...N.

▶️ Video explanation
🌐 NEX project page
📝 NEX paper
⏱ Realtime demo

💠 Multiplane Images (MPI)
💠 NeRF: Representing Scenes as Neural Radiance Fields for View Synthesis
#paper_explained #cv #video_exp

🧑‍🎓 Some NEF implementation details:
- Fine details or high-frequency content tends to come from the surface texture itself and not necessarily from a complex scene geometry. Authors found that simply storing the first coefficient k0, or “base color,” explicitly helps ease the network’s burden of compressing and reproducing detail and leads to sharper results, also in fewer iterations.
- k0 is optimized explicitly as a learnable parameter with a total variation regularizer.
- computing and storing all N + 1 coefficients (k0, k1, ... kN) for all pixels for all D depth planes can be expensive for both training and rendering. So authors use a coefficient sharing scheme where every M consecutive planes will share the same coefficients, but not the alphas
- Total reconstruction loss = Photometric loss + difference of the image gradients.
- For a scene with 17 input photos of resolution 1008x756, the training takes ~ 18 hours using 1xNVIDIA V100 with a batch size of 1.
- To render one pixel, NEX uses 0.16 MFLOPs, whereas NeRF uses 226 MFLOPs.
- f() is an MLP with 6 FC layers, each with 384 hidden nodes.
- g() is an MLP with 3 FC layers, each with 64 hidden nodes.
- Positional encodings (sin and cos with different frequencies) of the inputs x,y,z, and v are used instead of the raw values. Such a mapping into a higher dimensional space enables the MLPs to more easily approximate a high-frequency function of variation in color and geometry of the scene.

​​Controllable Neural Text Generation

Self-supervised pretraining of Language Models has become a de-facto standard nowadays. When generating sentences from a Language Model by iteratively sampling the next token, we do not have much control over attributes of the output text, such as the topic, the style, the sentiment, etc. Many applications would demand good control over the model output. For example, if we plan to use LM to generate reading materials for kids, we would like to guide the output stories to be safe, educational, and easily understood by children.

How to steer a powerful unconditioned language model? Note that model steerability is still an open research question. In this blogpost,
Lilian Weng (OpenAI) discusses several approaches for acontrolled content generation with an unconditioned language model:
- Apply guided decoding strategies and select desired outputs at test time.
- Optimize for the most desired outcomes via good prompt design.
- Fine-tune the base model or steerable layers to do conditioned content generation.

🌀 Blogpost link

--
P.S. Lilian Weng has a very informative blog with a lot of interesting posts mostly on Reinforcement Learning and Natural Language Processing.
#NLP

​​FastNeRF: High-Fidelity Neural Rendering at 200FPS

Smart ideas do not come in the only head. FastNeRF has the same idea as in NeX, but a bit different implementation.
The main idea is to factorize the voxel color representation into two independent components: one that depends only on positions p=(x,y,z) of the voxel and one that depends only on the ray directions v.
Essentially you predict K different (R,G,B) values for ever voxel and K weighting scalars H_i(v) for each of them:
color(x,y,z) = RGB_1 * H_1 + RGB_2 * H_2 + ... + RGB_K * H_K.
Then 2 neural networks f(x,y,z) = [RGB_1, ... RGB_K] and h(v) = [H1, .. H_K] are learn to predict color components an their weights. After that the values of these functions are cached for every voxel in the volume which enables swift online rendering.

⚔️ NeX(➖) vs FastNeRF(➕):
➖ NeX achieves ~60Fps and PSNR 27 on the Real Forward-Facing dataset using Nvidia 2080Ti . While FastNeRF - 50-80 fps and PSNR 26 using Nvidia RTX 3090 GPU, fps rate varies from scene to scene. 200fps is only achieved on synthetic datasets or lower resolution.
➖FastNeRF requires a bit more memory to cache their scene representation because NeX uses sparse representation along the depth direction (only 192 slices) and share RGB_i values between every 12 consecutive depth planes.
➖ The same idea - factorize color representation. Predict K RGB values for every voxel instead of a single one.
➕ FastNeRF attempted to justify such factorization by the Rendering equation (see the image with an intuitive explanation below).
➕ Very similar architecture. NeX has one extra learnable Tensor which represents average RGB colors independent of ray direction. All other components are learned by neural networks. FastNeRF learns everything with neural nets.
➖ NeX has more extensive experiments and also experiments on fixed basis functions (i.e., compute H_i(v) using the Fourier’s series, spherical harmonics, etc). Interestingly, using the Fourier's series instead of neural network g(v)yields only a slightly worse PSNR score and even better LPIPS score.
➖ NeX introduces and evaluated on more challenging Shiny objects dataset. Would be interesting to see the results on FastNeRF on the same dataset as well.


❗️👉 Overall, I would say the approaches are very similar with some minor implementation differences. One would need to combine both implementations to get the best result.

🌐 FastNeRF paper
Unfortunately there are video results available and no code of FastNeRF yet.

This Marilyn Monroe never existed 👌🏻.
StyleGAN + Latent space regressor + CLIP (probably).

Thanks @metasemantic on Twitter

DONeRF: Towards Real-Time Rendering of Neural Radiance Fields using Depth Oracle Networks
(Graz Uni, FRL)

❗️ Another attempt to speedup NeRF: 15 FPS at 800x800.

The number of samples required for each view ray can be significantly reduced when local samples are placed around surfaces in the scene. Authors propose a depth oracle network, which predicts ray sample locations for each view ray with a single network evaluation. They show that using a classification network around logarithmically discretized and spherically warped depth values is essential to encode surface locations rather than directly estimating depth.

DONeRF is a combination of these techniques: A dual network design with a depth oracle network as a first step and a locally sampled shading network for ray accumulation.

➕ 48x speedup compared to NeRF, while equal or better quality compared to NeRF. Obviously it is not that fast as NeF or FastNeRF, but the approach is different.

📝 Paper
🌐 Proj page