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nsff_pl

Neural Scene Flow Fields using pytorch-lightning. This repo reimplements the NSFF idea, but modifies several operations based on observation of NSFF results and discussions with the authors. For discussion details, please see the issues of the original repo. The code is based on my previous implementation.

The main modifications are the followings:

1. Remove the blending weight in static NeRF. I adopt the addition strategy in NeRF-W.
2. Remove disocclusion head. I use warped dynamic weights as an indicator of whether occlusion occurs. At the beginning of training, this indicator acts reliably as shown below:

As training goes, the disocclusion tends to get close to 1 almost everywhere, i.e. occlusion does not exist even in warping. In my opinion, this means the empty space learns to "move a little" to avoid the space occupied by dynamic objects (although the network has never been trained to do so).

3. Compose static dynamic also in image warping.

Implementation details are in models/rendering.py.

The implementation is verified on several sequences, and produces visually plausible results. Qualitatively, these modifications produces better result on the kid-running scene compared to the original repo.

Full reconstruction

Background reconstruction

Fix-view-change-time (view 8, times from 0 to 16)

Fix-time-change-view (time 8, views from 0 to 16)

Novel view synthesis (view 8, spiral)

Time interpolation (view 8, add 10 frames between each integer time from time 0 to 29)

The color of our method is more vivid and closer to the GT images both qualitatively and quantitatively (not because of gif compression). Also, even without any kind of supervision (either direct or self supervision), the network learns to separate the foreground and the background more cleanly than the original implementation, which is unexpected! Bad fg/bg separation not only means the background actually changes each frame, but also the color information is not leverage across time, so the reconstruction quality degrades, as can be shown in the original NSFF result towards the end.

Bonus - Depth

Our method also produces smoother depths, although it might not have direct impact on image quality.

More results

:computer: Installation

Hardware

• OS: Ubuntu 18.04
• NVIDIA GPU with CUDA>=10.2 (tested with 1 RTX2080Ti)

Software

• Clone this repo by git clone --recursive https://github.com/kwea123/nsff_pl
• Python>=3.7 (installation via anaconda is recommended, use conda create -n nsff_pl python=3.7 to create a conda environment and activate it by conda activate nsff_pl)
• Install core requirements by pip install -r requirements.txt
• Install cupy via pip install cupy-cudaxxx by replacing xxx with your cuda version.

:key: Training

Data preparation

Create a root directory (e.g. foobar), create a folder named frames and prepare your images (it is recommended to have at least 30 images) under it, so the structure looks like:

└── foobar
    └── frames
        ├── 00000.png
        ...
        └── 00029.png

The image names can be arbitrary, but the lexical order should be the same as time order! E.g. you can name the images as a.png, c.png, dd.png but the time order must be a -> c -> dd.

Motion Mask

In order to correctly reconstruct the camera poses, we must first filter out the dynamic areas so that feature points in these areas are not matched during estimation.

I use maskrcnn from detectron2. Only semantic masks are used, as I find flow-based masks too noisy.

Install detectron2 by python -m pip install detectron2 -f https://dl.fbaipublicfiles.com/detectron2/wheels/cu102/torch1.8/index.html.

Modify the DYNAMIC_CATEGORIES variable in third_party/predict_mask.py to the dynamic classes in your data (only COCO classes are supported).

Next, NSFF requires depth and optical flows. We'll use some SOTA methods to perform the prediction.

Depth

The instructions and code are borrowed from DPT.

Download the model weights from here and put it in third_party/depth/weights/.

Optical Flow

The instructions and code are borrowed from RAFT.

Download raft-things.pth from google drive and put it in third_party/flow/models/.

Prediction

Thanks to owang, after preparing the images and the model weights, we can automate the whole process by a single command python preprocess.py --root_dir <path/to/foobar>.

Finally, your root directory will have all of this:

└── foobar
    ├── frames (original images, not used, you can delete)
    │   ├── 00000.png
    │   ...
    │   └── 00029.png
    ├── images_resized (resized images, not used, you can delete)
    │   ├── 00000.png
    │   ...
    │   └── 00029.png
    ├── images (the images to use in training)
    │   ├── 00000.png
    │   ...
    │   └── 00029.png
    ├── masks (not used but do not delete)
    │   ├── 00000.png.png
    │   ...
    │   └── 00029.png.png
    ├── database.db
    ├── sparse
    │   └── 0
    │       ├── cameras.bin
    │       ├── images.bin
    │       ├── points3D.bin
    │       └── project.ini
    ├── disps
    │   ├── 00000.png
    │   ...
    │   └── 00029.png
    ├── flow_fw
    │   ├── 00000.flo
    │   ...
    │   └── 00028.flo
    └── flow_bw
        ├── 00001.flo
        ...
        └── 00029.flo

Now you can start training!

Train!

Run the following command (modify the parameters according to opt.py):

python train.py \
  --dataset_name monocular --root_dir $ROOT_DIR \
  --img_wh 512 288 --start_end 0 30 \
  --N_samples 128 --N_importance 0 --encode_t --use_viewdir \
  --num_epochs 50 --batch_size 512 \
  --optimizer adam --lr 5e-4 --lr_scheduler cosine \
  --exp_name exp

I also implemented a hard sampling strategy to improve the quality of the hard regions. Add --hard_sampling to enable it.

Specifically, I compute the SSIM between the prediction and the GT at the end of each epoch, and use 1-SSIM as the sampling probability for the next epoch. This allows rays with larger errors to be sampled more frequently, and thus improve the result. The choice of SSIM is that it reflects more visual quality, and is less sensible to noise or small pixel displacement like PSNR.

:mag_right: Testing

See test.ipynb for scene reconstruction, scene decomposition, fix-time-change-view, ..., etc. You can get almost everything out of this notebook. I will add more instructions inside in the future.

Use eval.py to create the whole sequence of moving views. E.g.

python eval.py \
  --dataset_name monocular --root_dir $ROOT_DIR \
  --N_samples 128 --N_importance 0 --img_wh 512 288 --start_end 0 30 \
  --encode_t --output_transient \
  --split test --video_format gif --fps 5 \
  --ckpt_path kid.ckpt --scene_name kid_reconstruction

More specifically, the split argument specifies which novel view to generate:

• test: test on training pose and times
• test_spiral: spiral path over the whole sequence, with time gradually advances (integer time for now)
• test_spiralX: fix the time to X and generate spiral path around training view X.
• test_fixviewX_interpY: fix the view to training pose X and interpolate the time from start to end, adding Y frames between each integer timestamps.

:warning: Other differences with the original paper

1. I add entropy loss as suggested here. This allows the person to be "thin" and produces less artifact when the camera is far from the original pose.
2. I explicitly zero the flow at far regions (where z>0.95).
3. I add a cross entropy loss with thickness to encourage static and dynamic weights to peak at different locations (i.e. one sample points is either static or dynamic). The thickness specifies how many intervals should the peaks be separated. Empirically I found 15 to be a good value. Explanation about this loss is, for example for the "kid playing bubble scene", the kid is rotating around himself with the rotation axis almost fixed, so if no prior is added, the network learns the central part around the body as static (for example you can imagine a flag rotating around its pole, only the flag moves but the pole doesn't). This doesn't cause problem for reconstruction, but when it comes to novel view, the wrongly estimated static part causes artifacts. In order to make the network learn that the whole body is moving, I add this cross entropy loss to force the static peak to be at least thickness//2 far from the dynamic peak.
4. In pose reconstruction, the original authors use the entire image to reconstruct the poses, without masking out the dynamic region. In my opinion this strategy might lead to totally wrong pose estimation in some cases, so I opt to reconstruct the poses with dynamic region masked out. In order to set the near plane correctly, I use COLMAP combined with monodepth to get the minimum depth.

TODO

• Add COLMAP reconstruction tutorial (mask out dynamic region).
• Remove NSFF dependency for data preparation. More precisely, the original code needs quite a lot modifications to work on own data, and the depth/flow are calculated on resized images, which might reduce their accuracy.
• Add spiral path for testing.
• Add mask hard mining at the beginning of training. Or prioritized experience replay.

Acknowledgment

Thank to the authors of the NSFF paper, owang zhengqili sniklaus, for fruitful discussions and supports!