BEVFormer-HoP

Intro

• Abstract
• Methods
• Contribution
• Getting Started
  • Installation
  • Dataset and Weights
  • Run and Eval
• Experiments
• Results
• Conclusion

Abstract

The objective is to develop a deep learning model that can transform monocular camera images of the surrounding into a bird’s eye view map. The output of the model will be bird’s eye view map that can be used to train the object detection, tracking and predictions algorithms of the Tesla Autopilot system. Detailed introduction, litterature review and problem statement are available in report.pdf.

Methods

BEVFormer: Learning Bird's-Eye-View Representation from Multi-Camera Images via Spatiotemporal Transformers, ECCV 2022

• Paper in arXiv, 13 Jul 2022

BEVFormer models have been shown to achieve state-of-the-art performance on a variety of benchmarks for BEV map generation in autonomous driving applications. This model consists of a convolutional neural network (CNN) backbone that extracts features from the input images and a transformer-based architecture that converts these features into a top-down representation of the surrounding environment. The overall architecture of BEVFormer is shown in bellow:

Figure 1: BEVFormer architecture

This repository is based on the official implementation of BEVFormer available here: https://github.com/fundamentalvision/BEVFormer.

Recall that in the context of our project, (i.e. building Tesla Autopilot system) we are only interested in extracing a bird-eye-view features map from surrounding cameras. From the figure above we therefore only want to extract BEV Bt that will be fed into downstream tasks (i.e. trajectories prediction and sim agents). However, to assess the quality of the BEV features map, it is more convenient to tie it up with 3d detection tasks or segmentation tasks. Here, only 3d detection is used.

Contribution

Temporal Enhanced Training of Multi-view 3D Object Detector via Historical Object Prediction

• Paper in arXiv, 3 Apr 2023

Very recent paper above comes up with a new paradigm, named Historical Object Prediction (HoP) for multi-view 3D detection to leverage temporal information more effectively. The overall architecture of HoP is shown in bellow:

Figure 2: Historical Object Prediction (HoP) architecture

This method allows state-of-the-art architectures to perform even better by generating a pseudo BEV feature map of timestamp (t−k) from its adjacent frames and utilize this feature to predict the object set at timestamp (t−k). HoP is performed only during training and thus, does not introduce extra overheads during inference. HoP is described as a plug-and-play approach and can be easily incorporated into state-of-the-art BEV detection frameworks including BEVFormer. As the paper is very recent, the implementation is yet not available and therefore the main contribution of this repo is the implementation of the proposed HoP method including it to the BEVFormer model.

Our implementation of HoP can be found at projects/mmdet3d_plugin/hop and is as follow:

├── hop/
    ├── __init__.py
    ├── detectors/
        ├── __init__.py
        ├── bevformer_hop.py/
    ├── modules/
        ├── __init__.py
        ├── hop.py/
        ├── object_decoder.py/
        ├── temporal_decoder.py/

HoP framework is implemented as a torch.nn.Module in hop.py. The TemporalEncoder that combines the outputs of both ShortTermTemporalDecoder and LongTermTemporalDecoder is implemented in temporal_decoder.py. For the theoritical background behind the implementation, please refer to the original paper. We have chosen to reuse the object detection head (aka. DetectionTransformerDecoder) of the existing BEVFormer as the ObjectDecoder (implemented in object_decoder.py) module in the HoP framework.

The HoP framework is then plugged to BEVFormer in bevformer_hop.py which implements the main class BEVFormer_HoP. Implementation of the forward pass using HoP, called forward_hop() and forward_train() as well as custom initialization from existing pre-trained weights is added to the exisiting BEVFormer class. Then BEVFormer_HoP is register to the detectors registry to comply with the existing repo of BEVFormer implemented using OpenMMLab.

Experiments

Recall that the main hypothesis behing HoP framework is that enforcing the detector to capture both the spatial location and temporal motion of objects occurring at historical timestamps can lead to more accurate BEV feature learning. The following experiments are here to check the sanity of our implementation as well as validating the main hypothesis.

Given the time and computational ressources limitations in the context of this project, here are the procedure used to obtain the results given bellow:

• Step 1: Only the tiny versions of BEVFormer, i.e. ResNet-50 instead of ResNet-101 as backbone and BEV feature map with shape 50x50 instead of 200x200, is considered. Existing pre-trained weights are loaded, see ckpts.
• Step 2: To get a decent weight initialization of HoP framework, we first only train the temporal and object decoder of HoP. To do so, from Figure 2, the image backbone as well as the image-view transform from the existing BEVFormer is freezed. Then, given the historical BEV feature maps from timestep t, the HoP branch is trained to generate the BEV maps of timestamp (t-k) from its adjacent frames and utilize this feature to predict the object set at timestamp (t-k).
• Step 3: From the weights of the HoP branch obtained after step 2, the image-view transform is unfreezed and BEVFormer is trained using both the prediction of the HoP branch and the prediction of the original BEV detection head. This is done by linear combination of the losses coming from both detection heads.

Finally, since we are training the models from pre-trained weights, the learning rate schedule is modify in consequence. Warmup steps are removed and the learning rate is decreased by a factor of 20.

To reproduce step 2 on the full dataset using 2 GPUs, run:

./tools/dist_train.sh ./projects/configs/bevformer_hop/bevformer_tiny_hop_only.py 2

We provide the logs of step 2. To visualize how the training went, run:

tensorboard --logdir=experiments/bevformer_tiny_hop_only/ --bind_all

To reproduce step 3 on the full dataset, using 2 GPUs, with a weight of 0.25 for the HoP branch, run:

./tools/dist_train.sh ./projects/configs/bevformer_hop/bevformer_tiny_hop_bi_loss_025.py 2

We provide the logs of step 3 for HoP weight of 0.25 and 0.5. To visualize how the training went, run:

tensorboard --logdir=experiments/bevformer_tiny_hop_bi_loss/ --bind_all

Results

Results of our experiments are shown bellow. For each experiments, the performances are computed on the validation set of the full-version of nuScenes. The performances metrics for the 3D object detection task are well defined HERE. We only report here:

• nuScenes detection score (NDS), weighted sum of mAP, mATE, mASE, mAOE, mAVE and mAAE
• mean Average Precision (mAP)

To assess the performances of our HoP framework implementation, we first established a baseline by training BEVFormer-tiny only (HoP weight loss of 0) on the nuScenes-train-mini from the pre-trained weights, over 10 epochs. Then we repeated the training under the same setup except that this time HoP framework is used with a weight of 0.5. A little increase in performances can be seen, tough not very relevant. Then, BEVFormer is trained for 2 epochs over the full dataset with a HoP weight of 0.5 and here the performances decrease. If we repeat the same training with a HoP weight of 0.25 the performances increase to 35% NDS.

Conclusion

From our experiments, we see that HoP framework can potentially increase the performances but the HoP weight is an important hyper-parameter. Also, we think that training the model with HoP from pre-trained weights is not optimal. Training the model from scratch with HoP should give better results as in the paper, but we could not afford that (training BEVFormer base, over 24 epochs with HoP would have taken around 16 days on SCITAS with 2 GPUs).

Our hypothesis is that, since we have trained BEVFormer from pre-trained weights, we have a model that has already converged to a good solution. However, the solution that one would obtain by training BEVFormer with HoP from scratch is likely very different since the temporal information is handled in a completly different way. Therefore, adding HoP to already pre-trained BEVFormer could lead to a decrease in performance first, and then to a convergence toward a solution that gives higher performances. This could explain the decrease in performance when training BEVFormer with HoP-weight set to 0.5 on 2 epochs. However, training only on 2 epochs is not enough to validate this hypothesis, and one should continue the training to assess its pertinence.

Inference

We also provide a script to make and render inference on video or single image using a trained model. Here is bellow an example of inference using bevformer base, a custom image config file and a score treshold of 0.15 for rendering:

python -m projects.inferences.inference ./projects/configs/bevformer/bevformer_base.py ./ckpts/bevformer_r101_dcn_24ep.pth ./projects/inferences/img_configs.py 0.15

To make inference on different video or images, simply change path and out_dir variables in your img_configs file.

Limitation: BEVFormer has been trained to generate BEV feature map from 6 well calibrated surrounding cameras. Therefore the model is not accurate at all using only the front camera. We have tried to duplicate the front image to simulate the surrounding environment, or use random images, but both methods failed. However, the pipeline and the rendering for single image as well as mp4 video works.