# C3D

[Learning Spatiotemporal Features with 3D Convolutional Networks](https://openaccess.thecvf.com/content_iccv_2015/html/Tran_Learning_Spatiotemporal_Features_ICCV_2015_paper.html)

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## Abstract

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We propose a simple, yet effective approach for spatiotemporal feature learning using deep 3-dimensional convolutional networks (3D ConvNets) trained on a large scale supervised video dataset. Our findings are three-fold: 1) 3D ConvNets are more suitable for spatiotemporal feature learning compared to 2D ConvNets; 2) A homogeneous architecture with small 3x3x3 convolution kernels in all layers is among the best performing architectures for 3D ConvNets; and 3) Our learned features, namely C3D (Convolutional 3D), with a simple linear classifier outperform state-of-the-art methods on 4 different benchmarks and are comparable with current best methods on the other 2 benchmarks. In addition, the features are compact: achieving 52.8% accuracy on UCF101 dataset with only 10 dimensions and also very efficient to compute due to the fast inference of ConvNets. Finally, they are conceptually very simple and easy to train and use.

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<img src="https://user-images.githubusercontent.com/34324155/143043383-8c26f5d6-d45e-47ae-be18-c23456eb84b9.png" width="800"/>
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## Results and Models

### UCF-101

| config | resolution | gpus | backbone | pretrain | top1 acc | top5 acc | testing protocol| inference_time(video/s)  | gpu_mem(M) | ckpt | log | json |
|:--|:--:|:--:|:--:|:--:|:--:|:--:|:--:|:--:|:--:|:--:|:--:|:--:|
|[c3d_sports1m_16x1x1_45e_ucf101_rgb.py](/configs/recognition/c3d/c3d_sports1m_16x1x1_45e_ucf101_rgb.py)|128x171|8| c3d | sports1m | 83.27 | 95.90 | 10 clips x 1 crop | x | 6053 | [ckpt](https://download.openmmlab.com/mmaction/recognition/c3d/c3d_sports1m_16x1x1_45e_ucf101_rgb/c3d_sports1m_16x1x1_45e_ucf101_rgb_20201021-26655025.pth)|[log](https://download.openmmlab.com/mmaction/recognition/c3d/c3d_sports1m_16x1x1_45e_ucf101_rgb/20201021_140429.log)|[json](https://download.openmmlab.com/mmaction/recognition/c3d/c3d_sports1m_16x1x1_45e_ucf101_rgb/20201021_140429.log.json)|

:::{note}

1. The author of C3D normalized UCF-101 with volume mean and used SVM to classify videos, while we normalized the dataset with RGB mean value and used a linear classifier.
2. The **gpus** indicates the number of gpu (32G V100) we used to get the checkpoint. It is noteworthy that the configs we provide are used for 8 gpus as default.
   According to the [Linear Scaling Rule](https://arxiv.org/abs/1706.02677), you may set the learning rate proportional to the batch size if you use different GPUs or videos per GPU,
   e.g., lr=0.01 for 4 GPUs x 2 video/gpu and lr=0.08 for 16 GPUs x 4 video/gpu.
3. The **inference_time** is got by this [benchmark script](/tools/analysis/benchmark.py), where we use the sampling frames strategy of the test setting and only care about the model inference time,
   not including the IO time and pre-processing time. For each setting, we use 1 gpu and set batch size (videos per gpu) to 1 to calculate the inference time.

:::

For more details on data preparation, you can refer to UCF-101 in [Data Preparation](/docs/data_preparation.md).

## Train

You can use the following command to train a model.

```shell
python tools/train.py ${CONFIG_FILE} [optional arguments]
```

Example: train C3D model on UCF-101 dataset in a deterministic option with periodic validation.

```shell
python tools/train.py configs/recognition/c3d/c3d_sports1m_16x1x1_45e_ucf101_rgb.py \
    --validate --seed 0 --deterministic
```

For more details, you can refer to **Training setting** part in [getting_started](/docs/getting_started.md#training-setting).

## Test

You can use the following command to test a model.

```shell
python tools/test.py ${CONFIG_FILE} ${CHECKPOINT_FILE} [optional arguments]
```

Example: test C3D model on UCF-101 dataset and dump the result to a json file.

```shell
python tools/test.py configs/recognition/c3d/c3d_sports1m_16x1x1_45e_ucf101_rgb.py \
    checkpoints/SOME_CHECKPOINT.pth --eval top_k_accuracy
```

For more details, you can refer to **Test a dataset** part in [getting_started](/docs/getting_started.md#test-a-dataset).

## Citation

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```BibTeX
@ARTICLE{2014arXiv1412.0767T,
author = {Tran, Du and Bourdev, Lubomir and Fergus, Rob and Torresani, Lorenzo and Paluri, Manohar},
title = {Learning Spatiotemporal Features with 3D Convolutional Networks},
keywords = {Computer Science - Computer Vision and Pattern Recognition},
year = 2014,
month = dec,
eid = {arXiv:1412.0767}
}
```
