Table Of Contents
Table Of Contents

8. Extracting video features from pre-trained models

Feature extraction is a very useful tool when you don’t have large annotated dataset or don’t have the computing resources to train a model from scratch for your use case. It’s also useful to visualize what the model have learned. In this tutorial, we provide a simple unified solution. The only thing you need to prepare is a text file containing the information of your videos (e.g., the path to your videos), we will take care of the rest. You can extract strong video features from many popular pre-trained models (e.g., I3D, I3D-nonlocal, SlowFast) using a single command line.

Note

Feel free to skip the tutorial because the feature extraction script is self-complete and ready to launch.

Download Full Python Script: feat_extract.py

For more command options, please run python feat_extract.py -h Please checkout the model_zoo to select your preferred pretrained model.

Prepare Data

Your data can be stored in any hierarchy. The only thing you need to prepare is a text file, video.txt, which should look like

/home/ubuntu/your_data/video_001.mp4
/home/ubuntu/your_data/video_001.mp4
/home/ubuntu/your_data/video_002.mp4
/home/ubuntu/your_data/video_003.mp4
/home/ubuntu/your_data/video_004.mp4
......
/home/ubuntu/your_data/video_100.mp4

Each line is the path to each video you want to extract features from.

Or you can also use the format we used for training models in other tutorials,

/home/ubuntu/your_data/video_001.mp4 200 0
/home/ubuntu/your_data/video_001.mp4 300 1
/home/ubuntu/your_data/video_002.mp4 100 2
/home/ubuntu/your_data/video_003.mp4 400 2
/home/ubuntu/your_data/video_004.mp4 200 1
......
/home/ubuntu/your_data/video_100.mp4.100 3

Each line has three things, the path to each video, the number of video frames and the video label. However, the second and third things are not gonna used in the code, they are just a placeholder. So you can put any postive number in these two places.

Note that, at this moment, we only support extracting features from videos directly.

Once you prepare the video.txt, you can start extracting feature by:

python feat_extract.py --data-list video.txt --model i3d_resnet50_v1_kinetics400 --save-dir ./features

The extracted features will be saved to the features directory. Each video will have one feature file. For example, video_001.mp4 will have a feature named i3d_resnet50_v1_kinetics400_video_001.mp4_feat.npy. The feature is extracted from the center of the video by using a 32-frames clip.

If you want a stronger feature by covering more temporal information. For example, you want to extract features from 10 segments of the video and combine them. You can do

python feat_extract.py --data-list video.txt --model i3d_resnet50_v1_kinetics400 --save-dir ./features --num-segments 10

If you you want to extract features from 10 segments of the video, select 64-frame clip from each segment, and combine them. You can do

python feat_extract.py --data-list video.txt --model i3d_resnet50_v1_kinetics400 --save-dir ./features --num-segments 10 --new-length 64

If you you want to extract features from 10 segments of the video, select 64-frame clip from each segment, perform three-cropping technology, and combine them. You can do

python feat_extract.py --data-list video.txt --model i3d_resnet50_v1_kinetics400 --save-dir ./features --num-segments 10 --new-length 64 --three-crop

We also provide pre-trained SlowFast models for you to extract video features. SlowFast is a recent state-of-the-art video model that achieves the best accuracy-efficiency tradeoff. For example, if you want to extract features from model slowfast_4x16_resnet50_kinetics400,

python feat_extract.py --data-list video.txt --model slowfast_4x16_resnet50_kinetics400 --save-dir ./features --slowfast --slow-temporal-stride 16 --fast-temporal-stride 2

The model requires the input to be a 64-frame video clip. We select 4 frames for the slow branch (temporal_stride = 16) and 32 frames for the fast branch (temporal_stride = 2).

Similarly, you can specify num_segments, new_legnth, etc. to obtain stronger features. There are many other options and other models you can choose, please check feat_extract.py for more usage information.

Total running time of the script: ( 0 minutes 0.000 seconds)

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