# Training Identification Nets¶

This example will present how to train nets with identification loss on popular datasets.

The objective of training an identification net is to learn good feature representation for persons. If the features of the same person are similar, while the features of different people are dissimilar, then querying a target person from a gallery database would become easy.

Different loss functions could be adopted for this purpose, for example,

## Head First Example¶

After cloning the repository, we can start with training an Inception net on VIPeR with Softmax loss from scratch

python examples/softmax_loss.py -d viper -b 64 -j 2 -a inception --logs-dir logs/softmax-loss/viper-inception


This script automatically downloads the VIPeR dataset and starts training, with batch size of 64 and two processes for data loading. Softmax cross entropy is used as the loss function. The training log should be print to screen as well as saved to logs/softmax-loss/viper-inception/log.txt. When training ends, it will evaluate the best model (the one with best validation performance) on the test set, and report several commonly used metrics.

## Training Options¶

Many training options are available through command line arguments. See all the options by python examples/softmax_loss.py -h. Here we elaborate on several commonly used options.

### Datasets¶

Specify the dataset by -d name, where name can be one of cuhk03, cuhk01, market1501, dukemtmc, and viper currently. For some datasets that cannot be downloaded automatically, running the script will raise an error with a link to the dataset. One may need to manually download it and put it to the directory instructed by the error message.

### Model Architectures¶

Specify the model architecture by -a name, where name can be one of resnet18, resnet34, resnet50, resnet101, resnet152, and inception currently. For resnet*, running the scripts will download an ImageNet pretrained model automatically, and then finetune from it. For inception, the scripts just train the net from scratch.

### Multi-GPU and Batch Size¶

All the examples support data parallel training on multiple GPUs. By default, the program will use all the GPUs listed in nvidia-smi. To control which GPUs to be used, one need to specify the environment variable CUDA_VISIBLE_DEVICES before running the python script. For example,

# 4 GPUs, with effective batch size of 256
CUDA_VISIBLE_DEVICES=0,1,2,3 python examples/softmax_loss.py -d viper -b 256 --lr 0.1 ...

# 1 GPU, reduce the batch size to 64, lr to 0.025
CUDA_VISIBLE_DEVICES=0 python examples/softmax_loss.py -d viper -b 64 --lr 0.025 ...


Note that the effective batch size specified by the -b option will be divided by the number of GPUs automatically for each GPU. For example, 4 GPUs with -b 256 will have 64 minibatch samples on each one.

In the second command above, we reduce the batch size and initial learning rate to 1/4, in order to adapt the original 4 GPUs setting to only 1 GPU.

### Resume from Checkpoints¶

After each training epoch, the script would save a latest checkpoint.pth.tar in the specified logs directory, and update a model_best.pth.tar if the model achieves the best validation performance so far. To resume from this checkpoint, just run the script with --resume /path/to/checkpoint.pth.tar.

### Evaluate a Trained Model¶

To evaluate a trained model, just run the script with --resume /path/to/model_best.pth.tar --evaluate. Different evaluation metrics, especially different versions of CMC could lead to drastically different numbers.

## Tips and Tricks¶

Training a baseline network can be tricky. Many options and parameters could (significantly) affect the reported performance number. Here we list some tips and tricks for experiments.

Combine train and val
One can first use separate training and validation set to tune the hyperparameters, then fix the hyperparameters and combine both sets together to train a final model. This can be done by appending an option --combine-trainval, and could lead to much better performance on the test set.
Input size
Larger input image size could benefit the performance. It depends on the network architecture. You may specify it by --height and --width. By default, we use 256x128 for resnet* and 144x56 for inception.
Multi-scale multi-crop test
Using multi-scale multi-crop for test normally guarantees performance gain. However, it sacrifices the running speed significantly. We have not implemented this yet.
Classifier initialization for softmax cross entropy loss
We found that initializing the softmax classifier weight with normal distribution std=0.001 generally leads to better performance. It is also important to use larger learning rate for the classifier if underlying CNN is already pretrained.

## References¶

 [zheng2016person] Zheng, Y. Yang, and A.G. Hauptmann. Person Re-identification: Past, Present and Future. arXiv:1610.02984, 2014.
 [xiao2016learning] Xiao, H. Li, W. Ouyang, and X. Wang. Learning deep feature representations with domain guided dropout for person re-identification. In CVPR, 2016.
 [xiaoli2017joint] Xiao*, S. Li*, B. Wang, L. Lin, and X. Wang. Joint Detection and Identification Feature Learning for Person Search. In CVPR, 2017.
 [hermans2017in] Hermans, L. Beyer, and B. Leibe. In Defense of the Triplet Loss for Person Re-Identification. arXiv:1703.07737, 2017.