(train)=

# Train

The train command is a simple interface to pretrain a large number of models using
different SSL methods. An example command looks like this:

````{tab} Python
```python
import lightly_train

if __name__ == "__main__":
    lightly_train.train(
        out="out/my_experiment",
        data="my_data_dir",
        model="torchvision/resnet50",
        method="distillation",
        epochs=100,
        batch_size=128,
    )
````

````{tab} Command Line
```bash
lightly-train train out="out/my_experiment" data="my_data_dir" model="torchvision/resnet50" method="distillation" epochs=100 batch_size=128
````

```{important}
The default pretraining method `distillation` is recommended, as it consistently outperforms others in extensive experiments. Batch sizes between `128` and `1536` strike a good balance between speed and performance. Moreover, long training runs, such as 2,000 epochs on COCO, significantly improve results. Check the [Methods](#methods-comparison) page for more details why `distillation` is the best choice.
```

This will pretrain a ResNet-50 model from TorchVision using images from `my_data_dir`
and the DINOv2 distillation pretraining method. All training logs, model exports, and
checkpoints are saved to the output directory at `out/my_experiment`.

```{tip}
See {meth}`lightly_train.train` for a complete list of available arguments.
```

(train-output)=

## Out

The `out` argument specifies the output directory where all training logs, model exports,
and checkpoints are saved. It looks like this after training:

```text
out/my_experiment
├── checkpoints
│   ├── epoch=99-step=123.ckpt                          # Intermediate checkpoint
│   └── last.ckpt                                       # Last checkpoint
├── events.out.tfevents.1721899772.host.1839736.0       # TensorBoard logs
├── exported_models
|   └── exported_last.pt                                # Final model exported
├── metrics.jsonl                                       # Training metrics
└── train.log                                           # Training logs
```

The final model checkpoint is saved to `out/my_experiment/checkpoints/last.ckpt`. The
file `out/my_experiment/exported_models/exported_last.pt` contains the final model,
exported in the default format (`package_default`) of the used library (see
{ref}`export format <export-format>` for more details).

```{tip}
Create a new output directory for each experiment to keep training logs, model exports,
and checkpoints organized.
```

(train-data)=

## Data

Lightly**Train** expects a folder containing images or a list of (possibly mixed) folders and image files.
Any folder will be recursively traversed and finds all image files within it (even in
nested subdirectories).

The following image formats are supported:

- jpg
- jpeg
- png
- ppm
- bmp
- pgm
- tif
- tiff
- webp

Example of passing a single folder `my_data_dir`:

```bash
my_data_dir
├── dir0
│   ├── image0.jpg
│   └── image1.jpg
└── dir1
    └── image0.jpg
```

````{tab} Python
```python skip_ruff
lightly_train.train(
    out="out/my_experiment",            # Output directory
    data="my_data_dir",                 # Directory with images
    model="torchvision/resnet18",       # Model to train
)
```
````

````{tab} Command Line
```bash
lightly-train train out="out/my_experiment" data="my_data_dir" model="torchvision/resnet18"
```
````

Example of passing a (mixed) list of files and folders:

```bash
├── image2.jpg
├── image3.jpg
└── my_data_dir
    ├── dir0
    │   ├── image0.jpg
    │   └── image1.jpg
    └── dir1
        └── image0.jpg
```

````{tab} Python
```python skip_ruff
lightly_train.train(
    out="out/my_experiment",            # Output directory
    data=["image2.jpg", "image3.jpg", "my_data_dir"],                 # Directory with images
    model="torchvision/resnet18",       # Model to train
)
```
````

````{tab} Command Line
```bash
lightly-train train out="out/my_experiment" data='["image2.jpg", "image3.jpg", "my_data_dir"]' model="torchvision/resnet18"
```
````

## Model

See [supported libraries](#models-supported-libraries) in the Models page for a detailed list of all supported libraries and their respective docs pages for all supported models.

## Method

See [](#methods) for a list of all supported methods.

(logging)=

## Loggers

Logging is configured with the `loggers` argument. The following loggers are
supported:

- [`jsonl`](#jsonl): Logs training metrics to a .jsonl file (enabled by default)
- [`mlflow`](#mlflow): Logs training metrics to MLflow (disabled by
  default, requires MLflow to be installed)
- [`tensorboard`](#tensorboard): Logs training metrics to TensorBoard (enabled by
  default, requires TensorBoard to be installed)
- [`wandb`](#wandb): Logs training metrics to Weights & Biases (disabled by
  default, requires Weights & Biases to be installed)

(jsonl)=

### JSONL

The JSONL logger is enabled by default and logs training metrics to a .jsonl file
at `out/my_experiment/metrics.jsonl`.

Disable the JSONL logger with:

````{tab} Python
```python
loggers={"jsonl": None}
````

````{tab} Command Line
```bash
loggers.jsonl=null
````

(mlflow)=

### MLflow

```{important}
MLflow must be installed with `pip install "lightly-train[mlflow]"`.
```

The mlflow logger can be configured with the following arguments:

````{tab} Python
```python
import lightly_train

if __name__ == "__main__":
    lightly_train.train(
        out="out/my_experiment",
        data="my_data_dir",
        model="torchvision/resnet50",
        loggers={
            "mlflow": {
                "experiment_name": "my_experiment",
                "run_name": "my_run",
                "tracking_uri": "tracking_uri",
                # "run_id": "my_run_id",  # Use if resuming a training with resume_interrupted=True
                # "log_model": True,      # Currently not supported
            },
        },
    )
````

````{tab} Command Line
```bash
lightly-train train out="out/my_experiment" data="my_data_dir" model="torchvision/resnet50" loggers.mlflow.experiment_name="my_experiment" loggers.mlflow.run_name="my_run" loggers.mlflow.tracking_uri=tracking_uri
````

(tensorboard)=

### TensorBoard

TensorBoard logs are automatically saved to the output directory. Run TensorBoard in
a new terminal to visualize the training progress:

```bash
tensorboard --logdir out/my_experiment
```

Disable the TensorBoard logger with:

````{tab} Python
```python
loggers={"tensorboard": None}
````

````{tab} Command Line
```bash
loggers.tensorboard=null
````

(wandb)=

### Weights & Biases

```{important}
Weights & Biases must be installed with `pip install "lightly-train[wandb]"`.
```

The Weights & Biases logger can be configured with the following arguments:

````{tab} Python
```python
import lightly_train

if __name__ == "__main__":
    lightly_train.train(
        out="out/my_experiment",
        data="my_data_dir",
        model="torchvision/resnet50",
        loggers={
            "wandb": {
                "project": "my_project",
                "name": "my_experiment",
                "log_model": False,              # Set to True to upload model checkpoints
            },
        },
    )
````

````{tab} Command Line
```bash
lightly-train train out="out/my_experiment" data="my_data_dir" model="torchvision/resnet50" loggers.wandb.project="my_project" loggers.wandb.name="my_experiment" loggers.wandb.log_model=False
````

More configuration options are available through the Weights & Biases environment
variables. See the [Weights & Biases documentation](https://docs.wandb.ai/guides/track/environment-variables/)
for more information.

Disable the Weights & Biases logger with:

````{tab} Python
```python
loggers={"wandb": None}
````

````{tab} Command Line
```bash
loggers.wandb=null
````

## Resume Training

There are two distinct ways to continue training, depending on your intention.

### Resume Interrupted Training

Use `resume_interrupted=True` to **resume a previously interrupted or crashed training run**.
This will pick up exactly where the training left off.

- You **must use the same `out` directory** as the original run.
- You **must not change any training parameters** (e.g., learning rate, batch size, data, etc.).
- This is intended for continuing the *same* run without modification.

### Load Weights for a New Run

Use `checkpoint` to further pretrain a model from a previous run. The checkpoint must
be a path to a checkpoint file created by a previous training run, for example
`checkpoint="out/my_experiment/checkpoints/last.ckpt"`. This will only load the model
weights from the previous run. All other training state (e.g. optimizer state, epochs)
from the previous run are not loaded. Instead, a new run is started with the model
weights from the checkpoint.

- You are free to **change training parameters**.
- This is useful for continuing training with a different setup.

### General Notes

```{important}
- `resume_interrupted=True` and `checkpoint=...` are mutually exclusive and cannot be
  used together.
- If `overwrite=True` is set, training will start fresh, overwriting existing outputs or
  checkpoints in the specified output directory.
```

## Advanced Options

### Input Image Resolution

The input image resolution can be set with the transform_args argument. By default a
resolution of 224x224 pixels is used. A custom resolution can be set like this:

````{tab} Python
```python
import lightly_train

if __name__ == "__main__":
    lightly_train.train(
        out="out/my_experiment",            # Output directory
        data="my_data_dir",                 # Directory with images
        model="torchvision/resnet18",       # Model to train
        transform_args={"image_size": (448, 448)}, # (height, width)
    )
```
````

````{tab} Command Line
```bash
lightly-train train out="out/my_experiment" data="my_data_dir" model="torchvision/resnet18" transform_args.image_size="[448,448]"
```
````

```{warning}
Not all models support all image sizes.
```

### Image Transforms

See {ref}`method-transform-args` on how to configure image transformations.

(method-args)=

### Method Arguments

```{warning}
In 99% of cases, it is not necessary to modify the default method arguments in
LightlyTrain. The default settings are carefully tuned to work well for most use cases.
```

The method arguments can be set with the `method_args` argument:

````{tab} Python
```python
import lightly_train

if __name__ == "__main__":
    lightly_train.train(
        out="out/my_experiment",            # Output directory
        data="my_data_dir",                 # Directory with images
        model="torchvision/resnet18",       # Model to train
        method="distillation",              # Pretraining method
        method_args={                       # Override the default teacher model
            "teacher": "dinov2/vitl14",
        },
    )
```
````

````{tab} Command Line
```bash
lightly-train train out="out/my_experiment" data="my_data_dir" model="torchvision/resnet18" method="distillation" method_args.teacher="dinov2/vitl14"
```
````

Each pretraining method has its own set of arguments that can be configured. LightlyTrain
provides sensible defaults that are adjusted depending on the dataset and model used.
The defaults for each method are listed in the respective {ref}`methods` documentation
pages.

### Performance Optimizations

For performance optimizations, e.g. using accelerators, multi-GPU, multi-node, and half precision training, see the [performance](#performance) page.

```{toctree}
---
hidden:
maxdepth: 1
---
method_transform_args
```