Object Detection with Ultralytics’ YOLO

This tutorial demonstrates how to pretrain a YOLO model using LightlyTrain and then fine-tune it for object detection using the ultralytics framework. We will perform both steps on the PASCAL VOC dataset.

Warning

Using Ultralytics models might require a commercial Ultralytics license. See the Ultralytics website for more information.

Install Dependencies

Install the required packages:

• lightly-train for pretraining, with support for ultralytics’ YOLO models

• supervision to visualize some of the annotated pictures

pip install "lightly-train[ultralytics]" "supervision==0.25.1"

Download the Dataset

We can download the dataset directly using Ultralytics’ API with the check_det_dataset function:

from ultralytics.data.utils import check_det_dataset

dataset = check_det_dataset("VOC.yaml")

Ultralytics always downloads your datasets to a fixed location, which you can fetch via their settings module:

from ultralytics import settings

print(settings["datasets_dir"])

Inside that directory (), you will now have the following structure of images and labels:

tree -d <DATASET-DIR>/VOC -I VOCdevkit

>    datasets/VOC
>    ├── images
>    │   ├── test2007
>    │   ├── train2007
>    │   ├── train2012
>    │   ├── val2007
>    │   └── val2012
>    └── labels
>        ├── test2007
>        ├── train2007
>        ├── train2012
>        ├── val2007
>        └── val2012

Note

Labels are not required for self-supervised pretraining. We will use the labels only for finetuning.

Inspect a few Images

Let’s use supervision and look at a few of the annotated samples to get a feeling of what the data looks like:

import random

import matplotlib.pyplot as plt
import supervision as sv
import yaml
from ultralytics import settings
from ultralytics.data.utils import check_det_dataset

dataset = check_det_dataset("VOC.yaml")

detections = sv.DetectionDataset.from_yolo(
    data_yaml_path=dataset["yaml_file"],
    images_directory_path=f"{settings['datasets_dir']}/VOC/images/train2012",
    annotations_directory_path=f"{settings['datasets_dir']}/VOC/labels/train2012",
)

with open(dataset["yaml_file"], "r") as f:
    data = yaml.safe_load(f)

names = data["names"]

box_annotator = sv.BoxAnnotator()
label_annotator = sv.LabelAnnotator()

fig, ax = plt.subplots(2, 2, figsize=(10, 10))
ax = ax.flatten()

detections = [detections[random.randint(0, len(detections))] for _ in range(4)]

for i, (path, image, annotation) in enumerate(detections):
    annotated_image = box_annotator.annotate(scene=image, detections=annotation)
    annotated_image = label_annotator.annotate(
        scene=annotated_image,
        detections=annotation,
        labels=[names[elem] for elem in annotation.class_id],
    )
    ax[i].imshow(annotated_image[..., ::-1])
    ax[i].axis("off")

fig.tight_layout()
fig.show()

Pretrain and Fine-tune

We will use LightlyTrain to pretrain a YOLO11 model.

The following scripts or CLI commands will:

• Initialize a YOLO11s model with random weights.

• Pretrain the YOLO11s model on the training images of PASCAL VOC using distillation pretraining.

• Export the pretrained YOLO11s model.

• Fine-tune the pretrained model on PASCAL VOC dataset using labels.

Python

# pretrain_yolo.py
import lightly_train
from ultralytics import settings

data_path = f"{settings["datasets_dir"]}/VOC/images/train2012"

if __name__ == "__main__":
    # Pretrain with LightlyTrain.
    lightly_train.train(
        out="out/my_experiment",            # Output directory.
        model="ultralytics/yolo11s.yaml",   # Pass the YOLO model.
        data=data_path,                     # Path to a directory with training images.
        epochs=100,                         # Adjust epochs for faster training.
        batch_size=64,                      # Adjust batch size based on hardware.
    )

# finetune_yolo.py

from ultralytics import YOLO

if __name__ == "__main__":
    # Load the exported model.
    model = YOLO("out/my_experiment/exported_models/exported_last.pt")

    # Fine-tune with ultralytics.
    model.train(data="VOC.yaml", epochs=100)

Command Line

lightly-train train out="out/my_experiment" data="<DATASET-DIR>/VOC/images/train2012" model="ultralytics/yolo11s.yaml" epochs=100 batch_size=64

yolo detect train model="out/my_experiment/exported_models/exported_last.pt" data="VOC.yaml" epochs=100

Congratulations! You have successfully pretrained a model using LightlyTrain and fine-tuned it for object detection using Ultralytics.

For more advanced options, explore the LightlyTrain Python API and Ultralytics documentation.

Next Steps

• Go beyond the default distillation pretraining and experiment other pretraining learning methods in LightlyTrain. Check Methods for more information.

• Try various YOLO models (YOLOv5, YOLOv6, YOLOv8).

• Use the pretrained model for other tasks, like image embeddings.