Model Fine Tuning#

The Smart City Blueprint uses two classes of models: computer vision models for object detection based on RT-DETR and Mask Grounding DINO, and vision language models (VLMs) based on NVIDIA CR2 model. Usage of these models is described in the Blueprint Deep Dive section Blueprint Deep Dive.

Users can fine tune these models to further specialize their object detection accuracy for specific object classes (for CV models). This section describes procedure for fine tuning the CV model; fine tuning recipe for the CR2 model is forthcoming.

CV model fine tuning#

Fine-tuning TrafficCamNet Transformer Lite with TAO RT-DETR#

The TrafficCamNet Transformer Lite model is based on the RT-DETR (Real-Time DEtection TRansformer) architecture and can be fine-tuned using NVIDIA TAO Toolkit to improve detection accuracy for specific traffic scenarios or new object classes.

Prerequisites#

  1. NVIDIA TAO Toolkit: Install TAO Toolkit following the official documentation.

  2. Dataset: Prepare your dataset in COCO format with:

    • Directory of training/validation images

    • Annotation files in COCO JSON format

    • Class mapping file (labels.txt)

  3. NGC CLI: Install NGC CLI following the NGC CLI documentation.

  4. Base Model: Download the TrafficCamNet Transformer Lite pretrained model from the TAO Model Zoo using the NGC CLI.

    ngc registry model download-version nvidia/trafficcamnet_transformer_lite:trainable_resnet50_v2.0 --destination /path/to/model

Dataset Preparation#

Organize your dataset in the following structure:

dataset/
├── train/
│   ├── images/
│   └── annotations.json
├── val/
│   ├── images/
│   └── annotations.json
└── labels.txt

The annotations should follow COCO format with bounding boxes for objects like vehicles, pedestrians, and cyclists.

Creating the Experiment Spec File#

Create a YAML specification file (e.g., rtdetr_trafficcam_finetune.yaml) with the following configuration:

dataset:
  train_data_sources:
    - image_dir: /path/to/dataset/train/images
      json_file: /path/to/dataset/train/annotations.json
  val_data_sources:
    image_dir: /path/to/dataset/val/images
    json_file: /path/to/dataset/val/annotations.json
  batch_size: 8
  workers: 4
  num_classes: 4  # e.g., car, person, bicycle, truck
  augmentation:
    multi_scales:
    - - 480
      - 832
    - - 512
      - 896
    - - 544
      - 960
    - - 544
      - 960
    - - 544
      - 960
    - - 576
      - 992
    - - 608
      - 1056
    - - 672
      - 1184
    - - 704
      - 1216
    - - 736
      - 1280
    - - 768
      - 1344
    - - 800
      - 1408
    train_spatial_size: [544, 960]
    eval_spatial_size: [544, 960]
    distortion_prob: 0.8
    iou_crop_prob: 0.8

model:
  backbone: resnet_50
  train_backbone: true
  pretrained_backbone_path: /path/to/trafficcamnet_transformer_lite.pth
  num_queries: 300
  dec_layers: 6
  enc_layers: 1

train:
  optim:
    lr: 0.0001
    lr_backbone: 0.00001
    lr_scheduler: MultiStep
    lr_steps: [30, 45]
    lr_decay: 0.1
    optimizer: AdamW
    weight_decay: 0.0001
  num_epochs: 50
  checkpoint_interval: 5
  validation_interval: 5
  num_gpus: 1
  gpu_ids: [0]
  pretrained_model_path: /path/to/trafficcamnet_transformer_lite.pth

Running Fine-Tuning#

Execute the training command:

tao model rtdetr train -e rtdetr_trafficcam_finetune.yaml \
                       results_dir=/results/trafficcam_finetuned

The training process will:

  • Load the pretrained TrafficCamNet Transformer Lite weights

  • Fine-tune on your custom dataset

  • Save checkpoints every 5 epochs

  • Run validation every 5 epochs

Resuming Training#

If training is interrupted, resume from a checkpoint:

tao model rtdetr train -e rtdetr_trafficcam_finetune.yaml \
                       train.resume_training_checkpoint_path=/results/trafficcam_finetuned/model_epoch_020.pth

Evaluating the Fine-Tuned Model#

After training, evaluate the model on your validation set:

tao model rtdetr evaluate -e rtdetr_trafficcam_finetune.yaml \
                          evaluate.checkpoint=/results/trafficcam_finetuned/model_epoch_050.pth

This will output mAP (mean Average Precision) and per-class detection metrics.

Exporting for Deployment#

Export the fine-tuned model to ONNX format for deployment:

tao model rtdetr export -e rtdetr_trafficcam_finetune.yaml \
                        export.checkpoint=/results/trafficcam_finetuned/model_epoch_050.pth \
                        export.onnx_file=/results/trafficcam_finetuned.onnx \
                        export.input_width=960 \
                        export.input_height=544

The exported ONNX model can then be converted to TensorRT for optimized inference in production environments.

Fine-Tuning Tips#

  • Learning Rate: Start with a lower learning rate (0.0001) when fine-tuning from a pretrained model

  • Data Augmentation: Adjust distortion_prob and iou_crop_prob based on your dataset characteristics

  • Epochs: Monitor validation mAP; typically 30-50 epochs are sufficient for fine-tuning

  • Backbone Freezing: Set train_backbone: false to freeze backbone weights if you have limited data

  • Multi-GPU Training: Increase num_gpus and adjust batch_size proportionally for faster training

For more detailed information on RT-DETR configuration options, refer to the TAO RT-DETR documentation.