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Transfer Learning for Computer Vision Tutorial#
Created On: Mar 24, 2017 | Last Updated: Jan 27, 2025 | Last Verified: Nov 05, 2024
Author: Sasank Chilamkurthy
In this tutorial, you will learn how to train a convolutional neural network for image classification using transfer learning. You can read more about the transfer learning at cs231n notes
Quoting these notes,
In practice, very few people train an entire Convolutional Network from scratch (with random initialization), because it is relatively rare to have a dataset of sufficient size. Instead, it is common to pretrain a ConvNet on a very large dataset (e.g. ImageNet, which contains 1.2 million images with 1000 categories), and then use the ConvNet either as an initialization or a fixed feature extractor for the task of interest.
These two major transfer learning scenarios look as follows:
Finetuning the ConvNet: Instead of random initialization, we initialize the network with a pretrained network, like the one that is trained on imagenet 1000 dataset. Rest of the training looks as usual.
ConvNet as fixed feature extractor: Here, we will freeze the weights for all of the network except that of the final fully connected layer. This last fully connected layer is replaced with a new one with random weights and only this layer is trained.
# License: BSD
# Author: Sasank Chilamkurthy
import torch
import torch.nn as nn
import torch.optim as optim
from torch.optim import lr_scheduler
import torch.backends.cudnn as cudnn
import numpy as np
import torchvision
from torchvision import datasets, models, transforms
import matplotlib.pyplot as plt
import time
import os
from PIL import Image
from tempfile import TemporaryDirectory
cudnn.benchmark = True
plt.ion() # interactive mode
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Load Data#
We will use torchvision and torch.utils.data packages for loading the data.
The problem we’re going to solve today is to train a model to classify ants and bees. We have about 120 training images each for ants and bees. There are 75 validation images for each class. Usually, this is a very small dataset to generalize upon, if trained from scratch. Since we are using transfer learning, we should be able to generalize reasonably well.
This dataset is a very small subset of imagenet.
Note
Download the data from here and extract it to the current directory.
# Data augmentation and normalization for training
# Just normalization for validation
data_transforms = {
'train': transforms.Compose([
transforms.RandomResizedCrop(224),
transforms.RandomHorizontalFlip(),
transforms.ToTensor(),
transforms.Normalize([0.485, 0.456, 0.406], [0.229, 0.224, 0.225])
]),
'val': transforms.Compose([
transforms.Resize(256),
transforms.CenterCrop(224),
transforms.ToTensor(),
transforms.Normalize([0.485, 0.456, 0.406], [0.229, 0.224, 0.225])
]),
}
data_dir = 'data/hymenoptera_data'
image_datasets = {x: datasets.ImageFolder(os.path.join(data_dir, x),
data_transforms[x])
for x in ['train', 'val']}
dataloaders = {x: torch.utils.data.DataLoader(image_datasets[x], batch_size=4,
shuffle=True, num_workers=4)
for x in ['train', 'val']}
dataset_sizes = {x: len(image_datasets[x]) for x in ['train', 'val']}
class_names = image_datasets['train'].classes
# We want to be able to train our model on an `accelerator <https://pytorch.org/docs/stable/torch.html#accelerators>`__
# such as CUDA, MPS, MTIA, or XPU. If the current accelerator is available, we will use it. Otherwise, we use the CPU.
device = torch.accelerator.current_accelerator().type if torch.accelerator.is_available() else "cpu"
print(f"Using {device} device")
Using cuda device
Visualize a few images#
Let’s visualize a few training images so as to understand the data augmentations.
def imshow(inp, title=None):
"""Display image for Tensor."""
inp = inp.numpy().transpose((1, 2, 0))
mean = np.array([0.485, 0.456, 0.406])
std = np.array([0.229, 0.224, 0.225])
inp = std * inp + mean
inp = np.clip(inp, 0, 1)
plt.imshow(inp)
if title is not None:
plt.title(title)
plt.pause(0.001) # pause a bit so that plots are updated
# Get a batch of training data
inputs, classes = next(iter(dataloaders['train']))
# Make a grid from batch
out = torchvision.utils.make_grid(inputs)
imshow(out, title=[class_names[x] for x in classes])
![['ants', 'ants', 'bees', 'ants']](../_images/sphx_glr_transfer_learning_tutorial_001.png)
Training the model#
Now, let’s write a general function to train a model. Here, we will illustrate:
Scheduling the learning rate
Saving the best model
In the following, parameter scheduler is an LR scheduler object from
torch.optim.lr_scheduler.
def train_model(model, criterion, optimizer, scheduler, num_epochs=25):
since = time.time()
# Create a temporary directory to save training checkpoints
with TemporaryDirectory() as tempdir:
best_model_params_path = os.path.join(tempdir, 'best_model_params.pt')
torch.save(model.state_dict(), best_model_params_path)
best_acc = 0.0
for epoch in range(num_epochs):
print(f'Epoch {epoch}/{num_epochs - 1}')
print('-' * 10)
# Each epoch has a training and validation phase
for phase in ['train', 'val']:
if phase == 'train':
model.train() # Set model to training mode
else:
model.eval() # Set model to evaluate mode
running_loss = 0.0
running_corrects = 0
# Iterate over data.
for inputs, labels in dataloaders[phase]:
inputs = inputs.to(device)
labels = labels.to(device)
# zero the parameter gradients
optimizer.zero_grad()
# forward
# track history if only in train
with torch.set_grad_enabled(phase == 'train'):
outputs = model(inputs)
_, preds = torch.max(outputs, 1)
loss = criterion(outputs, labels)
# backward + optimize only if in training phase
if phase == 'train':
loss.backward()
optimizer.step()
# statistics
running_loss += loss.item() * inputs.size(0)
running_corrects += torch.sum(preds == labels.data)
if phase == 'train':
scheduler.step()
epoch_loss = running_loss / dataset_sizes[phase]
epoch_acc = running_corrects.double() / dataset_sizes[phase]
print(f'{phase} Loss: {epoch_loss:.4f} Acc: {epoch_acc:.4f}')
# deep copy the model
if phase == 'val' and epoch_acc > best_acc:
best_acc = epoch_acc
torch.save(model.state_dict(), best_model_params_path)
print()
time_elapsed = time.time() - since
print(f'Training complete in {time_elapsed // 60:.0f}m {time_elapsed % 60:.0f}s')
print(f'Best val Acc: {best_acc:4f}')
# load best model weights
model.load_state_dict(torch.load(best_model_params_path, weights_only=True))
return model
Visualizing the model predictions#
Generic function to display predictions for a few images
def visualize_model(model, num_images=6):
was_training = model.training
model.eval()
images_so_far = 0
fig = plt.figure()
with torch.no_grad():
for i, (inputs, labels) in enumerate(dataloaders['val']):
inputs = inputs.to(device)
labels = labels.to(device)
outputs = model(inputs)
_, preds = torch.max(outputs, 1)
for j in range(inputs.size()[0]):
images_so_far += 1
ax = plt.subplot(num_images//2, 2, images_so_far)
ax.axis('off')
ax.set_title(f'predicted: {class_names[preds[j]]}')
imshow(inputs.cpu().data[j])
if images_so_far == num_images:
model.train(mode=was_training)
return
model.train(mode=was_training)
Finetuning the ConvNet#
Load a pretrained model and reset final fully connected layer.
model_ft = models.resnet18(weights='IMAGENET1K_V1')
num_ftrs = model_ft.fc.in_features
# Here the size of each output sample is set to 2.
# Alternatively, it can be generalized to ``nn.Linear(num_ftrs, len(class_names))``.
model_ft.fc = nn.Linear(num_ftrs, 2)
model_ft = model_ft.to(device)
criterion = nn.CrossEntropyLoss()
# Observe that all parameters are being optimized
optimizer_ft = optim.SGD(model_ft.parameters(), lr=0.001, momentum=0.9)
# Decay LR by a factor of 0.1 every 7 epochs
exp_lr_scheduler = lr_scheduler.StepLR(optimizer_ft, step_size=7, gamma=0.1)
Downloading: "https://download.pytorch.org/models/resnet18-f37072fd.pth" to /var/lib/ci-user/.cache/torch/hub/checkpoints/resnet18-f37072fd.pth
0%| | 0.00/44.7M [00:00<?, ?B/s]
92%|█████████▏| 41.2M/44.7M [00:00<00:00, 432MB/s]
100%|██████████| 44.7M/44.7M [00:00<00:00, 433MB/s]
Train and evaluate#
It should take around 15-25 min on CPU. On GPU though, it takes less than a minute.
model_ft = train_model(model_ft, criterion, optimizer_ft, exp_lr_scheduler,
num_epochs=25)
Epoch 0/24
----------
train Loss: 0.5558 Acc: 0.7213
val Loss: 0.3325 Acc: 0.8758
Epoch 1/24
----------
train Loss: 0.5005 Acc: 0.7828
val Loss: 0.3528 Acc: 0.8824
Epoch 2/24
----------
train Loss: 0.5238 Acc: 0.7787
val Loss: 0.2799 Acc: 0.8889
Epoch 3/24
----------
train Loss: 0.7393 Acc: 0.7213
val Loss: 0.2539 Acc: 0.8954
Epoch 4/24
----------
train Loss: 0.5331 Acc: 0.7869
val Loss: 0.2882 Acc: 0.8693
Epoch 5/24
----------
train Loss: 0.5506 Acc: 0.7746
val Loss: 0.2236 Acc: 0.9020
Epoch 6/24
----------
train Loss: 0.4585 Acc: 0.8074
val Loss: 1.2237 Acc: 0.7059
Epoch 7/24
----------
train Loss: 0.7083 Acc: 0.7623
val Loss: 0.2409 Acc: 0.9150
Epoch 8/24
----------
train Loss: 0.2590 Acc: 0.8934
val Loss: 0.2289 Acc: 0.9412
Epoch 9/24
----------
train Loss: 0.3133 Acc: 0.8566
val Loss: 0.2350 Acc: 0.9477
Epoch 10/24
----------
train Loss: 0.3000 Acc: 0.8811
val Loss: 0.2032 Acc: 0.9346
Epoch 11/24
----------
train Loss: 0.2973 Acc: 0.8770
val Loss: 0.1790 Acc: 0.9281
Epoch 12/24
----------
train Loss: 0.2712 Acc: 0.8852
val Loss: 0.1723 Acc: 0.9477
Epoch 13/24
----------
train Loss: 0.3758 Acc: 0.8443
val Loss: 0.1936 Acc: 0.9281
Epoch 14/24
----------
train Loss: 0.3541 Acc: 0.8443
val Loss: 0.1807 Acc: 0.9412
Epoch 15/24
----------
train Loss: 0.2667 Acc: 0.8893
val Loss: 0.1800 Acc: 0.9346
Epoch 16/24
----------
train Loss: 0.2789 Acc: 0.8730
val Loss: 0.1818 Acc: 0.9412
Epoch 17/24
----------
train Loss: 0.2722 Acc: 0.8893
val Loss: 0.2006 Acc: 0.9346
Epoch 18/24
----------
train Loss: 0.2729 Acc: 0.9016
val Loss: 0.1872 Acc: 0.9281
Epoch 19/24
----------
train Loss: 0.3554 Acc: 0.8361
val Loss: 0.1814 Acc: 0.9412
Epoch 20/24
----------
train Loss: 0.2122 Acc: 0.8975
val Loss: 0.1841 Acc: 0.9412
Epoch 21/24
----------
train Loss: 0.2274 Acc: 0.9098
val Loss: 0.1806 Acc: 0.9281
Epoch 22/24
----------
train Loss: 0.2574 Acc: 0.9016
val Loss: 0.1861 Acc: 0.9346
Epoch 23/24
----------
train Loss: 0.2364 Acc: 0.8893
val Loss: 0.1819 Acc: 0.9346
Epoch 24/24
----------
train Loss: 0.3160 Acc: 0.8566
val Loss: 0.2078 Acc: 0.9412
Training complete in 0m 40s
Best val Acc: 0.947712
visualize_model(model_ft)
ConvNet as fixed feature extractor#
Here, we need to freeze all the network except the final layer. We need
to set requires_grad = False to freeze the parameters so that the
gradients are not computed in backward().
You can read more about this in the documentation here.
model_conv = torchvision.models.resnet18(weights='IMAGENET1K_V1')
for param in model_conv.parameters():
param.requires_grad = False
# Parameters of newly constructed modules have requires_grad=True by default
num_ftrs = model_conv.fc.in_features
model_conv.fc = nn.Linear(num_ftrs, 2)
model_conv = model_conv.to(device)
criterion = nn.CrossEntropyLoss()
# Observe that only parameters of final layer are being optimized as
# opposed to before.
optimizer_conv = optim.SGD(model_conv.fc.parameters(), lr=0.001, momentum=0.9)
# Decay LR by a factor of 0.1 every 7 epochs
exp_lr_scheduler = lr_scheduler.StepLR(optimizer_conv, step_size=7, gamma=0.1)
Train and evaluate#
On CPU this will take about half the time compared to previous scenario. This is expected as gradients don’t need to be computed for most of the network. However, forward does need to be computed.
model_conv = train_model(model_conv, criterion, optimizer_conv,
exp_lr_scheduler, num_epochs=25)
Epoch 0/24
----------
train Loss: 0.6416 Acc: 0.6434
val Loss: 0.2829 Acc: 0.8889
Epoch 1/24
----------
train Loss: 0.3945 Acc: 0.7910
val Loss: 0.2091 Acc: 0.9477
Epoch 2/24
----------
train Loss: 0.5168 Acc: 0.7541
val Loss: 0.2621 Acc: 0.9085
Epoch 3/24
----------
train Loss: 0.3692 Acc: 0.8525
val Loss: 0.2386 Acc: 0.9216
Epoch 4/24
----------
train Loss: 0.4537 Acc: 0.7828
val Loss: 0.3414 Acc: 0.8758
Epoch 5/24
----------
train Loss: 0.4436 Acc: 0.8033
val Loss: 0.2418 Acc: 0.9346
Epoch 6/24
----------
train Loss: 0.3430 Acc: 0.8361
val Loss: 0.2868 Acc: 0.8824
Epoch 7/24
----------
train Loss: 0.3765 Acc: 0.8279
val Loss: 0.1919 Acc: 0.9542
Epoch 8/24
----------
train Loss: 0.4020 Acc: 0.8156
val Loss: 0.1956 Acc: 0.9477
Epoch 9/24
----------
train Loss: 0.3729 Acc: 0.8484
val Loss: 0.1970 Acc: 0.9542
Epoch 10/24
----------
train Loss: 0.4358 Acc: 0.7910
val Loss: 0.2059 Acc: 0.9477
Epoch 11/24
----------
train Loss: 0.3921 Acc: 0.8279
val Loss: 0.1998 Acc: 0.9477
Epoch 12/24
----------
train Loss: 0.2982 Acc: 0.8566
val Loss: 0.2158 Acc: 0.9412
Epoch 13/24
----------
train Loss: 0.3521 Acc: 0.8238
val Loss: 0.2675 Acc: 0.9216
Epoch 14/24
----------
train Loss: 0.2636 Acc: 0.8934
val Loss: 0.1976 Acc: 0.9542
Epoch 15/24
----------
train Loss: 0.3777 Acc: 0.8033
val Loss: 0.1949 Acc: 0.9542
Epoch 16/24
----------
train Loss: 0.3006 Acc: 0.8689
val Loss: 0.1803 Acc: 0.9542
Epoch 17/24
----------
train Loss: 0.3126 Acc: 0.8607
val Loss: 0.1869 Acc: 0.9542
Epoch 18/24
----------
train Loss: 0.3118 Acc: 0.8402
val Loss: 0.1971 Acc: 0.9542
Epoch 19/24
----------
train Loss: 0.4113 Acc: 0.8156
val Loss: 0.2081 Acc: 0.9477
Epoch 20/24
----------
train Loss: 0.3038 Acc: 0.8607
val Loss: 0.1960 Acc: 0.9542
Epoch 21/24
----------
train Loss: 0.3177 Acc: 0.8566
val Loss: 0.2071 Acc: 0.9542
Epoch 22/24
----------
train Loss: 0.4137 Acc: 0.8156
val Loss: 0.1955 Acc: 0.9477
Epoch 23/24
----------
train Loss: 0.2506 Acc: 0.8852
val Loss: 0.2140 Acc: 0.9281
Epoch 24/24
----------
train Loss: 0.3255 Acc: 0.8443
val Loss: 0.1917 Acc: 0.9542
Training complete in 0m 28s
Best val Acc: 0.954248
visualize_model(model_conv)
plt.ioff()
plt.show()
Inference on custom images#
Use the trained model to make predictions on custom images and visualize the predicted class labels along with the images.
def visualize_model_predictions(model,img_path):
was_training = model.training
model.eval()
img = Image.open(img_path)
img = data_transforms['val'](img)
img = img.unsqueeze(0)
img = img.to(device)
with torch.no_grad():
outputs = model(img)
_, preds = torch.max(outputs, 1)
ax = plt.subplot(2,2,1)
ax.axis('off')
ax.set_title(f'Predicted: {class_names[preds[0]]}')
imshow(img.cpu().data[0])
model.train(mode=was_training)
visualize_model_predictions(
model_conv,
img_path='data/hymenoptera_data/val/bees/72100438_73de9f17af.jpg'
)
plt.ioff()
plt.show()
Further Learning#
If you would like to learn more about the applications of transfer learning, checkout our Quantized Transfer Learning for Computer Vision Tutorial.
Total running time of the script: (1 minutes 10.412 seconds)
