bit.ipynb

Kernel: Python 3

Image Classification using BigTransfer (BiT)

Author: Sayan Nath
Date created: 2021/09/24
Last modified: 2023/12/22
Description: BigTransfer (BiT) State-of-the-art transfer learning for image classification.

Introduction

BigTransfer (also known as BiT) is a state-of-the-art transfer learning method for image classification. Transfer of pre-trained representations improves sample efficiency and simplifies hyperparameter tuning when training deep neural networks for vision. BiT revisit the paradigm of pre-training on large supervised datasets and fine-tuning the model on a target task. The importance of appropriately choosing normalization layers and scaling the architecture capacity as the amount of pre-training data increases.

BigTransfer(BiT) is trained on public datasets, along with code in TF2, Jax and Pytorch. This will help anyone to reach state of the art performance on their task of interest, even with just a handful of labeled images per class.

You can find BiT models pre-trained on ImageNet and ImageNet-21k in TFHub as TensorFlow2 SavedModels that you can use easily as Keras Layers. There are a variety of sizes ranging from a standard ResNet50 to a ResNet152x4 (152 layers deep, 4x wider than a typical ResNet50) for users with larger computational and memory budgets but higher accuracy requirements.

Figure: The x-axis shows the number of images used per class, ranging from 1 to the full dataset. On the plots on the left, the curve in blue above is our BiT-L model, whereas the curve below is a ResNet-50 pre-trained on ImageNet (ILSVRC-2012).

Setup

import os
os.environ["KERAS_BACKEND"] = "tensorflow"
import numpy as np
import pandas as pd
import matplotlib.pyplot as plt

import keras
from keras import ops
import tensorflow as tf
import tensorflow_hub as hub
import tensorflow_datasets as tfds

tfds.disable_progress_bar()

SEEDS = 42

keras.utils.set_random_seed(SEEDS)

Gather Flower Dataset

train_ds, validation_ds = tfds.load(
    "tf_flowers",
    split=["train[:85%]", "train[85%:]"],
    as_supervised=True,
)

Visualise the dataset

plt.figure(figsize=(10, 10))
for i, (image, label) in enumerate(train_ds.take(9)):
    ax = plt.subplot(3, 3, i + 1)
    plt.imshow(image)
    plt.title(int(label))
    plt.axis("off")

Define hyperparameters

RESIZE_TO = 384
CROP_TO = 224
BATCH_SIZE = 64
STEPS_PER_EPOCH = 10
AUTO = tf.data.AUTOTUNE  # optimise the pipeline performance
NUM_CLASSES = 5  # number of classes
SCHEDULE_LENGTH = (
    500  # we will train on lower resolution images and will still attain good results
)
SCHEDULE_BOUNDARIES = [
    200,
    300,
    400,
]  # more the dataset size the schedule length increase

The hyperparamteres like SCHEDULE_LENGTH and SCHEDULE_BOUNDARIES are determined based on empirical results. The method has been explained in the original paper and in their Google AI Blog Post.

The SCHEDULE_LENGTH is aslo determined whether to use MixUp Augmentation or not. You can also find an easy MixUp Implementation in Keras Coding Examples.

Define preprocessing helper functions

SCHEDULE_LENGTH = SCHEDULE_LENGTH * 512 / BATCH_SIZE

random_flip = keras.layers.RandomFlip("horizontal")
random_crop = keras.layers.RandomCrop(CROP_TO, CROP_TO)

def preprocess_train(image, label):
    image = random_flip(image)
    image = ops.image.resize(image, (RESIZE_TO, RESIZE_TO))
    image = random_crop(image)
    image = image / 255.0
    return (image, label)


def preprocess_test(image, label):
    image = ops.image.resize(image, (RESIZE_TO, RESIZE_TO))
    image = ops.cast(image, dtype="float32")
    image = image / 255.0
    return (image, label)


DATASET_NUM_TRAIN_EXAMPLES = train_ds.cardinality().numpy()

repeat_count = int(
    SCHEDULE_LENGTH * BATCH_SIZE / DATASET_NUM_TRAIN_EXAMPLES * STEPS_PER_EPOCH
)
repeat_count += 50 + 1  # To ensure at least there are 50 epochs of training

Define the data pipeline

# Training pipeline
pipeline_train = (
    train_ds.shuffle(10000)
    .repeat(repeat_count)  # Repeat dataset_size / num_steps
    .map(preprocess_train, num_parallel_calls=AUTO)
    .batch(BATCH_SIZE)
    .prefetch(AUTO)
)

# Validation pipeline
pipeline_validation = (
    validation_ds.map(preprocess_test, num_parallel_calls=AUTO)
    .batch(BATCH_SIZE)
    .prefetch(AUTO)
)

Visualise the training samples

image_batch, label_batch = next(iter(pipeline_train))

plt.figure(figsize=(10, 10))
for n in range(25):
    ax = plt.subplot(5, 5, n + 1)
    plt.imshow(image_batch[n])
    plt.title(label_batch[n].numpy())
    plt.axis("off")

Load pretrained TF-Hub model into a KerasLayer

bit_model_url = "https://tfhub.dev/google/bit/m-r50x1/1"
bit_module = hub.load(bit_model_url)

Create BigTransfer (BiT) model

To create the new model, we:

1. Cut off the BiT model’s original head. This leaves us with the “pre-logits” output. We do not have to do this if we use the ‘feature extractor’ models (i.e. all those in subdirectories titled feature_vectors), since for those models the head has already been cut off.

2. Add a new head with the number of outputs equal to the number of classes of our new task. Note that it is important that we initialise the head to all zeroes.

class MyBiTModel(keras.Model):
    def __init__(self, num_classes, module, **kwargs):
        super().__init__(**kwargs)

        self.num_classes = num_classes
        self.head = keras.layers.Dense(num_classes, kernel_initializer="zeros")
        self.bit_model = module

    def call(self, images):
        bit_embedding = self.bit_model(images)
        return self.head(bit_embedding)


model = MyBiTModel(num_classes=NUM_CLASSES, module=bit_module)

Define optimizer and loss

learning_rate = 0.003 * BATCH_SIZE / 512

# Decay learning rate by a factor of 10 at SCHEDULE_BOUNDARIES.
lr_schedule = keras.optimizers.schedules.PiecewiseConstantDecay(
    boundaries=SCHEDULE_BOUNDARIES,
    values=[
        learning_rate,
        learning_rate * 0.1,
        learning_rate * 0.01,
        learning_rate * 0.001,
    ],
)
optimizer = keras.optimizers.SGD(learning_rate=lr_schedule, momentum=0.9)

loss_fn = keras.losses.SparseCategoricalCrossentropy(from_logits=True)

Compile the model

model.compile(optimizer=optimizer, loss=loss_fn, metrics=["accuracy"])

Set up callbacks

train_callbacks = [
    keras.callbacks.EarlyStopping(
        monitor="val_accuracy", patience=2, restore_best_weights=True
    )
]

Train the model

history = model.fit(
    pipeline_train,
    batch_size=BATCH_SIZE,
    epochs=int(SCHEDULE_LENGTH / STEPS_PER_EPOCH),
    steps_per_epoch=STEPS_PER_EPOCH,
    validation_data=pipeline_validation,
    callbacks=train_callbacks,
)

Plot the training and validation metrics

def plot_hist(hist):
    plt.plot(hist.history["accuracy"])
    plt.plot(hist.history["val_accuracy"])
    plt.plot(hist.history["loss"])
    plt.plot(hist.history["val_loss"])
    plt.title("Training Progress")
    plt.ylabel("Accuracy/Loss")
    plt.xlabel("Epochs")
    plt.legend(["train_acc", "val_acc", "train_loss", "val_loss"], loc="upper left")
    plt.show()


plot_hist(history)

Evaluate the model

accuracy = model.evaluate(pipeline_validation)[1] * 100
print("Accuracy: {:.2f}%".format(accuracy))

Conclusion

BiT performs well across a surprisingly wide range of data regimes -- from 1 example per class to 1M total examples. BiT achieves 87.5% top-1 accuracy on ILSVRC-2012, 99.4% on CIFAR-10, and 76.3% on the 19 task Visual Task Adaptation Benchmark (VTAB). On small datasets, BiT attains 76.8% on ILSVRC-2012 with 10 examples per class, and 97.0% on CIFAR-10 with 10 examples per class.

You can experiment further with the BigTransfer Method by following the original paper.

Example available on HuggingFace

Trained Model	Demo