src/ai_image_detector/model.py

from __future__ import annotations

import tensorflow as tf
from tensorflow.keras import Sequential, layers
from tensorflow.keras.applications import MobileNetV2
from tensorflow.keras.optimizers import Adam

from .config import IMAGE_SIZE, SEED


def compile_model(model: tf.keras.Model, learning_rate: float) -> None:
    model.compile(
        optimizer=Adam(learning_rate=learning_rate),
        loss="binary_crossentropy",
        metrics=[
            "accuracy",
            tf.keras.metrics.Precision(name="precision"),
            tf.keras.metrics.Recall(name="recall"),
            tf.keras.metrics.AUC(name="auc"),
        ],
    )


def build_model() -> tf.keras.Model:
    tf.keras.utils.set_random_seed(SEED)

    base_model = MobileNetV2(
        input_shape=(*IMAGE_SIZE, 3),
        include_top=False,
        weights="imagenet",
    )
    base_model.trainable = False

    model = Sequential(
        [
            base_model,
            layers.GlobalAveragePooling2D(),
            layers.Dropout(0.3),
            layers.Dense(128, activation="relu"),
            layers.Dropout(0.2),
            layers.Dense(1, activation="sigmoid"),
        ]
    )

    compile_model(model, learning_rate=1e-4)
    return model


def unfreeze_for_fine_tuning(model: tf.keras.Model, trainable_layers: int = 30) -> None:
    base_model = model.layers[0]
    base_model.trainable = True

    for layer in base_model.layers[:-trainable_layers]:
        layer.trainable = False

    for layer in base_model.layers[-trainable_layers:]:
        if isinstance(layer, tf.keras.layers.BatchNormalization):
            layer.trainable = False

    compile_model(model, learning_rate=1e-5)