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def add_new_last_layer(base_model, nb_classes): """Add last layer to the convnet Args: base_model: keras model excluding top nb_classes: # of classes Returns: new keras model with last layer """ x = base_model.output x = GlobalAveragePooling2D()(x) x = Dense(FC_SIZE, activation='relu')(x) predictions = Dense(nb_classes, activation='softmax')(x) model = Model(input=base_model.input, output=predictions) return model
载入预训练模型作为前端的网络,在自己的数据集上进行微调,最好按照以下两步进行:
- Transfer learning:freeze all but the penultimate layer and re-train the last
Dense
layer - Fine-tuning:un-freeze the lower convolutional layers and retrain more layers
Doing both, in that order, will ensure a more stable and consistent training. This is because the large gradient updates triggered by randomly initialized weights could wreck the learned weights in the convolutional base if not frozen. Once the last layer has stabilized (transfer learning), then we move onto retraining more layers (fine-tuning).
Transfer learning
def setup_to_transfer_learn(model, base_model): """Freeze all layers and compile the model""" for layer in base_model.layers: layer.trainable = False model.compile(optimizer='rmsprop', loss='categorical_crossentropy', metrics=['accuracy'])
Fine-tune
def setup_to_finetune(model): """Freeze the bottom NB_IV3_LAYERS and retrain the remaining top layers. note: NB_IV3_LAYERS corresponds to the top 2 inception blocks in the inceptionv3 architecture Args: model: keras model """ for layer in model.layers[:NB_IV3_LAYERS_TO_FREEZE]: layer.trainable = False for layer in model.layers[NB_IV3_LAYERS_TO_FREEZE:]: layer.trainable = True model.compile(optimizer=SGD(lr=0.0001, momentum=0.9), loss='categorical_crossentropy')
When fine-tuning, it’s important to lower your learning rate relative to the rate that was used when training from scratch (lr=0.0001
), otherwise, the optimization could destabilize and the loss diverge.
Training
Now we’re all set for training. Usefit_generator
for both transfer learning and fine-tuning. 分两个阶段依次进行训练
history = model.fit_generator( train_generator, samples_per_epoch=nb_train_samples, nb_epoch=nb_epoch, validation_data=validation_generator, nb_val_samples=nb_val_samples, class_weight='auto')
model.save(args.output_model_file)
在keras2.0版本以上时,函数参数做了改变
datagen = ImageDataGenerator( featurewise_center=False, # set input mean to 0 over the dataset samplewise_center=False, # set each sample mean to 0 featurewise_std_normalization=False, # divide inputs by std of the dataset samplewise_std_normalization=False, # divide each input by its std zca_whitening=False, # apply ZCA whitening rotation_range=0, # randomly rotate images in the range (degrees, 0 to 180) width_shift_range=0.1, # randomly shift images horizontally (fraction of total width) height_shift_range=0.1, # randomly shift images vertically (fraction of total height) horizontal_flip=True, # randomly flip images vertical_flip=False) # randomly flip images # Compute quantities required for feature-wise normalization # (std, mean, and principal components if ZCA whitening is applied). datagen.fit(x_train) # Fit the model on the batches generated by datagen.flow(). model.fit_generator(datagen.flow(x_train, y_train, batch_size=batch_size), steps_per_epoch=x_train.shape[0] // batch_size, epochs=epochs, validation_data=(x_test, y_test))
预测函数:
def predict(model, img, target_size, top_n=3): """Run model prediction on image Args: model: keras model img: PIL format image target_size: (width, height) tuple top_n: # of top predictions to return Returns: list of predicted labels and their probabilities """ if img.size != target_size: img = img.resize(target_size)
x = image.img_to_array(img) x = np.expand_dims(x, axis=0) # 插入这一个轴是关键,因为keras中的model的tensor的shape是(bath_size, h, w, c),如果是tf后台 x = preprocess_input(x) preds = model.predict(x) return decode_predictions(preds, top=top_n)[0]
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