# Keras Model question for Pre trained model extension

I want to add a few more layers to a Resnet50 model and my question is - do I need to compile it and train it on new data or can I just use it as it is? Will it just give me the Resnet50 results?

Here is what I am trying:

def base_model():
resnet = resnet50.ResNet50(weights="imagenet", include_top=False)
x = resnet.output
x = Conv2D(128, (3, 3), activation='relu',padding='same')(x)
x = Conv2D(128, (3, 3), activation='relu',padding='same')(x)
x = MaxPooling2D((2, 2), strides=(2, 2))(x)
x = Conv2D(256, (3, 3), activation='relu',padding='same')(x)
x = Conv2D(256, (3, 3), activation='relu',padding='same')(x)
x = MaxPooling2D((2, 2), strides=(2, 2))(x)
x = Conv2D(512, (3, 3), activation='relu',padding='same')(x)
x = Conv2D(512, (3, 3), activation='relu',padding='same')(x)
x = MaxPooling2D((2, 2), strides=(2, 2))(x)
x = GlobalAveragePooling2D()(x)
x = Dense(4096, activation='relu')(x)
x = Dropout(0.6)(x)
x = Dense(4096, activation='relu')(x)
x = Dropout(0.6)(x)
x = Lambda(lambda  x_: K.l2_normalize(x,axis=1))(x)
return Model(inputs=resnet.input, outputs=x)


And then use it like this :

enhanced_resnet = base_model()
img = image.load_img(file, target_size=(224, 224))
img = image.img_to_array(img)
x = resnet50.preprocess_input(img)
x = np.array([x])
feature = enhanced_resnet.predict(x)


What I want as return value is the features from the image and not a prediction, as I am using a distance equation to tell the similarity between images.

Short answer - Yes.

The pretrained Resnet model has trained parameters (weights and biases). When you add extra layers on top of it, the parameters of these layers are randomly initialized. Therefore the final model will give random results.

You need to train the new model after adding extra layers on your new dataset. You don't need to train all the previous layers, just few of them.

For further reading search for - transfer learning. That's what you're trying to do.

In transfer learning techniques, if you are adding new layers always make sure to compile your model.

As your project is based on finding similarity of the image my suggestion is to do only flatten after adding new Conv2D layers. Most of the popular architecture does not contain Dense layer in case of similarity.

• How would you set up the labels for feature extraction?
– MNM
Jul 3, 2019 at 23:19