How does TensorFlow compute the gradient of vgg19.preprocess_input?

Question

I am following the tutorial on neural style transfer. The style transfer is done by minimizing a loss function with respect to an image (initialized with the content image). What confuses me is the following piece of code:

preprocessed_input = tf.keras.applications.vgg19.preprocess_input(inputs)

which is part of the call method in the StyleContentModel class. How does TensorFlow know the gradient of this operation? I have checked if this operation has a gradient function using get_gradient_function in the module tensorflow.python.framework.ops, and as far as I can tell it does not.

Answer 1

This has nothing to do with the model or gradients. What this function does is scale the input images so the pixels are in the range from -1 to +1. This is a common requirement for many models used in transfer learning like VGG and MobileNet. If you use the ImageDataGenerator it has a parameter preprocessing_function which the generator calls to preprocess the images. Make sure if you preprocess the training images you do the same for the test and validation images.

Answer 2

It is very simple, the function internally uses symbolic tensor operations that are differentiable. TensorFlow can compute gradients through functions that internally use TensorFlow operations, there is no need to manually define a gradient for each function.

You can confirm by looking at the code of that function here, specially if you look at the _preprocess_symbolic_function here which is using normal scalar operations and Keras backend functions (which are just TensorFlow functions in tf.keras).