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What they’re and how one can use them

Knowledge pre-processing: What you do to the information earlier than feeding it to the mannequin.
— A easy definition that, in follow, leaves open many questions. The place, precisely, ought to pre-processing cease, and the mannequin start? Are steps like normalization, or varied numerical transforms, a part of the mannequin, or the pre-processing? What about knowledge augmentation? In sum, the road between what’s pre-processing and what’s modeling has at all times, on the edges, felt considerably fluid.

On this state of affairs, the arrival of keras pre-processing layers modifications a long-familiar image.

In concrete phrases, with keras, two options tended to prevail: one, to do issues upfront, in R; and two, to assemble a tfdatasets pipeline. The previous utilized every time we wanted the entire knowledge to extract some abstract info. For instance, when normalizing to a imply of zero and a normal deviation of 1. However usually, this meant that we needed to rework back-and-forth between normalized and un-normalized variations at a number of factors within the workflow. The tfdatasets method, then again, was elegant; nevertheless, it might require one to put in writing a whole lot of low-level tensorflow code.

Pre-processing layers, accessible as of keras model 2.6.1, take away the necessity for upfront R operations, and combine properly with tfdatasets. However that isn’t all there may be to them. On this publish, we need to spotlight 4 important features:

  1. Pre-processing layers considerably scale back coding effort. You might code these operations your self; however not having to take action saves time, favors modular code, and helps to keep away from errors.
  2. Pre-processing layers – a subset of them, to be exact – can produce abstract info earlier than coaching correct, and make use of a saved state when referred to as upon later.
  3. Pre-processing layers can pace up coaching.
  4. Pre-processing layers are, or might be made, a part of the mannequin, thus eradicating the necessity to implement unbiased pre-processing procedures within the deployment surroundings.

Following a brief introduction, we’ll increase on every of these factors. We conclude with two end-to-end examples (involving pictures and textual content, respectively) that properly illustrate these 4 features.

Pre-processing layers in a nutshell

Like different keras layers, those we’re speaking about right here all begin with layer_, and could also be instantiated independently of mannequin and knowledge pipeline. Right here, we create a layer that can randomly rotate pictures whereas coaching, by as much as 45 levels in each instructions:

aug_layer <- layer_random_rotation(issue = 0.125)

As soon as we now have such a layer, we will instantly take a look at it on some dummy picture.

[[1. 0. 0. 0. 0.]
 [0. 1. 0. 0. 0.]
 [0. 0. 1. 0. 0.]
 [0. 0. 0. 1. 0.]
 [0. 0. 0. 0. 1.]], form=(5, 5), dtype=float32)

“Testing the layer” now actually means calling it like a perform:

[[0.         0.         0.         0.         0.        ]
 [0.44459596 0.32453176 0.05410459 0.         0.        ]
 [0.15844001 0.4371609  1.         0.4371609  0.15844001]
 [0.         0.         0.05410453 0.3245318  0.44459593]
 [0.         0.         0.         0.         0.        ]], form=(5, 5), dtype=float32)

As soon as instantiated, a layer can be utilized in two methods. Firstly, as a part of the enter pipeline.

In pseudocode:

# pseudocode
train_ds <- ... # outline dataset
preprocessing_layer <- ... # instantiate layer

train_ds <- train_ds %>%
  dataset_map(perform(x, y) record(preprocessing_layer(x), y))

Secondly, the way in which that appears most pure, for a layer: as a layer contained in the mannequin. Schematically:

# pseudocode
enter <- layer_input(form = input_shape)

output <- enter %>%
  preprocessing_layer() %>%

mannequin <- keras_model(enter, output)

In reality, the latter appears so apparent that you just is likely to be questioning: Why even permit for a tfdatasets-integrated different? We’ll increase on that shortly, when speaking about efficiency.

Stateful layers – who’re particular sufficient to deserve their personal part – can be utilized in each methods as effectively, however they require a further step. Extra on that beneath.

How pre-processing layers make life simpler

Devoted layers exist for a large number of data-transformation duties. We are able to subsume them below two broad classes, function engineering and knowledge augmentation.

Function engineering

The necessity for function engineering might come up with all kinds of knowledge. With pictures, we don’t usually use that time period for the “pedestrian” operations which might be required for a mannequin to course of them: resizing, cropping, and such. Nonetheless, there are assumptions hidden in every of those operations , so we really feel justified in our categorization. Be that as it might, layers on this group embody layer_resizing(), layer_rescaling(), and layer_center_crop().

With textual content, the one performance we couldn’t do with out is vectorization. layer_text_vectorization() takes care of this for us. We’ll encounter this layer within the subsequent part, in addition to within the second full-code instance.

Now, on to what’s usually seen as the area of function engineering: numerical and categorical (we would say: “spreadsheet”) knowledge.

First, numerical knowledge usually must be normalized for neural networks to carry out effectively – to attain this, use layer_normalization(). Or perhaps there’s a cause we’d prefer to put steady values into discrete classes. That’d be a activity for layer_discretization().

Second, categorical knowledge are available in varied codecs (strings, integers …), and there’s at all times one thing that must be performed with the intention to course of them in a significant method. Usually, you’ll need to embed them right into a higher-dimensional area, utilizing layer_embedding(). Now, embedding layers anticipate their inputs to be integers; to be exact: consecutive integers. Right here, the layers to search for are layer_integer_lookup() and layer_string_lookup(): They’ll convert random integers (strings, respectively) to consecutive integer values. In a special state of affairs, there is likely to be too many classes to permit for helpful info extraction. In such instances, use layer_hashing() to bin the information. And at last, there’s layer_category_encoding() to provide the classical one-hot or multi-hot representations.

Knowledge augmentation

Within the second class, we discover layers that execute [configurable] random operations on pictures. To call only a few of them: layer_random_crop(), layer_random_translation(), layer_random_rotation() … These are handy not simply in that they implement the required low-level performance; when built-in right into a mannequin, they’re additionally workflow-aware: Any random operations shall be executed throughout coaching solely.

Now we now have an concept what these layers do for us, let’s concentrate on the precise case of state-preserving layers.

Pre-processing layers that preserve state

A layer that randomly perturbs pictures doesn’t must know something in regards to the knowledge. It simply must observe a rule: With likelihood (p), do (x). A layer that’s alleged to vectorize textual content, then again, must have a lookup desk, matching character strings to integers. The identical goes for a layer that maps contingent integers to an ordered set. And in each instances, the lookup desk must be constructed upfront.

With stateful layers, this information-buildup is triggered by calling adapt() on a freshly-created layer occasion. For instance, right here we instantiate and “situation” a layer that maps strings to consecutive integers:

colours <- c("cyan", "turquoise", "celeste");

layer <- layer_string_lookup()
layer %>% adapt(colours)

We are able to test what’s within the lookup desk:

[1] "[UNK]"     "turquoise" "cyan"      "celeste"  

Then, calling the layer will encode the arguments:

layer(c("azure", "cyan"))
tf.Tensor([0 2], form=(2,), dtype=int64)

layer_string_lookup() works on particular person character strings, and consequently, is the transformation enough for string-valued categorical options. To encode entire sentences (or paragraphs, or any chunks of textual content) you’d use layer_text_vectorization() as a substitute. We’ll see how that works in our second end-to-end instance.

Utilizing pre-processing layers for efficiency

Above, we mentioned that pre-processing layers may very well be utilized in two methods: as a part of the mannequin, or as a part of the information enter pipeline. If these are layers, why even permit for the second method?

The primary cause is efficiency. GPUs are nice at common matrix operations, similar to these concerned in picture manipulation and transformations of uniformly-shaped numerical knowledge. Due to this fact, you probably have a GPU to coach on, it’s preferable to have picture processing layers, or layers similar to layer_normalization(), be a part of the mannequin (which is run fully on GPU).

However, operations involving textual content, similar to layer_text_vectorization(), are finest executed on the CPU. The identical holds if no GPU is obtainable for coaching. In these instances, you’d transfer the layers to the enter pipeline, and try to learn from parallel – on-CPU – processing. For instance:

# pseudocode

preprocessing_layer <- ... # instantiate layer

dataset <- dataset %>%
  dataset_map(~record(text_vectorizer(.x), .y),
              num_parallel_calls = tf$knowledge$AUTOTUNE) %>%
mannequin %>% match(dataset)

Accordingly, within the end-to-end examples beneath, you’ll see picture knowledge augmentation taking place as a part of the mannequin, and textual content vectorization, as a part of the enter pipeline.

Exporting a mannequin, full with pre-processing

Say that for coaching your mannequin, you discovered that the tfdatasets method was the perfect. Now, you deploy it to a server that doesn’t have R put in. It will appear to be that both, you need to implement pre-processing in another, accessible, know-how. Alternatively, you’d need to depend on customers sending already-pre-processed knowledge.

Luckily, there’s something else you are able to do. Create a brand new mannequin particularly for inference, like so:

# pseudocode

enter <- layer_input(form = input_shape)

output <- enter %>%
  preprocessing_layer(enter) %>%

inference_model <- keras_model(enter, output)

This method makes use of the practical API to create a brand new mannequin that prepends the pre-processing layer to the pre-processing-less, unique mannequin.

Having targeted on a number of issues particularly “good to know”, we now conclude with the promised examples.

Instance 1: Picture knowledge augmentation

Our first instance demonstrates picture knowledge augmentation. Three kinds of transformations are grouped collectively, making them stand out clearly within the total mannequin definition. This group of layers shall be lively throughout coaching solely.


# Load CIFAR-10 knowledge that include keras
c(c(x_train, y_train), ...) %<-% dataset_cifar10()
input_shape <- dim(x_train)[-1] # drop batch dim
lessons <- 10

# Create a tf_dataset pipeline 
train_dataset <- tensor_slices_dataset(record(x_train, y_train)) %>%

# Use a (non-trained) ResNet structure
resnet <- application_resnet50(weights = NULL,
                               input_shape = input_shape,
                               lessons = lessons)

# Create an information augmentation stage with horizontal flipping, rotations, zooms
data_augmentation <-
  keras_model_sequential() %>%
  layer_random_flip("horizontal") %>%
  layer_random_rotation(0.1) %>%

enter <- layer_input(form = input_shape)

# Outline and run the mannequin
output <- enter %>%
  layer_rescaling(1 / 255) %>%   # rescale inputs
  data_augmentation() %>%

mannequin <- keras_model(enter, output) %>%
  compile(optimizer = "rmsprop", loss = "sparse_categorical_crossentropy") %>%
  match(train_dataset, steps_per_epoch = 5)

Instance 2: Textual content vectorization

In pure language processing, we frequently use embedding layers to current the “workhorse” (recurrent, convolutional, self-attentional, what have you ever) layers with the continual, optimally-dimensioned enter they want. Embedding layers anticipate tokens to be encoded as integers, and rework textual content to integers is what layer_text_vectorization() does.

Our second instance demonstrates the workflow: You have got the layer be taught the vocabulary upfront, then name it as a part of the pre-processing pipeline. As soon as coaching has completed, we create an “all-inclusive” mannequin for deployment.


# Instance knowledge
textual content <- as_tensor(c(
  "From every in keeping with his capability, to every in keeping with his wants!",
  "Act that you just use humanity, whether or not in your personal particular person or within the particular person of every other, at all times concurrently an finish, by no means merely as a method.",
  "Cause is, and ought solely to be the slave of the passions, and may by no means fake to every other workplace than to serve and obey them."

# Create and adapt layer
text_vectorizer <- layer_text_vectorization(output_mode="int")
text_vectorizer %>% adapt(textual content)

# Verify
as.array(text_vectorizer("To every in keeping with his wants"))

# Create a easy classification mannequin
enter <- layer_input(form(NULL), dtype="int64")

output <- enter %>%
  layer_embedding(input_dim = text_vectorizer$vocabulary_size(),
                  output_dim = 16) %>%
  layer_gru(8) %>%
  layer_dense(1, activation = "sigmoid")

mannequin <- keras_model(enter, output)

# Create a labeled dataset (which incorporates unknown tokens)
train_dataset <- tensor_slices_dataset(record(
    c("From every in keeping with his capability", "There may be nothing larger than cause."),
    c(1L, 0L)

# Preprocess the string inputs
train_dataset <- train_dataset %>%
  dataset_batch(2) %>%
  dataset_map(~record(text_vectorizer(.x), .y),
              num_parallel_calls = tf$knowledge$AUTOTUNE)

# Prepare the mannequin
mannequin %>%
  compile(optimizer = "adam", loss = "binary_crossentropy") %>%

# export inference mannequin that accepts strings as enter
enter <- layer_input(form = 1, dtype="string")
output <- enter %>%
  text_vectorizer() %>%

end_to_end_model <- keras_model(enter, output)

# Check inference mannequin
test_data <- as_tensor(c(
  "To every in keeping with his wants!",
  "Cause is, and ought solely to be the slave of the passions."
test_output <- end_to_end_model(test_data)


With this publish, our objective was to name consideration to keras’ new pre-processing layers, and present how – and why – they’re helpful. Many extra use instances might be discovered within the vignette.

Thanks for studying!

Picture by Henning Borgersen on Unsplash



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