Consideration-based Picture Captioning with Keras

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In picture captioning, an algorithm is given a picture and tasked with producing a wise caption. It’s a difficult job for a number of causes, not the least being that it includes a notion of saliency or relevance. Because of this latest deep studying approaches largely embrace some “consideration” mechanism (generally even multiple) to assist specializing in related picture options.

On this publish, we show a formulation of picture captioning as an encoder-decoder drawback, enhanced by spatial consideration over picture grid cells. The concept comes from a latest paper on Neural Picture Caption Era with Visible Consideration (Xu et al. 2015), and employs the identical type of consideration algorithm as detailed in our publish on machine translation.

We’re porting Python code from a latest Google Colaboratory pocket book, utilizing Keras with TensorFlow keen execution to simplify our lives.


The code proven right here will work with the present CRAN variations of tensorflow, keras, and tfdatasets. Test that you just’re utilizing no less than model 1.9 of TensorFlow. If that isn’t the case, as of this writing, this

will get you model 1.10.

When loading libraries, please be sure to’re executing the primary 4 strains on this actual order. We want to verify we’re utilizing the TensorFlow implementation of Keras (tf.keras in Python land), and we’ve got to allow keen execution earlier than utilizing TensorFlow in any method.

No must copy-paste any code snippets – you’ll discover the entire code (so as vital for execution) right here: eager-image-captioning.R.

The dataset

MS-COCO (“Frequent Objects in Context”) is one among, maybe the, reference dataset in picture captioning (object detection and segmentation, too). We’ll be utilizing the coaching photographs and annotations from 2014 – be warned, relying in your location, the obtain can take a lengthy time.

After unpacking, let’s outline the place the photographs and captions are.

annotation_file <- "train2014/annotations/captions_train2014.json"
image_path <- "train2014/train2014"

The annotations are in JSON format, and there are 414113 of them! Fortunately for us we didn’t need to obtain that many photographs – each picture comes with 5 completely different captions, for higher generalizability.

annotations <- fromJSON(file = annotation_file)
annot_captions <- annotations[[4]]

num_captions <- size(annot_captions)

We retailer each annotations and picture paths in lists, for later loading.

all_captions <- vector(mode = "record", size = num_captions)
all_img_names <- vector(mode = "record", size = num_captions)

for (i in seq_len(num_captions)) {
  caption <- paste0("<begin> ",
                    " <finish>"
  image_id <- annot_captions[[i]][["image_id"]]
  full_coco_image_path <- sprintf(
  all_img_names[[i]] <- full_coco_image_path
  all_captions[[i]] <- caption

Relying in your computing setting, you’ll for positive wish to limit the variety of examples used. This publish will use 30000 captioned photographs, chosen randomly, and put aside 20% for validation.

Beneath, we take random samples, cut up into coaching and validation elements. The companion code may also retailer the indices on disk, so you may decide up on verification and evaluation later.

num_examples <- 30000

random_sample <- pattern(1:num_captions, measurement = num_examples)
train_indices <- pattern(random_sample, measurement = size(random_sample) * 0.8)
validation_indices <- setdiff(random_sample, train_indices)

sample_captions <- all_captions[random_sample]
sample_images <- all_img_names[random_sample]
train_captions <- all_captions[train_indices]
train_images <- all_img_names[train_indices]
validation_captions <- all_captions[validation_indices]
validation_images <- all_img_names[validation_indices]


Earlier than actually diving into the technical stuff, let’s take a second to replicate on this job. In typical image-related deep studying walk-throughs, we’re used to seeing well-defined issues – even when in some circumstances, the answer could also be onerous. Take, for instance, the stereotypical canine vs. cat drawback. Some canines might seem like cats and a few cats might seem like canines, however that’s about it: All in all, within the common world we reside in, it needs to be a roughly binary query.

If, alternatively, we ask individuals to explain what they see in a scene, it’s to be anticipated from the outset that we’ll get completely different solutions. Nonetheless, how a lot consensus there may be will very a lot depend upon the concrete dataset we’re utilizing.

Let’s check out some picks from the very first 20 coaching objects sampled randomly above.

Now this picture doesn’t go away a lot room for resolution what to concentrate on, and acquired a really factual caption certainly: “There’s a plate with one slice of bacon a half of orange and bread.” If the dataset have been all like this, we’d suppose a machine studying algorithm ought to do fairly properly right here.

Choosing one other one from the primary 20:

What can be salient data to you right here? The caption supplied goes “A smiling little boy has a checkered shirt.” Is the look of the shirt as vital as that? You may as properly concentrate on the surroundings, – and even one thing on a very completely different degree: The age of the picture, or it being an analog one.

Let’s take a closing instance.

What would you say about this scene? The official label we sampled right here is “A gaggle of individuals posing in a humorous method for the digicam.” Effectively …

Please don’t neglect that for every picture, the dataset consists of 5 completely different captions (though our n = 30000 samples in all probability gained’t). So this isn’t saying the dataset is biased – under no circumstances. As an alternative, we wish to level out the ambiguities and difficulties inherent within the job. Really, given these difficulties, it’s all of the extra wonderful that the duty we’re tackling right here – having a community mechanically generate picture captions – needs to be doable in any respect!

Now let’s see how we are able to do that.

For the encoding a part of our encoder-decoder community, we are going to make use of InceptionV3 to extract picture options. In precept, which options to extract is as much as experimentation, – right here we simply use the final layer earlier than the absolutely related high:

image_model <- application_inception_v3(
  include_top = FALSE,
  weights = "imagenet"

For a picture measurement of 299×299, the output might be of measurement (batch_size, 8, 8, 2048), that’s, we’re making use of 2048 characteristic maps.

InceptionV3 being a “massive mannequin,” the place each cross by way of the mannequin takes time, we wish to precompute options upfront and retailer them on disk. We’ll use tfdatasets to stream photographs to the mannequin. This implies all our preprocessing has to make use of tensorflow capabilities: That’s why we’re not utilizing the extra acquainted image_load from keras beneath.

Our customized load_image will learn in, resize and preprocess the photographs as required to be used with InceptionV3:

load_image <- perform(image_path) {
  img <-
    tf$read_file(image_path) %>%
    tf$picture$decode_jpeg(channels = 3) %>%
    tf$picture$resize_images(c(299L, 299L)) %>%
  record(img, image_path)

Now we’re prepared to save lots of the extracted options to disk. The (batch_size, 8, 8, 2048)-sized options might be flattened to (batch_size, 64, 2048). The latter form is what our encoder, quickly to be mentioned, will obtain as enter.

preencode <- distinctive(sample_images) %>% unlist() %>% kind()
num_unique <- size(preencode)

# adapt this in accordance with your system's capacities  
batch_size_4save <- 1
image_dataset <-
  tensor_slices_dataset(preencode) %>%
  dataset_map(load_image) %>%
save_iter <- make_iterator_one_shot(image_dataset)
  save_count <- save_count + batch_size_4save
  batch_4save <- save_iter$get_next()
  img <- batch_4save[[1]]
  path <- batch_4save[[2]]
  batch_features <- image_model(img)
  batch_features <- tf$reshape(
    record(dim(batch_features)[1], -1L, dim(batch_features)[4]
  for (i in 1:dim(batch_features)[1]) {
            batch_features[i, , ]$numpy())

Earlier than we get to the encoder and decoder fashions although, we have to maintain the captions.

Processing the captions

We’re utilizing keras text_tokenizer and the textual content processing capabilities texts_to_sequences and pad_sequences to rework ascii textual content right into a matrix.

# we are going to use the 5000 most frequent phrases solely
top_k <- 5000
tokenizer <- text_tokenizer(
  num_words = top_k,
  oov_token = "<unk>",
  filters = '!"#$%&()*+.,-/:;=?@[]^_`~ ')

train_captions_tokenized <-
  tokenizer %>% texts_to_sequences(train_captions)
validation_captions_tokenized <-
  tokenizer %>% texts_to_sequences(validation_captions)

# pad_sequences will use 0 to pad all captions to the identical size
tokenizer$word_index["<pad>"] <- 0

# create a lookup dataframe that permits us to go in each instructions
word_index_df <- information.body(
  phrase = tokenizer$word_index %>% names(),
  index = tokenizer$word_index %>% unlist(use.names = FALSE),
  stringsAsFactors = FALSE
word_index_df <- word_index_df %>% prepare(index)

decode_caption <- perform(textual content) {
  paste(map(textual content, perform(quantity)
    word_index_df %>%
      filter(index == quantity) %>%
      choose(phrase) %>%
    collapse = " ")

# pad all sequences to the identical size (the utmost size, in our case)
# might experiment with shorter padding (truncating the very longest captions)
caption_lengths <- map(
  perform(c) str_split(c," ")[[1]] %>% size()
  ) %>% unlist()
max_length <- fivenum(caption_lengths)[5]

train_captions_padded <-  pad_sequences(
  maxlen = max_length,
  padding = "publish",
  truncating = "publish"

validation_captions_padded <- pad_sequences(
  maxlen = max_length,
  padding = "publish",
  truncating = "publish"

Loading the info for coaching

Now that we’ve taken care of pre-extracting the options and preprocessing the captions, we’d like a technique to stream them to our captioning mannequin. For that, we’re utilizing tensor_slices_dataset from tfdatasets, passing within the record of paths to the photographs and the preprocessed captions. Loading the photographs is then carried out as a TensorFlow graph operation (utilizing tf$pyfunc).

The unique Colab code additionally shuffles the info on each iteration. Relying in your {hardware}, this may increasingly take a very long time, and given the scale of the dataset it isn’t strictly essential to get cheap outcomes. (The outcomes reported beneath have been obtained with out shuffling.)

batch_size <- 10
buffer_size <- num_examples

map_func <- perform(img_name, cap) {
  p <- paste0(img_name$decode("utf-8"), ".npy")
  img_tensor <- np$load(p)
  img_tensor <- tf$solid(img_tensor, tf$float32)
  record(img_tensor, cap)

train_dataset <-
  tensor_slices_dataset(record(train_images, train_captions_padded)) %>%
    perform(item1, item2) tf$py_func(map_func, record(item1, item2), record(tf$float32, tf$int32))
  ) %>%
  # optionally shuffle the dataset
  # dataset_shuffle(buffer_size) %>%

Captioning mannequin

The mannequin is mainly the identical as that mentioned within the machine translation publish. Please confer with that article for a proof of the ideas, in addition to an in depth walk-through of the tensor shapes concerned at each step. Right here, we offer the tensor shapes as feedback within the code snippets, for fast overview/comparability.

Nevertheless, in case you develop your individual fashions, with keen execution you may merely insert debugging/logging statements at arbitrary locations within the code – even in mannequin definitions. So you may have a perform

maybecat <- perform(context, x) {
  if (debugshapes) {
    title <- enexpr(x)
    dims <- paste0(dim(x), collapse = " ")
    cat(context, ": form of ", title, ": ", dims, "n", sep = "")

And in case you now set

you may hint – not solely tensor shapes, however precise tensor values by way of your fashions, as proven beneath for the encoder. (We don’t show any debugging statements after that, however the pattern code has many extra.)


Now it’s time to outline some some sizing-related hyperparameters and housekeeping variables:

# for encoder output
embedding_dim <- 256
# decoder (LSTM) capability
gru_units <- 512
# for decoder output
vocab_size <- top_k
# variety of characteristic maps gotten from Inception V3
features_shape <- 2048
# form of consideration options (flattened from 8x8)
attention_features_shape <- 64

The encoder on this case is only a absolutely related layer, taking within the options extracted from Inception V3 (in flattened kind, as they have been written to disk), and embedding them in 256-dimensional area.

cnn_encoder <- perform(embedding_dim, title = NULL) {
  keras_model_custom(title = title, perform(self) {
    self$fc <- layer_dense(models = embedding_dim, activation = "relu")
    perform(x, masks = NULL) {
      # enter form: (batch_size, 64, features_shape)
      maybecat("encoder enter", x)
      # form after fc: (batch_size, 64, embedding_dim)
      x <- self$fc(x)
      maybecat("encoder output", x)

Consideration module

In contrast to within the machine translation publish, right here the eye module is separated out into its personal customized mannequin. The logic is identical although:

attention_module <- perform(gru_units, title = NULL) {
  keras_model_custom(title = title, perform(self) {
    self$W1 = layer_dense(models = gru_units)
    self$W2 = layer_dense(models = gru_units)
    self$V = layer_dense(models = 1)
    perform(inputs, masks = NULL) {
      options <- inputs[[1]]
      hidden <- inputs[[2]]
      # options(CNN_encoder output) form == (batch_size, 64, embedding_dim)
      # hidden form == (batch_size, gru_units)
      # hidden_with_time_axis form == (batch_size, 1, gru_units)
      hidden_with_time_axis <- k_expand_dims(hidden, axis = 2)
      # rating form == (batch_size, 64, 1)
      rating <- self$V(k_tanh(self$W1(options) + self$W2(hidden_with_time_axis)))
      # attention_weights form == (batch_size, 64, 1)
      attention_weights <- k_softmax(rating, axis = 2)
      # context_vector form after sum == (batch_size, embedding_dim)
      context_vector <- k_sum(attention_weights * options, axis = 2)
      record(context_vector, attention_weights)


The decoder at every time step calls the eye module with the options it acquired from the encoder and its final hidden state, and receives again an consideration vector. The eye vector will get concatenated with the present enter and additional processed by a GRU and two absolutely related layers, the final of which supplies us the (unnormalized) possibilities for the subsequent phrase within the caption.

The present enter at every time step right here is the earlier phrase: the proper one throughout coaching (trainer forcing), the final generated one throughout inference.

rnn_decoder <- perform(embedding_dim, gru_units, vocab_size, title = NULL) {
  keras_model_custom(title = title, perform(self) {
    self$gru_units <- gru_units
    self$embedding <- layer_embedding(input_dim = vocab_size, 
                                      output_dim = embedding_dim)
    self$gru <- if (tf$take a look at$is_gpu_available()) {
        models = gru_units,
        return_sequences = TRUE,
        return_state = TRUE,
        recurrent_initializer = 'glorot_uniform'
    } else {
        models = gru_units,
        return_sequences = TRUE,
        return_state = TRUE,
        recurrent_initializer = 'glorot_uniform'
    self$fc1 <- layer_dense(models = self$gru_units)
    self$fc2 <- layer_dense(models = vocab_size)
    self$consideration <- attention_module(self$gru_units)
    perform(inputs, masks = NULL) {
      x <- inputs[[1]]
      options <- inputs[[2]]
      hidden <- inputs[[3]]
      c(context_vector, attention_weights) %<-% 
        self$consideration(record(options, hidden))
      # x form after passing by way of embedding == (batch_size, 1, embedding_dim)
      x <- self$embedding(x)
      # x form after concatenation == (batch_size, 1, 2 * embedding_dim)
      x <- k_concatenate(record(k_expand_dims(context_vector, 2), x))
      # passing the concatenated vector to the GRU
      c(output, state) %<-% self$gru(x)
      # form == (batch_size, 1, gru_units)
      x <- self$fc1(output)
      # x form == (batch_size, gru_units)
      x <- k_reshape(x, c(-1, dim(x)[[3]]))
      # output form == (batch_size, vocab_size)
      x <- self$fc2(x)
      record(x, state, attention_weights)

Loss perform, and instantiating all of it

Now that we’ve outlined our mannequin (constructed of three customized fashions), we nonetheless want to truly instantiate it (being exact: the 2 courses we are going to entry from outdoors, that’s, the encoder and the decoder).

We additionally must instantiate an optimizer (Adam will do), and outline our loss perform (categorical crossentropy). Notice that tf$nn$sparse_softmax_cross_entropy_with_logits expects uncooked logits as a substitute of softmax activations, and that we’re utilizing the sparse variant as a result of our labels aren’t one-hot-encoded.

encoder <- cnn_encoder(embedding_dim)
decoder <- rnn_decoder(embedding_dim, gru_units, vocab_size)

optimizer = tf$practice$AdamOptimizer()

cx_loss <- perform(y_true, y_pred) {
  masks <- 1 - k_cast(y_true == 0L, dtype = "float32")
  loss <- tf$nn$sparse_softmax_cross_entropy_with_logits(
    labels = y_true,
    logits = y_pred
  ) * masks


Coaching the captioning mannequin is a time-consuming course of, and you’ll for positive wish to save the mannequin’s weights! How does this work with keen execution?

We create a tf$practice$Checkpoint object, passing it the objects to be saved: In our case, the encoder, the decoder, and the optimizer. Later, on the finish of every epoch, we are going to ask it to jot down the respective weights to disk.

restore_checkpoint <- FALSE

checkpoint_dir <- "./checkpoints_captions"
checkpoint_prefix <- file.path(checkpoint_dir, "ckpt")
checkpoint <- tf$practice$Checkpoint(
  optimizer = optimizer,
  encoder = encoder,
  decoder = decoder

As we’re simply beginning to practice the mannequin, restore_checkpoint is about to false. Later, restoring the weights might be as simple as

if (restore_checkpoint) {

The coaching loop is structured identical to within the machine translation case: We loop over epochs, batches, and the coaching targets, feeding within the right earlier phrase at each timestep. Once more, tf$GradientTape takes care of recording the ahead cross and calculating the gradients, and the optimizer applies the gradients to the mannequin’s weights. As every epoch ends, we additionally save the weights.

num_epochs <- 20

if (!restore_checkpoint) {
  for (epoch in seq_len(num_epochs)) {
    total_loss <- 0
    progress <- 0
    train_iter <- make_iterator_one_shot(train_dataset)
      batch <- iterator_get_next(train_iter)
      loss <- 0
      img_tensor <- batch[[1]]
      target_caption <- batch[[2]]
      dec_hidden <- k_zeros(c(batch_size, gru_units))
      dec_input <- k_expand_dims(
        rep(record(word_index_df[word_index_df$word == "<start>", "index"]), 
      with(tf$GradientTape() %as% tape, {
        options <- encoder(img_tensor)
        for (t in seq_len(dim(target_caption)[2] - 1)) {
          c(preds, dec_hidden, weights) %<-%
            decoder(record(dec_input, options, dec_hidden))
          loss <- loss + cx_loss(target_caption[, t], preds)
          dec_input <- k_expand_dims(target_caption[, t])
      total_loss <-
        total_loss + loss / k_cast_to_floatx(dim(target_caption)[2])
      variables <- c(encoder$variables, decoder$variables)
      gradients <- tape$gradient(loss, variables)
      optimizer$apply_gradients(purrr::transpose(record(gradients, variables)),
                                global_step = tf$practice$get_or_create_global_step()
      "nnTotal loss (epoch): ",
      ": ",
      (total_loss / k_cast_to_floatx(buffer_size)) %>% as.double() %>% spherical(4),
    checkpoint$save(file_prefix = checkpoint_prefix)

Peeking at outcomes

Similar to within the translation case, it’s fascinating to have a look at mannequin efficiency throughout coaching. The companion code has that performance built-in, so you may watch mannequin progress for your self.

The fundamental perform right here is get_caption: It will get handed the trail to a picture, hundreds it, obtains its options from Inception V3, after which asks the encoder-decoder mannequin to generate a caption. If at any level the mannequin produces the finish image, we cease early. In any other case, we proceed till we hit the predefined most size.

Anderson et al. 2017) use object detection strategies to bottom-up isolate fascinating objects, and an LSTM stack whereby the primary LSTM computes top-down consideration guided by the output phrase generated by the second.

One other fascinating method involving consideration is utilizing a multimodal attentive translator (Liu et al. 2017), the place the picture options are encoded and offered in a sequence, such that we find yourself with sequence fashions each on the encoding and the decoding sides.

One other different is so as to add a realized subject to the knowledge enter (Zhu, Xue, and Yuan 2018), which once more is a top-down characteristic present in human cognition.

When you discover one among these, or one more, method extra convincing, an keen execution implementation, within the type of the above, will possible be a sound method of implementing it.

Anderson, Peter, Xiaodong He, Chris Buehler, Damien Teney, Mark Johnson, Stephen Gould, and Lei Zhang. 2017. “Backside-up and High-down Consideration for Picture Captioning and VQA.” CoRR abs/1707.07998.
Liu, Chang, Fuchun Solar, Changhu Wang, Feng Wang, and Alan L. Yuille. 2017. “A Multimodal Attentive Translator for Picture Captioning.” CoRR abs/1702.05658.
Xu, Kelvin, Jimmy Ba, Ryan Kiros, Kyunghyun Cho, Aaron C. Courville, Ruslan Salakhutdinov, Richard S. Zemel, and Yoshua Bengio. 2015. “Present, Attend and Inform: Neural Picture Caption Era with Visible Consideration.” CoRR abs/1502.03044.
Zhu, Zhihao, Zhan Xue, and Zejian Yuan. 2018. “A Subject-Guided Consideration for Picture Captioning.” CoRR abs/1807.03514v1.


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