We’ve seen fairly a couple of examples of unsupervised studying (or self-supervised studying, to decide on the extra right however much less fashionable time period) on this weblog.

Usually, these concerned *Variational Autoencoders (VAEs)*, whose attraction lies in them permitting to mannequin a *latent area* of underlying, impartial (ideally) elements that decide the seen options. A potential draw back could be the inferior high quality of generated samples. Generative Adversarial Networks (GANs) are one other fashionable strategy. Conceptually, these are extremely engaging as a result of their game-theoretic framing. Nevertheless, they are often tough to coach. *PixelCNN* variants, however – we’ll subsume all of them right here beneath PixelCNN – are typically recognized for his or her good outcomes. They appear to contain some extra alchemy although. Below these circumstances, what may very well be extra welcome than a simple method of experimenting with them? By means of TensorFlow Likelihood (TFP) and its R wrapper, tfprobability, we now have such a method.

This put up first provides an introduction to PixelCNN, concentrating on high-level ideas (leaving the small print for the curious to look them up within the respective papers). We’ll then present an instance of utilizing `tfprobability`

to experiment with the TFP implementation.

## PixelCNN ideas

### Autoregressivity, or: We want (some) order

The essential thought in PixelCNN is autoregressivity. Every pixel is modeled as relying on all prior pixels. Formally:

[p(mathbf{x}) = prod_{i}p(x_i|x_0, x_1, …, x_{i-1})]

Now wait a second – what even *are* prior pixels? Final I noticed one pictures have been two-dimensional. So this implies we’ve got to impose an *order* on the pixels. Generally this can be *raster scan* order: row after row, from left to proper. However when coping with colour pictures, there’s one thing else: At every place, we even have *three* depth values, one for every of purple, inexperienced, and blue. The unique PixelCNN paper(Oord, Kalchbrenner, and Kavukcuoglu 2016) carried by means of autoregressivity right here as nicely, with a pixel’s depth for purple relying on simply prior pixels, these for inexperienced relying on these similar prior pixels however moreover, the present worth for purple, and people for blue relying on the prior pixels in addition to the present values for purple and inexperienced.

[p(x_i|mathbf{x}<i) = p(x_{i,R}|mathbf{x}<i) p(x_{i,G}|mathbf{x}<i, x_{i,R}) p(x_{i,B}|mathbf{x}<i, x_{i,R}, x_{i,G})]

Right here, the variant applied in TFP, PixelCNN++(Salimans et al. 2017) , introduces a simplification; it factorizes the joint distribution in a much less compute-intensive method.

Technically, then, we all know how autoregressivity is realized; intuitively, it might nonetheless appear shocking that imposing a raster scan order “simply works” (to me, a minimum of, it’s). Possibly that is a kind of factors the place compute energy efficiently compensates for lack of an equal of a cognitive prior.

### Masking, or: The place to not look

Now, PixelCNN ends in “CNN” for a motive – as typical in picture processing, convolutional layers (or blocks thereof) are concerned. However – is it not the very nature of a convolution that it computes a median of some types, trying, for every output pixel, not simply on the corresponding enter but in addition, at its spatial (or temporal) environment? How does that rhyme with the look-at-just-prior-pixels technique?

Surprisingly, this downside is simpler to unravel than it sounds. When making use of the convolutional kernel, simply multiply with a masks that zeroes out any “forbidden pixels” – like on this instance for a 5×5 kernel, the place we’re about to compute the convolved worth for row 3, column 3:

[left[begin{array}

{rrr}

1 & 1 & 1 & 1 & 1

1 & 1 & 1 & 1 & 1

1 & 1 & 1 & 0 & 0

0 & 0 & 0 & 0 & 0

0 & 0 & 0 & 0 & 0

end{array}right]

]

This makes the algorithm sincere, however introduces a distinct downside: With every successive convolutional layer consuming its predecessor’s output, there’s a repeatedly rising *blind spot* (so-called in analogy to the blind spot on the retina, however situated within the high proper) of pixels which are by no means *seen* by the algorithm. Van den Oord et al. (2016)(Oord et al. 2016) repair this by utilizing two completely different convolutional stacks, one continuing from high to backside, the opposite from left to proper.

### Conditioning, or: Present me a kitten

To date, we’ve all the time talked about “producing pictures” in a purely generic method. However the actual attraction lies in creating samples of some specified sort – one of many courses we’ve been coaching on, or orthogonal info fed into the community. That is the place PixelCNN turns into *Conditional PixelCNN*(Oord et al. 2016), and it’s also the place that feeling of magic resurfaces. Once more, as “basic math” it’s not onerous to conceive. Right here, (mathbf{h}) is the extra enter we’re conditioning on:

[p(mathbf{x}| mathbf{h}) = prod_{i}p(x_i|x_0, x_1, …, x_{i-1}, mathbf{h})]

However how does this translate into neural community operations? It’s simply one other matrix multiplication ((V^T mathbf{h})) added to the convolutional outputs ((W mathbf{x})).

[mathbf{y} = tanh(W_{k,f} mathbf{x} + V^T_{k,f} mathbf{h}) odot sigma(W_{k,g} mathbf{x} + V^T_{k,g} mathbf{h})]

(For those who’re questioning in regards to the second half on the fitting, after the Hadamard product signal – we received’t go into particulars, however in a nutshell, it’s one other modification launched by (Oord et al. 2016), a switch of the “gating” precept from recurrent neural networks, comparable to GRUs and LSTMs, to the convolutional setting.)

So we see what goes into the choice of a pixel worth to pattern. However how is that call truly *made*?

### Logistic combination probability , or: No pixel is an island

Once more, that is the place the TFP implementation doesn’t observe the unique paper, however the latter PixelCNN++ one. Initially, pixels have been modeled as discrete values, selected by a softmax over 256 (0-255) potential values. (That this truly labored looks like one other occasion of deep studying magic. Think about: On this mannequin, 254 is as removed from 255 as it’s from 0.)

In distinction, PixelCNN++ assumes an underlying steady distribution of colour depth, and rounds to the closest integer. That underlying distribution is a mix of logistic distributions, thus permitting for multimodality:

[nu sim sum_{i} pi_i logistic(mu_i, sigma_i)]

### General structure and the PixelCNN distribution

General, PixelCNN++, as described in (Salimans et al. 2017), consists of six blocks. The blocks collectively make up a UNet-like construction, successively downsizing the enter after which, upsampling once more:

In TFP’s PixelCNN distribution, the variety of blocks is configurable as `num_hierarchies`

, the default being 3.

Every block consists of a customizable variety of layers, known as *ResNet layers* because of the residual connection (seen on the fitting) complementing the convolutional operations within the horizontal stack:

In TFP, the variety of these layers per block is configurable as `num_resnet`

.

`num_resnet`

and `num_hierarchies`

are the parameters you’re most definitely to experiment with, however there are a couple of extra you’ll be able to try within the documentation. The variety of logistic distributions within the combination can be configurable, however from my experiments it’s finest to maintain that quantity relatively low to keep away from producing `NaN`

s throughout coaching.

Let’s now see a whole instance.

## Finish-to-end instance

Our playground can be QuickDraw, a dataset – nonetheless rising – obtained by asking folks to attract some object in at most twenty seconds, utilizing the mouse. (To see for your self, simply try the web site). As of at this time, there are greater than a fifty million situations, from 345 completely different courses.

Before everything, these information have been chosen to take a break from MNIST and its variants. However identical to these (and plenty of extra!), QuickDraw could be obtained, in `tfdatasets`

-ready type, through tfds, the R wrapper to TensorFlow datasets. In distinction to the MNIST “household” although, the “actual samples” are themselves extremely irregular, and sometimes even lacking important components. So to anchor judgment, when displaying generated samples we all the time present eight precise drawings with them.

### Getting ready the information

The dataset being gigantic, we instruct `tfds`

to load the primary 500,000 drawings “solely.”

To hurry up coaching additional, we then zoom in on twenty courses. This successfully leaves us with ~ 1,100 – 1,500 drawings per class.

```
# bee, bicycle, broccoli, butterfly, cactus,
# frog, guitar, lightning, penguin, pizza,
# rollerskates, sea turtle, sheep, snowflake, solar,
# swan, The Eiffel Tower, tractor, prepare, tree
courses <- c(26, 29, 43, 49, 50,
125, 134, 172, 218, 225,
246, 255, 258, 271, 295,
296, 308, 320, 322, 323
)
classes_tensor <- tf$solid(courses, tf$int64)
train_ds <- train_ds %>%
dataset_filter(
perform(report) tf$reduce_any(tf$equal(classes_tensor, report$label), -1L)
)
```

The PixelCNN distribution expects values within the vary from 0 to 255 – no normalization required. Preprocessing then consists of simply casting pixels and labels every to `float`

:

```
preprocess <- perform(report) {
report$picture <- tf$solid(report$picture, tf$float32)
report$label <- tf$solid(report$label, tf$float32)
record(tuple(report$picture, report$label))
}
batch_size <- 32
prepare <- train_ds %>%
dataset_map(preprocess) %>%
dataset_shuffle(10000) %>%
dataset_batch(batch_size)
```

### Creating the mannequin

We now use tfd_pixel_cnn to outline what would be the loglikelihood utilized by the mannequin.

```
dist <- tfd_pixel_cnn(
image_shape = c(28, 28, 1),
conditional_shape = record(),
num_resnet = 5,
num_hierarchies = 3,
num_filters = 128,
num_logistic_mix = 5,
dropout_p =.5
)
image_input <- layer_input(form = c(28, 28, 1))
label_input <- layer_input(form = record())
log_prob <- dist %>% tfd_log_prob(image_input, conditional_input = label_input)
```

This practice loglikelihood is added as a loss to the mannequin, after which, the mannequin is compiled with simply an optimizer specification solely. Throughout coaching, loss first decreased shortly, however enhancements from later epochs have been smaller.

```
mannequin <- keras_model(inputs = record(image_input, label_input), outputs = log_prob)
mannequin$add_loss(-tf$reduce_mean(log_prob))
mannequin$compile(optimizer = optimizer_adam(lr = .001))
mannequin %>% match(prepare, epochs = 10)
```

To collectively show actual and faux pictures:

```
for (i in courses) {
real_images <- train_ds %>%
dataset_filter(
perform(report) report$label == tf$solid(i, tf$int64)
) %>%
dataset_take(8) %>%
dataset_batch(8)
it <- as_iterator(real_images)
real_images <- iter_next(it)
real_images <- real_images$picture %>% as.array()
real_images <- real_images[ , , , 1]/255
generated_images <- dist %>% tfd_sample(8, conditional_input = i)
generated_images <- generated_images %>% as.array()
generated_images <- generated_images[ , , , 1]/255
pictures <- abind::abind(real_images, generated_images, alongside = 1)
png(paste0("draw_", i, ".png"), width = 8 * 28 * 10, peak = 2 * 28 * 10)
par(mfrow = c(2, 8), mar = c(0, 0, 0, 0))
pictures %>%
purrr::array_tree(1) %>%
purrr::map(as.raster) %>%
purrr::iwalk(plot)
dev.off()
}
```

From our twenty courses, right here’s a alternative of six, every exhibiting actual drawings within the high row, and faux ones under.

We most likely wouldn’t confuse the primary and second rows, however then, the precise human drawings exhibit huge variation, too. And nobody ever stated PixelCNN was an structure for idea studying. Be happy to mess around with different datasets of your alternative – TFP’s PixelCNN distribution makes it simple.

## Wrapping up

On this put up, we had `tfprobability`

/ TFP do all of the heavy lifting for us, and so, may give attention to the underlying ideas. Relying in your inclinations, this may be an excellent scenario – you don’t lose sight of the forest for the timber. Then again: Must you discover that altering the offered parameters doesn’t obtain what you need, you might have a reference implementation to begin from. So regardless of the consequence, the addition of such higher-level performance to TFP is a win for the customers. (For those who’re a TFP developer studying this: Sure, we’d like extra :-)).

To everybody although, thanks for studying!

*CoRR*abs/1601.06759. http://arxiv.org/abs/1601.06759.

*CoRR*abs/1606.05328. http://arxiv.org/abs/1606.05328.

Salimans, Tim, Andrej Karpathy, Xi Chen, and Diederik P. Kingma. 2017. “PixelCNN++: A PixelCNN Implementation with Discretized Logistic Combination Probability and Different Modifications.” In *ICLR*.