tfp.bijectors.Glow

Implements the Glow Bijector from Kingma & Dhariwal (2018)[1].

Inherits From: Chain, Composition, AutoCompositeTensorBijector

tfp.bijectors.Glow(
    output_shape=(32, 32, 3), num_glow_blocks=3, num_steps_per_block=32,
    coupling_bijector_fn=None, exit_bijector_fn=None, grab_after_block=None,
    use_actnorm=True, seed=None, validate_args=False, name='glow'
)

Overview: Glow is a chain of bijectors which transforms a rank-1 tensor (vector) into a rank-3 tensor (e.g. an RGB image). Glow does this by chaining together an alternating series of "Blocks," "Squeezes," and "Exits" which are each themselves special chains of other bijectors. The intended use of Glow is as part of a tfp.distributions.TransformedDistribution, in which the base distribution over the vector space is used to generate samples in the image space. In the paper, an Independent Normal distribution is used as the base distribution.

A "Block" (implemented as the GlowBlock Bijector) performs much of the transformations which allow glow to produce sophisticated and complex mappings between the image space and the latent space and therefore achieve rich image generation performance. A Block is composed of num_steps_per_block steps, which are each implemented as a Chain containing an ActivationNormalization (ActNorm) bijector, followed by an (invertible) OneByOneConv bijector, and finally a coupling bijector. The coupling bijector is an instance of a RealNVP bijector, and uses the coupling_bijector_fn function to instantiate the coupling bijector function which is given to the RealNVP. This function returns a bijector which defines the coupling (e.g. Shift(Scale) for affine coupling or Shift for additive coupling).

A "Squeeze" converts spatial features into channel features. It is implemented using the Expand bijector. The difference in names is due to the fact that the forward function from glow is meant to ultimately correspond to sampling from a tfp.util.TransformedDistribution object, which would use Expand (Squeeze is just Invert(Expand)). The Expand bijector takes a tensor with shape [H, W, C] and returns a tensor with shape [2H, 2W, C / 4], such that each 2x2x1 spatial tile in the output is composed from a single 1x1x4 tile in the input tensor, as depicted in the figure below.

                     Forward pass (Expand)
                    ______        __________
                    \     \       \    \    \
                    \\     \ ----> \  1 \  2 \
                    \\\__1__\       \____\____\
                    \\\__2__\        \    \    \
                     \\__3__\  <----  \  3 \  4 \
                      \__4__\          \____\____\
                         Inverse pass (Squeeze)

This is implemented using a chain of Reshape -> Transpose -> Reshape bijectors. Note that on an inverse pass through the bijector, each Squeeze will cause the width/height of the image to decrease by a factor of 2. Therefore, the input image must be evenly divisible by 2 at least num_glow_blocks times, since it will pass through a Squeeze step that many times.

An "Exit" is simply a junction at which some of the tensor "exits" from the glow bijector and therefore avoids any further alteration. Each exit is implemented as a Blockwise bijector, where some channels are given to the rest of the glow model, and the rest are given to a bypass implemented using the Identity bijector. The fraction of channels to be removed at each exit is determined by the grab_after_block arg, indicates the fraction of remaining channels which join the identity bypass. The fraction is converted to an integer number of channels by multiplying by the remaining number of channels and rounding.

Additionally, at each exit, glow couples the tensor exiting the highway to the tensor continuing onward. This makes small scale features in the image dependent on larger scale features, since the larger scale features dictate the mean and scale of the distribution over the smaller scale features. This coupling is done similarly to the Coupling bijector in each step of the flow (i.e. using a RealNVP bijector). However for the exit bijector, the coupling is instantiated using exit_bijector_fn rather than coupling bijector fn, allowing for different behaviors between standard coupling and exit coupling. Also note that because the exit utilizes a coupling bijector, there are two special cases (all channels exiting and no channels exiting).

The full Glow bijector consists of num_glow_blocks Blocks each of which contains num_steps_per_block steps. Each step implements a coupling using bijector_coupling_fn. Between blocks, glow converts between spatial pixels and channels using the Expand Bijector, and splits channels out of the bijector using the Exit Bijector. The channels which have exited continue onward through Identity bijectors and those which have not exited are given to the next block. After passing through all Blocks, the tensor is reshaped to a rank-1 tensor with the same number of elements. This is where the distribution will be defined.

A schematic diagram of Glow is shown below. The forward function of the bijector starts from the bottom and goes upward, while the inverse function starts from the top and proceeds downward.

==============================================================================
                         Glow Schematic Diagram

Input Image     ########################   shape = [H, W, C]

                \                      /<- Expand Bijector turns spatial
                 \                    /    dimensions into channels.
                _
               |  XXXXXXXXXXXXXXXXXXXX
               |  XXXXXXXXXXXXXXXXXXXX
               |  XXXXXXXXXXXXXXXXXXXX     A single step of the flow consists
 Glow Block  - |  XXXXXXXXXXXXXXXXXXXX  <- of ActNorm -> 1x1Conv -> Coupling.
               |  XXXXXXXXXXXXXXXXXXXX     there are num_steps_per_block
               |  XXXXXXXXXXXXXXXXXXXX     steps of the flow in each block.
               |_ XXXXXXXXXXXXXXXXXXXX

                  \                  / <-- Expand bijectors follow each glow
                   \                /      block

                    XXXXXXXX\\\\\\\\   <-- Exit Bijector removes channels
                _                    _     from additional alteration.
               |    XXXXXXXX !  |  !
               |    XXXXXXXX !  |  !
               |    XXXXXXXX !  |  !       After exiting, channels are passed
 Glow Block  - |    XXXXXXXX !  |  !  <--- downward using the Blockwise and
               |    XXXXXXXX !  |  !       Identify bijectors.
               |    XXXXXXXX !  |  !
               |_   XXXXXXXX !  |  !

                    \              / <---- Expand Bijector
                     \            /

                      XXX\\\    | !  <---- Exit Bijector
                _
               |      XXX ! |   | !
               |      XXX ! |   | !
               |      XXX ! |   | !
 Glow Block  - |      XXX ! |   | !
               |      XXX ! |   | !
               |      XXX ! |   | !
               |_     XXX ! |   | !

                      XX\ ! |   | ! <----- (Optional) Exit Bijector

                       |    |   |
                       v    v   v
Output Distribution    ##########          shape = [H * W * C]
                                                   _________________________
                                                  |         Legend          |
                                                  | XX  = Step of flow      |
                                                  | X\  = Exit bijector     |
                                                  | \/  = Expand bijector   |
                                                  | !|! = Identity bijector |
                                                  |                         |
                                                  | up  = Forward pass      |
                                                  | dn  = Inverse pass      |
                                                  |_________________________|

==============================================================================

The default configuration for glow is meant to replicate the architecture in [1] for generating images from CIFAR-10.

Example usage:


from functools import reduce
from operator import mul
import tensorflow as tf
import tensorflow_datasets as tfds
import tensorflow_probability as tfp
tfb = tfp.bijectors
tfd = tfp.distributions

data, info = tfds.load('cifar10', with_info=True)
train_data, test_data = data['train'], data['test']

preprocess = lambda x: tf.cast(x['image'], tf.float32)
train_data = train_data.batch(4).map(preprocess)
test_data = test_data.batch(4).map(preprocess)

x = next(iter(train_data))

glow = tfb.Glow(output_shape=info.features['image'].shape,
                coupling_bijector_fn=tfb.GlowDefaultNetwork,
                exit_bijector_fn=tfb.GlowDefaultExitNetwork)

z_shape = glow.inverse_event_shape(info.features['image'].shape)

pz = tfd.Sample(tfd.Normal(0., 1.), z_shape)

# Calling glow on distribution p(z) creates our glow distribution over images.
px = glow(pz)

# Take samples from the distribution to get images from your dataset
images = px.sample(4)

# Map images to positions in the distribution
z = glow.inverse(x)

# Get the z's corresponding to each spatial scale. To do this, we have to
# find out how many zs are passed through blockwise at each stage that were
# not passed at the previous stage. This is encoded in the second element of
# each list of blockwise splits. However because the bijector iteratively
# converts spatial pixels to channels, we also need to multiply the size of
# that second element by the number of spatial-to-channel conversions that the
# tensor receives after exiting (including after any alteration).
ztake = [bs[1] * 4**(i+2) for i, bs in enumerate(glow.blockwise_splits)]
total_z_taken = sum(ztake)
split_sizes = [z_shape.as_list()[0]-total_z_taken] + ztake
zsplits = tf.split(z, num_or_size_splits=split_sizes, axis=-1)

References:

[1]: Diederik P Kingma, Prafulla Dhariwal, Glow: Generative Flow with Invertible 1x1 Convolutions. In Neural Information Processing Systems, 2018. https://arxiv.org/abs/1807.03039

[2]: Laurent Dinh, Jascha Sohl-Dickstein, and Samy Bengio. Density Estimation using Real NVP. In International Conference on Learning Representations, 2017. https://arxiv.org/abs/1605.08803

Args
`output_shape`	A list of integers, specifying the event shape of the output, of the bijectors forward pass (the image). Specified as [H, W, C]. Default Value: (32, 32, 3)
`num_glow_blocks`	An integer, specifying how many downsampling levels to include in the model. This must divide equally into both H and W, otherwise the bijector would not be invertible. Default Value: 3
`num_steps_per_block`	An integer specifying how many Affine Coupling and 1x1 convolution layers to include at each level of the spatial hierarchy. Default Value: 32 (i.e. the value used in the original glow paper).
`coupling_bijector_fn`	A function which takes the argument `input_shape` and returns a callable neural network (e.g. a keras.Sequential). The network should either return a tensor with the same event shape as `input_shape` (this will employ additive coupling), a tensor with the same height and width as `input_shape` but twice the number of channels (this will employ affine coupling), or a bijector which takes in a tensor with event shape `input_shape`, and returns a tensor with shape `input_shape`.
`exit_bijector_fn`	Similar to coupling_bijector_fn, exit_bijector_fn is a function which takes the argument `input_shape` and `output_chan` and returns a callable neural network. The neural network it returns should take a tensor of shape `input_shape` as the input, and return one of three options: A tensor with `output_chan` channels, a tensor with `2 * output_chan` channels, or a bijector. Additional details can be found in the documentation for ExitBijector.
`grab_after_block`	A tuple of floats, specifying what fraction of the remaining channels to remove following each glow block. Glow will take the integer floor of this number multiplied by the remaining number of channels. The default is half at each spatial hierarchy. Default value: None (this will take out half of the channels after each block.
`use_actnorm`	A bool deciding whether or not to use actnorm. Data-dependent initialization is used to initialize this layer. Default value: `False`
`seed`	A seed to control randomness in the 1x1 convolution initialization. Default value: `None` (i.e., non-reproducible sampling).
`validate_args`	Python `bool` indicating whether arguments should be checked for correctness. Default value: `False`
`name`	Python `str`, name given to ops managed by this object. Default value: `'glow'`.

Attributes
`bijectors`
`blockwise_splits`
`dtype`
`forward_min_event_ndims`	Returns the minimal number of dimensions bijector.forward operates on. Multipart bijectors return structured `ndims`, which indicates the expected structure of their inputs. Some multipart bijectors, notably Composites, may return structures of `None`.
`graph_parents`	Returns this `Bijector`'s graph_parents as a Python list.
`has_static_min_event_ndims`	Returns True if the bijector has statically-known `min_event_ndims`. (deprecated) Warning: THIS FUNCTION IS DEPRECATED. It will be removed after 2021-08-01. Instructions for updating: `min_event_ndims` is now static for all bijectors; this property is no longer needed.
`inverse_min_event_ndims`	Returns the minimal number of dimensions bijector.inverse operates on. Multipart bijectors return structured `event_ndims`, which indicates the expected structure of their outputs. Some multipart bijectors, notably Composites, may return structures of `None`.
`is_constant_jacobian`	Returns true iff the Jacobian matrix is not a function of x. Note: Jacobian matrix is either constant for both forward and inverse or neither.
`name`	Returns the string name of this `Bijector`.
`name_scope`	Returns a `tf.name_scope` instance for this class.
`non_trainable_variables`	Sequence of non-trainable variables owned by this module and its submodules. Note: this method uses reflection to find variables on the current instance and submodules. For performance reasons you may wish to cache the result of calling this method if you don't expect the return value to change.
`parameters`	Dictionary of parameters used to instantiate this `Bijector`.
`submodules`	Sequence of all sub-modules. Submodules are modules which are properties of this module, or found as properties of modules which are properties of this module (and so on). `a = tf.Module()` `b = tf.Module()` `c = tf.Module()` `a.b = b` `b.c = c` `list(a.submodules) == [b, c]` `True` `list(b.submodules) == [c]` `True` `list(c.submodules) == []` `True`
`trainable_variables`	Sequence of trainable variables owned by this module and its submodules. Note: this method uses reflection to find variables on the current instance and submodules. For performance reasons you may wish to cache the result of calling this method if you don't expect the return value to change.
`validate_args`	Returns True if Tensor arguments will be validated.
`validate_event_size`
`variables`	Sequence of variables owned by this module and its submodules. Note: this method uses reflection to find variables on the current instance and submodules. For performance reasons you may wish to cache the result of calling this method if you don't expect the return value to change.

Args
`x`	`Tensor` (structure). The input to the 'forward' evaluation.
`name`	The name to give this op.
`**kwargs`	Named arguments forwarded to subclass implementation.

Raises
`TypeError`	if `self.dtype` is specified and `x.dtype` is not `self.dtype`.
`NotImplementedError`	if `_forward` is not implemented.

Args
`input_shape`	`Tensor`, `int32` vector (structure) indicating event-portion shape passed into `forward` function.
`name`	name to give to the op

Args
`x`	`Tensor` (structure). The input to the 'forward' Jacobian determinant evaluation.
`event_ndims`	Optional number of dimensions in the probabilistic events being transformed; this must be greater than or equal to `self.forward_min_event_ndims`. If `event_ndims` is specified, the log Jacobian determinant is summed to produce a scalar log-determinant for each event. Otherwise (if `event_ndims` is `None`), no reduction is performed. Multipart bijectors require structured event_ndims, such that the batch rank `rank(y[i]) - event_ndims[i]` is the same for all elements `i` of the structured input. In most cases (with the exception of `tfb.JointMap`) they further require that `event_ndims[i] - self.inverse_min_event_ndims[i]` is the same for all elements `i` of the structured input. Default value: `None` (equivalent to `self.forward_min_event_ndims`).
`name`	The name to give this op.
`**kwargs`	Named arguments forwarded to subclass implementation.

Raises
`TypeError`	if `y`'s dtype is incompatible with the expected output dtype.
`NotImplementedError`	if neither `_forward_log_det_jacobian` nor {`_inverse`, `_inverse_log_det_jacobian`} are implemented, or this is a non-injective bijector.
`ValueError`	if the value of `event_ndims` is not valid for this bijector.

Raises
`TypeError`	if `y`'s structured dtype is incompatible with the expected output dtype.
`NotImplementedError`	if `_inverse` is not implemented.

Raises
`TypeError`	if `x`'s dtype is incompatible with the expected inverse-dtype.
`NotImplementedError`	if `_inverse_log_det_jacobian` is not implemented.
`ValueError`	if the value of `event_ndims` is not valid for this bijector.

Args
`value`	A `tfd.Distribution`, `tfb.Bijector`, or a (structure of) `Tensor`.
`name`	Python `str` name given to ops created by this function.
`**kwargs`	Additional keyword arguments passed into the created `tfd.TransformedDistribution`, `tfb.Bijector`, or `self.forward`.

tfp.bijectors.Glow

Example usage:

References:

Args

Attributes

Methods

forward

forward_dtype

forward_event_ndims

forward_event_shape

forward_event_shape_tensor

forward_log_det_jacobian

inverse

inverse_dtype

inverse_event_ndims

inverse_event_shape

inverse_event_shape_tensor

inverse_log_det_jacobian

parameter_properties

with_name_scope

__call__

Examples

__eq__

`forward`

`forward_dtype`

`forward_event_ndims`

`forward_event_shape`

`forward_event_shape_tensor`

`forward_log_det_jacobian`

`inverse`

`inverse_dtype`

`inverse_event_ndims`

`inverse_event_shape`

`inverse_event_shape_tensor`

`inverse_log_det_jacobian`

`parameter_properties`

`with_name_scope`

`call`

`eq`