Posit AI Weblog: Implementing rotation equivariance: Group-equivariant CNN from scratch

Convolutional neural networks (CNNs) are nice – they’re in a position to detect options in a picture regardless of the place. Nicely, not precisely. They’re not detached to simply any type of motion. Shifting up or down, or left or proper, is okay; rotating round an axis is just not. That’s due to how convolution works: traverse by row, then traverse by column (or the opposite method spherical). If we would like “extra” (e.g., profitable detection of an upside-down object), we have to prolong convolution to an operation that’s rotation-equivariant. An operation that’s equivariant to some sort of motion is not going to solely register the moved function per se, but additionally, preserve monitor of which concrete motion made it seem the place it’s.

That is the second put up in a collection that introduces group-equivariant CNNs (GCNNs). The first was a high-level introduction to why we’d need them, and the way they work. There, we launched the important thing participant, the symmetry group, which specifies what sorts of transformations are to be handled equivariantly. In case you haven’t, please check out that put up first, since right here I’ll make use of terminology and ideas it launched.

At the moment, we code a easy GCNN from scratch. Code and presentation tightly comply with a pocket book supplied as a part of College of Amsterdam’s 2022 Deep Studying Course. They will’t be thanked sufficient for making out there such glorious studying supplies.

In what follows, my intent is to elucidate the final considering, and the way the ensuing structure is constructed up from smaller modules, every of which is assigned a transparent objective. For that cause, I gained’t reproduce all of the code right here; as an alternative, I’ll make use of the package deal gcnn. Its strategies are closely annotated; so to see some particulars, don’t hesitate to have a look at the code.

As of as we speak, gcnn implements one symmetry group: (C_4), the one which serves as a working instance all through put up one. It’s straightforwardly extensible, although, making use of sophistication hierarchies all through.

Step 1: The symmetry group (C_4)

In coding a GCNN, the very first thing we have to present is an implementation of the symmetry group we’d like to make use of. Right here, it’s (C_4), the four-element group that rotates by 90 levels.

We are able to ask gcnn to create one for us, and examine its parts.

# remotes::install_github("skeydan/gcnn")
library(gcnn)
library(torch)

C_4 <- CyclicGroup(order = 4)
elems <- C_4$parts()
elems

torch_tensor
 0.0000
 1.5708
 3.1416
 4.7124
[ CPUFloatType{4} ]

Parts are represented by their respective rotation angles: (0), (frac{pi}{2}), (pi), and (frac{3 pi}{2}).

Teams are conscious of the id, and know tips on how to assemble a component’s inverse:

C_4$id

g1 <- elems[2]
C_4$inverse(g1)

torch_tensor
 0
[ CPUFloatType{1} ]

torch_tensor
4.71239
[ CPUFloatType{} ]

Right here, what we care about most is the group parts’ motion. Implementation-wise, we have to distinguish between them performing on one another, and their motion on the vector area (mathbb{R}^2), the place our enter photographs stay. The previous half is the simple one: It could merely be applied by including angles. The truth is, that is what gcnn does after we ask it to let g1 act on g2:

g2 <- elems[3]

# in C_4$left_action_on_H(), H stands for the symmetry group
C_4$left_action_on_H(torch_tensor(g1)$unsqueeze(1), torch_tensor(g2)$unsqueeze(1))

torch_tensor
 4.7124
[ CPUFloatType{1,1} ]

What’s with the unsqueeze()s? Since (C_4)’s final raison d’être is to be a part of a neural community, left_action_on_H() works with batches of parts, not scalar tensors.

Issues are a bit much less easy the place the group motion on (mathbb{R}^2) is worried. Right here, we’d like the idea of a group illustration. That is an concerned subject, which we gained’t go into right here. In our present context, it really works about like this: We now have an enter sign, a tensor we’d wish to function on in a roundabout way. (That “a way” will probably be convolution, as we’ll see quickly.) To render that operation group-equivariant, we first have the illustration apply the inverse group motion to the enter. That achieved, we go on with the operation as if nothing had occurred.

To present a concrete instance, let’s say the operation is a measurement. Think about a runner, standing on the foot of some mountain path, able to run up the climb. We’d wish to document their peak. One choice we’ve got is to take the measurement, then allow them to run up. Our measurement will probably be as legitimate up the mountain because it was down right here. Alternatively, we could be well mannered and never make them wait. As soon as they’re up there, we ask them to come back down, and once they’re again, we measure their peak. The consequence is similar: Physique peak is equivariant (greater than that: invariant, even) to the motion of working up or down. (In fact, peak is a reasonably uninteresting measure. However one thing extra fascinating, corresponding to coronary heart fee, wouldn’t have labored so effectively on this instance.)

Returning to the implementation, it seems that group actions are encoded as matrices. There’s one matrix for every group aspect. For (C_4), the so-called customary illustration is a rotation matrix:

[
begin{bmatrix} cos(theta) & -sin(theta) sin(theta) & cos(theta) end{bmatrix}
]

In gcnn, the operate making use of that matrix is left_action_on_R2(). Like its sibling, it’s designed to work with batches (of group parts in addition to (mathbb{R}^2) vectors). Technically, what it does is rotate the grid the picture is outlined on, after which, re-sample the picture. To make this extra concrete, that technique’s code appears about as follows.

Here’s a goat.

img_path <- system.file("imgs", "z.jpg", package deal = "gcnn")
img <- torchvision::base_loader(img_path) |> torchvision::transform_to_tensor()
img$permute(c(2, 3, 1)) |> as.array() |> as.raster() |> plot()

First, we name C_4$left_action_on_R2() to rotate the grid.

# Grid form is [2, 1024, 1024], for a second, 1024 x 1024 picture.
img_grid_R2 <- torch::torch_stack(torch::torch_meshgrid(
    record(
      torch::torch_linspace(-1, 1, dim(img)[2]),
      torch::torch_linspace(-1, 1, dim(img)[3])
    )
))

# Remodel the picture grid with the matrix illustration of some group aspect.
transformed_grid <- C_4$left_action_on_R2(C_4$inverse(g1)$unsqueeze(1), img_grid_R2)

Second, we re-sample the picture on the remodeled grid. The goat now appears as much as the sky.

transformed_img <- torch::nnf_grid_sample(
  img$unsqueeze(1), transformed_grid,
  align_corners = TRUE, mode = "bilinear", padding_mode = "zeros"
)

transformed_img[1,..]$permute(c(2, 3, 1)) |> as.array() |> as.raster() |> plot()

Step 2: The lifting convolution

We need to make use of present, environment friendly torch performance as a lot as doable. Concretely, we need to use nn_conv2d(). What we’d like, although, is a convolution kernel that’s equivariant not simply to translation, but additionally to the motion of (C_4). This may be achieved by having one kernel for every doable rotation.

Implementing that concept is precisely what LiftingConvolution does. The precept is similar as earlier than: First, the grid is rotated, after which, the kernel (weight matrix) is re-sampled to the remodeled grid.

Why, although, name this a lifting convolution? The same old convolution kernel operates on (mathbb{R}^2); whereas our prolonged model operates on combos of (mathbb{R}^2) and (C_4). In math communicate, it has been lifted to the semi-direct product (mathbb{R}^2rtimes C_4).

lifting_conv <- LiftingConvolution(
    group = CyclicGroup(order = 4),
    kernel_size = 5,
    in_channels = 3,
    out_channels = 8
  )

x <- torch::torch_randn(c(2, 3, 32, 32))
y <- lifting_conv(x)
y$form

[1]  2  8  4 28 28

Since, internally, LiftingConvolution makes use of a further dimension to comprehend the product of translations and rotations, the output is just not four-, however five-dimensional.

Step 3: Group convolutions

Now that we’re in “group-extended area”, we will chain a variety of layers the place each enter and output are group convolution layers. For instance:

group_conv <- GroupConvolution(
  group = CyclicGroup(order = 4),
    kernel_size = 5,
    in_channels = 8,
    out_channels = 16
)

z <- group_conv(y)
z$form

[1]  2 16  4 24 24

All that is still to be performed is package deal this up. That’s what gcnn::GroupEquivariantCNN() does.

Step 4: Group-equivariant CNN

We are able to name GroupEquivariantCNN() like so.

cnn <- GroupEquivariantCNN(
    group = CyclicGroup(order = 4),
    kernel_size = 5,
    in_channels = 1,
    out_channels = 1,
    num_hidden = 2, # variety of group convolutions
    hidden_channels = 16 # variety of channels per group conv layer
)

img <- torch::torch_randn(c(4, 1, 32, 32))
cnn(img)$form

[1] 4 1

At informal look, this GroupEquivariantCNN appears like every outdated CNN … weren’t it for the group argument.

Now, after we examine its output, we see that the extra dimension is gone. That’s as a result of after a sequence of group-to-group convolution layers, the module initiatives right down to a illustration that, for every batch merchandise, retains channels solely. It thus averages not simply over areas – as we usually do – however over the group dimension as effectively. A remaining linear layer will then present the requested classifier output (of dimension out_channels).

And there we’ve got the whole structure. It’s time for a real-world(ish) check.

Rotated digits!

The thought is to coach two convnets, a “regular” CNN and a group-equivariant one, on the standard MNIST coaching set. Then, each are evaluated on an augmented check set the place every picture is randomly rotated by a steady rotation between 0 and 360 levels. We don’t anticipate GroupEquivariantCNN to be “good” – not if we equip with (C_4) as a symmetry group. Strictly, with (C_4), equivariance extends over 4 positions solely. However we do hope it should carry out considerably higher than the shift-equivariant-only customary structure.

First, we put together the information; specifically, the augmented check set.

dir <- "/tmp/mnist"

train_ds <- torchvision::mnist_dataset(
  dir,
  obtain = TRUE,
  rework = torchvision::transform_to_tensor
)

test_ds <- torchvision::mnist_dataset(
  dir,
  prepare = FALSE,
  rework = operate(x) >
      torchvision::transform_to_tensor() 
)

train_dl <- dataloader(train_ds, batch_size = 128, shuffle = TRUE)
test_dl <- dataloader(test_ds, batch_size = 128)

How does it look?

test_images <- coro::gather(
  test_dl, 1
)[[1]]$x[1:32, 1, , ] |> as.array()

par(mfrow = c(4, 8), mar = rep(0, 4), mai = rep(0, 4))
test_images |>
  purrr::array_tree(1) |>
  purrr::map(as.raster) |>
  purrr::iwalk(~ {
    plot(.x)
  })

We first outline and prepare a standard CNN. It’s as much like GroupEquivariantCNN(), architecture-wise, as doable, and is given twice the variety of hidden channels, in order to have comparable capability total.

 default_cnn <- nn_module(
   "default_cnn",
   initialize = operate(kernel_size, in_channels, out_channels, num_hidden, hidden_channels) {
     self$conv1 <- torch::nn_conv2d(in_channels, hidden_channels, kernel_size)
     self$convs <- torch::nn_module_list()
     for (i in 1:num_hidden) {
       self$convs$append(torch::nn_conv2d(hidden_channels, hidden_channels, kernel_size))
     }
     self$avg_pool <- torch::nn_adaptive_avg_pool2d(1)
     self$final_linear <- torch::nn_linear(hidden_channels, out_channels)
   },
   ahead = operate(x) >
       self$conv1() 
 )

fitted <- default_cnn |>
    luz::setup(
      loss = torch::nn_cross_entropy_loss(),
      optimizer = torch::optim_adam,
      metrics = record(
        luz::luz_metric_accuracy()
      )
    ) |>
    luz::set_hparams(
      kernel_size = 5,
      in_channels = 1,
      out_channels = 10,
      num_hidden = 4,
      hidden_channels = 32
    ) %>%
    luz::set_opt_hparams(lr = 1e-2, weight_decay = 1e-4) |>
    luz::match(train_dl, epochs = 10, valid_data = test_dl) 

Prepare metrics: Loss: 0.0498 - Acc: 0.9843
Legitimate metrics: Loss: 3.2445 - Acc: 0.4479

Unsurprisingly, accuracy on the check set is just not that nice.

Subsequent, we prepare the group-equivariant model.

fitted <- GroupEquivariantCNN |>
  luz::setup(
    loss = torch::nn_cross_entropy_loss(),
    optimizer = torch::optim_adam,
    metrics = record(
      luz::luz_metric_accuracy()
    )
  ) |>
  luz::set_hparams(
    group = CyclicGroup(order = 4),
    kernel_size = 5,
    in_channels = 1,
    out_channels = 10,
    num_hidden = 4,
    hidden_channels = 16
  ) |>
  luz::set_opt_hparams(lr = 1e-2, weight_decay = 1e-4) |>
  luz::match(train_dl, epochs = 10, valid_data = test_dl)

Prepare metrics: Loss: 0.1102 - Acc: 0.9667
Legitimate metrics: Loss: 0.4969 - Acc: 0.8549

For the group-equivariant CNN, accuracies on check and coaching units are rather a lot nearer. That may be a good consequence! Let’s wrap up as we speak’s exploit resuming a thought from the primary, extra high-level put up.

A problem

Going again to the augmented check set, or reasonably, the samples of digits displayed, we discover an issue. In row two, column 4, there’s a digit that “beneath regular circumstances”, ought to be a 9, however, likely, is an upside-down 6. (To a human, what suggests that is the squiggle-like factor that appears to be discovered extra usually with sixes than with nines.) Nonetheless, you could possibly ask: does this have to be an issue? Perhaps the community simply must study the subtleties, the sorts of issues a human would spot?

The way in which I view it, all of it relies on the context: What actually ought to be achieved, and the way an utility goes for use. With digits on a letter, I’d see no cause why a single digit ought to seem upside-down; accordingly, full rotation equivariance can be counter-productive. In a nutshell, we arrive on the similar canonical crucial advocates of honest, simply machine studying preserve reminding us of:

All the time consider the best way an utility goes for use!

In our case, although, there’s one other facet to this, a technical one. gcnn::GroupEquivariantCNN() is a straightforward wrapper, in that its layers all make use of the identical symmetry group. In precept, there isn’t any want to do that. With extra coding effort, totally different teams can be utilized relying on a layer’s place within the feature-detection hierarchy.

Right here, let me lastly let you know why I selected the goat image. The goat is seen via a red-and-white fence, a sample – barely rotated, as a result of viewing angle – made up of squares (or edges, for those who like). Now, for such a fence, kinds of rotation equivariance corresponding to that encoded by (C_4) make a whole lot of sense. The goat itself, although, we’d reasonably not have look as much as the sky, the best way I illustrated (C_4) motion earlier than. Thus, what we’d do in a real-world image-classification activity is use reasonably versatile layers on the backside, and more and more restrained layers on the high of the hierarchy.

Thanks for studying!

Photograph by Marjan Blan | @marjanblan on Unsplash