by Subhaditya Mukherjee
Here we will talk about Generative Networks and implement a simple version of DC GAN.
GAN? -> Generative Adversarial Network. This came about as an attempt to have a neural network create something new from images instead of just using them to perform supervised tasks.
Why? -> Many reasons. Art being one of them. The second being the ability to have an almost infinte supply of data. Finally, this is one step closer to having an AI system that can build things beyond what it was taught. Beyond infact, what we can do as humans too.
The challenge? -> When you have too many examples of different things, it is very easy to “forget” what you wanted. Or end up with a mash up of things. This is called mode collapse and many attempts to do it have been made.
Before I go on to our simple GAN (on MNIST). Here are some results I got with the same GAN for different purposes.
This is trying to generate faces.
This is what we want to generate today (I know.. what even right?). Okay so I did cheat a bit, this animation was generated in Python a long time back when I first did DCGans there. But I get a bit of a waiver for this cool animation right??
So let us get to making it? Hey! You did not explain how it works. Well I am a boring person by nature. xD. I will explain by code. But I want theory!!! Go check out the Paper.
Let us first import whatever we need. We have covered all of these before so I will skip the explanations.
using Base.Iterators: partition using Flux using Flux.Optimise: update! using Flux: logitbinarycrossentropy using Images using MLDatasets using Statistics using Printf using Random using ImageView
So our aim is to take a random noise the size we need and generate a sensible image out of it. Everything that is possible in that is called it’s latent space.
We also need to define some constants. Batch size, the dimension of the space we are trying to recreate, Sizes we want. And different (or same) learning rates for the two parts of our network.
batch_size = 128 latent_dim = 300 verbose_freq = 1000 output_x = 6 output_y = 6 lr_dscr = 0.0002 lr_gen = 0.0002
Now we need a way to plot our function from
So our network is made up of two parts. The first is a vanilla classifier with a bit of drama added on. It tells us how good our created image is. We call this the “Discriminator” because.. because well that’s what it does.
If you notice, this is just a simple classifier. Nothing new here. Maybe except leaky relu. Why not relu? Because trial and error proved otherwise.(And a blog I linked towards the end of this article)
function Discriminator() return Chain( Conv((4, 4), 1 => 64; stride = 2, pad = 1), x->leakyrelu.(x, 0.2f0), Dropout(0.25), Conv((4, 4), 64 => 128; stride = 2, pad = 1), x->leakyrelu.(x, 0.2f0), Dropout(0.25), x->reshape(x, 7 * 7 * 128, :), Dense(7 * 7 * 128, 1)) end
Now for the main part. The main dude here. This would be a little weird to look at. So let us look at it first.
function Generator() return Chain( Dense(latent_dim, 7 * 7 * 256), BatchNorm(7 * 7 * 256, relu), x->reshape(x, 7, 7, 256, :), ConvTranspose((5, 5), 256 => 128; stride = 1, pad = 2), BatchNorm(128, relu), ConvTranspose((4, 4), 128 => 64; stride = 2, pad = 1), BatchNorm(64, relu), ConvTranspose((4, 4), 64 => 1, tanh; stride = 2, pad = 1), ) end
Okay so first. What is conv transpose? I didn’t know it myself so I looked it up. Great blog on it
Simply put, we take a smol image and try to reconstruct it to a bigger one. Why? Consider it to be an “up” convolution instead of a “down” convolution where the size reduces after the operation.
Okay but how? Consider our task of taking a matrix and a kernel. We pass the image through im2col to get our things together. After that we can do a matrix multiply if needed.
data = [1 2 3; 6 5 3; 1 4 1] col_data = im2col(tmp_cm, (3, 3)) kernel = [1 2 0 2 1 0 0 0 0; 0 1 2 0 2 1 0 0 0; 0 0 0 1 2 0 2 1 0; 0 0 0 0 1 2 0 2 1] new_data = col2im(kernel*col_data, (2,2))
Now what if we take the output of this and want to get the original back? To do that we basically pad the image. And then take the transpose of the kernel. And boom. This is something like a de - convolution. Except that is a misnomer.
Now armed with that knowlege, I would advice you to go back and look at our Generator function again.
Oh and before I forget. Notice the last Conv transpose’s activation function? Yes. It’s tanh. :) It seems to do better than relu for this layer due to some weight changes.
Discriminator -> We have a loss function which takes the loss for real and fake images and sums them up.
function discriminator_loss(real_output, fake_output) real_loss = mean(logitbinarycrossentropy.(real_output, 1f0)) fake_loss = mean(logitbinarycrossentropy.(fake_output, 0f0)) return real_loss + fake_loss end
Generator -> Simple loss function as above
generator_loss(fake_output) = mean(logitbinarycrossentropy.(fake_output, 1f0))
Now how about training?
Discriminator -> We first generate random noise. That will be our fake input. Then we can do a forward and backward pass on it.
function train_discriminator!(gen, dscr, x, opt_dscr) noise = randn!(similar(x, (latent_dim, batch_size))) fake_input = gen(noise) ps = Flux.params(dscr) # Taking gradient loss, back = Flux.pullback(ps) do discriminator_loss(dscr(x), dscr(fake_input)) end grad = back(1f0) update!(opt_dscr, ps, grad) return loss end
Generator -> For this we do almost the same exluding the loss function.
function train_generator!(gen, dscr, x, opt_gen) noise = randn!(similar(x, (latent_dim, batch_size))) ps = Flux.params(gen) # Taking gradient loss, back = Flux.pullback(ps) do generator_loss(dscr(gen(noise))) end grad = back(1f0) update!(opt_gen, ps, grad) return loss end
Then we can load the data.
images, _ = MLDatasets.MNIST.traindata(Float32) # Normalize to [-1, 1] image_tensor = reshape(@.(2f0 * images - 1f0), 28, 28, 1, :) # Partition into batches data = [image_tensor[:, :, :, r] |> gpu for r in partition(1:60000, batch_size)] fixed_noise = [randn(latent_dim, 1) |> gpu for _=1:output_x*output_y]
Let us train our network.
for ep in 1:epochs global train_steps @info "Epoch $ep" for x in data # Update discriminator and generator loss_dscr = train_discriminator!(gen, dscr, x, opt_dscr) loss_gen = train_generator!(gen, dscr, x, opt_gen) if train_steps % verbose_freq == 0 @info("Train step $(train_steps), Discriminator loss = $(loss_dscr), Generator loss = $(loss_gen)") # Save generated fake image # output_image = create_output_image(gen, fixed_noise) # save(@sprintf("output/dcgan_steps_%06d.png", train_steps), output_image) end train_steps += 1 end end
Now that we have everything we need, we need to grab our image from this hot mess. Do do that, we first set the network as not training. Then take the current fixed noise, pass it through the generator and pop them into the CPU. After that we set the mode to eval (aka generation) and convert this into an image array we can use.
function create_output_image(gen, fixed_noise) @eval Flux.istraining() = false fake_images = @. cpu(gen(fixed_noise)) @eval Flux.istraining() = true image_array = permutedims(dropdims(reduce(vcat, reduce.(hcat, partition(fake_images, output_y))); dims=(3, 4)), (2, 1)) image_array = @. Gray(image_array + 1f0) / 2f0 return image_array end output_image = create_output_image(gen, fixed_noise) imshow(output_image)
Aand… we are done :)
I actually wrote a blog. And even gave a webinar on this topic. So if you want to check them out..