Torchvision, Conversions, Samplers

On conversions and transforms
Author

Nenad Bozinovic

Published

November 3, 2022

Here we’ll take a problem of binary image classifing.

Code 
from IPython.display import display, HTML
display(HTML("<style>.container { width:80% !important; }</style>"))

import numpy as np
import torch
import torch.optim as optim
import torch.nn as nn
from shared.step_by_step import StepByStep
import platform
from PIL import Image
import datetime
import matplotlib.pyplot as plt
from matplotlib import cm
from torch.utils.data import DataLoader, Dataset, random_split, WeightedRandomSampler, SubsetRandomSampler
from torchvision.transforms import Compose, ToTensor, Normalize, ToPILImage, RandomHorizontalFlip, Resize

plt.style.use('fivethirtyeight')
Code 
def show_image(im, cmap=None):
    fig = plt.figure(figsize=(3,3))
    plt.imshow(im, cmap=cmap)
    plt.grid(False)
    plt.show()

def gen_img(start, target, fill=1, img_size=10):
    # Generates empty image
    img = np.zeros((img_size, img_size), dtype=float)

    start_row, start_col = None, None

    if start > 0:
        start_row = start
    else:
        start_col = np.abs(start)

    if target == 0:
        if start_row is None:
            img[:, start_col] = fill
        else:
            img[start_row, :] = fill
    else:
        if start_col == 0:
            start_col = 1
        
        if target == 1:
            if start_row is not None:
                up = (range(start_row, -1, -1), 
                      range(0, start_row + 1))
            else:
                up = (range(img_size - 1, start_col - 1, -1), 
                      range(start_col, img_size))
            img[up] = fill
        else:
            if start_row is not None:
                down = (range(start_row, img_size, 1), 
                        range(0, img_size - start_row))
            else:
                down = (range(0, img_size - 1 - start_col + 1), 
                        range(start_col, img_size))
            img[down] = fill
    
    return 255 * img.reshape(1, img_size, img_size)


def generate_dataset(img_size=10, n_images=100, binary=True, seed=17):
    np.random.seed(seed)

    starts = np.random.randint(-(img_size - 1), img_size, size=(n_images,))
    targets = np.random.randint(0, 3, size=(n_images,))
    
    images = np.array([gen_img(s, t, img_size=img_size) 
                       for s, t in zip(starts, targets)], dtype=np.uint8)
    
    if binary:
        targets = (targets > 0).astype(int)
    
    return images, targets

def plot_images(images, targets, n_plot=30):
    n_rows = n_plot // 6 + ((n_plot % 6) > 0)
    fig, axes = plt.subplots(n_rows, 6, figsize=(9, 1.5 * n_rows))
    axes = np.atleast_2d(axes)
    
    for i, (image, target) in enumerate(zip(images[:n_plot], targets[:n_plot])):
        row, col = i // 6, i % 6    
        ax = axes[row, col]
        ax.set_title('#{} - Label:{}'.format(i, target), {'size': 12})
        # plot filter channel in grayscale
        ax.imshow(image.squeeze(), cmap='gray', vmin=0, vmax=1)

    for ax in axes.flat:
        ax.set_xticks([])
        ax.set_yticks([])
        ax.label_outer()

    plt.tight_layout()
    return fig

class TransformedTensorDataset(Dataset):
    def __init__(self, x, y, transform=None):
        self.x = x
        self.y = y
        self.transform = transform
        
    def __getitem__(self, index):
        x = self.x[index]
        
        if self.transform:
            x = self.transform(x)
        
        return x, self.y[index]
        
    def __len__(self):
        return len(self.x)
Code 
images, labels = generate_dataset(img_size=5, n_images=300, binary=True, seed=13)
x_tensor = torch.as_tensor(images / 255.).float()
y_tensor = torch.as_tensor(labels.reshape(-1, 1)).float()  # reshaped this to (N,1) tensor
torch.manual_seed(13)
N = len(x_tensor)
n_train = int(.8*N)
n_val = N - n_train
train_subset, val_subset = random_split(x_tensor, [n_train, n_val])
train_idx = train_subset.indices
val_idx = val_subset.indices

#|code-fold: true
#|output: false
lr = 0.1

# Now we can create a model
model_deeper = nn.Sequential()
model_deeper.add_module('flatten', nn.Flatten())
model_deeper.add_module('linear1', nn.Linear(25, 10, bias=True))
model_deeper.add_module('relu', nn.ReLU())
model_deeper.add_module('linear2', nn.Linear(10, 1, bias=True))
model_deeper.add_module('sigmoid', nn.Sigmoid())

# Defines a SGD optimizer to update the parameters 
optimizer_deeper = optim.SGD(model_deeper.parameters(), lr=lr)

# Defines a binary cross entropy loss function
binary_loss_fn = nn.BCELoss()
sbs_deeper = StepByStep(model_deeper, optimizer_deeper, binary_loss_fn)

Note on NCHW vs NHWC formats

The format for images is different between libraries: - PyTorch uses NCHW - TensorFlow uses NHWC - PIL/Matplotlib images are HWC.

image_r  = np.zeros((5, 5), dtype=np.uint8)
image_r[:, 0] = 255
image_r[:, 1] = 128

image_g = np.zeros((5, 5), dtype=np.uint8)
image_g[:, 1] = 128
image_g[:, 2] = 255
image_g[:, 3] = 128

image_b = np.zeros((5, 5), dtype=np.uint8)
image_b[:, 3] = 128
image_b[:, 4] = 255
Code 
show_image(image_r, 'gray')
zeros = np.zeros((5,5), dtype=np.uint8)
stacked_red = np.zeros((5,5,3), dtype=np.uint8)  # the format is HxWxX (height X width X channel)
stacked_red[:,:,0] = image_r  
# also same thing can be achieved stacked_red = np.stack([image_r, zeros, zeros], axis=2)
show_image(stacked_red)
Code 
def image_channels(red, green, blue, rgb, gray, rows=(0, 1, 2)):
    fig, axs = plt.subplots(len(rows), 4, figsize=(15, 5.5))

    zeros = np.zeros((5, 5), dtype=np.uint8)

    titles1 = ['Red', 'Green', 'Blue', 'Grayscale Image']
    titles0 = ['image_r', 'image_g', 'image_b', 'image_gray']
    titles2 = ['as first channel', 'as second channel', 'as third channel', 'RGB Image']

    idx0 = np.argmax(np.array(rows) == 0)
    idx1 = np.argmax(np.array(rows) == 1)
    idx2 = np.argmax(np.array(rows) == 2)
    
    for i, m in enumerate([red, green, blue, gray]):
        if 0 in rows:
            axs[idx0, i].axis('off')
            axs[idx0, i].invert_yaxis()
            if (1 in rows) or (i < 3):
                axs[idx0, i].text(0.15, 0.25, str(m.astype(np.uint8)), verticalalignment='top')    
                axs[idx0, i].set_title(titles0[i], fontsize=16)

        if 1 in rows:
            axs[idx1, i].set_title(titles1[i], fontsize=16)
            axs[idx1, i].set_xlabel('5x5', fontsize=14)
            axs[idx1, i].imshow(m, cmap=plt.cm.gray)

        if 2 in rows:
            axs[idx2, i].set_title(titles2[i], fontsize=16)
            axs[idx2, i].set_xlabel(f'5x5x3 - {titles1[i][0]} only', fontsize=14)
            if i < 3:
                stacked = [zeros] * 3
                stacked[i] = m
                axs[idx2, i].imshow(np.stack(stacked, axis=2))
            else:
                axs[idx2, i].imshow(rgb)

        for r in [1, 2]:
            if r in rows:
                idx = idx1 if r == 1 else idx2
                axs[idx, i].set_xticks([])
                axs[idx, i].set_yticks([])
                for k, v in axs[idx, i].spines.items():
                    v.set_color('black')
                    v.set_linewidth(.8)

    if 1 in rows:
        axs[idx1, 0].set_ylabel('Single\nChannel\n(grayscale)', rotation=0, labelpad=40, fontsize=12)
        axs[idx1, 3].set_xlabel('5x5 = 0.21R + 0.72G + 0.07B')
    if 2 in rows:
        axs[idx2, 0].set_ylabel('Three\nChannels\n(color)', rotation=0, labelpad=40, fontsize=12)
        axs[idx2, 3].set_xlabel('5x5x3 = (R, G, B) stacked')
    fig.tight_layout()
    return fig
image_gray = .2126*image_r + .7152*image_g + .0722*image_b
image_rgb = np.stack([image_r, image_g, image_b], axis=2)
fig = image_channels(image_r, image_g, image_b, image_rgb, image_gray, rows=(0, 1))

fig = image_channels(image_r, image_g, image_b, image_rgb, image_gray, rows=(0, 2))

Note on Torchvision

torchvision has many existing Datasets, Models, and Transformations.

Conversion between ndarray, PIL, and tensor

images.shape
(300, 1, 5, 5)
example_chw = images[3]
example_chw
array([[[  0,   0, 255,   0,   0],
        [  0, 255,   0,   0,   0],
        [255,   0,   0,   0,   0],
        [  0,   0,   0,   0,   0],
        [  0,   0,   0,   0,   0]]], dtype=uint8)
example_chw.shape
(1, 5, 5)
example_hwc = np.transpose(example_chw, (1,2,0))
example_hwc.shape
(5, 5, 1)
show_image(example_hwc, 'gray')

ToTensor: Converts a PIL Image or numpy.ndarray (H x W x C) in the range [0, 255] to a torch.FloatTensor of shape (C x H x W) in the range [0.0, 1.0] if the PIL Image belongs to one of the modes (L, LA, P, I, F, RGB, YCbCr, RGBA, CMYK, 1) or if the numpy.ndarray has dtype = np.uint8

torch.as_tensor can work on N-dimensional arrays (ToTensor works only on 2D/3D images), but doesn’t apply scalling and, just fyi, it shares data i.e. doesn’t make a copy.

tensorizer = ToTensor()
example_tensor = tensorizer(example_hwc)
example_tensor.shape
torch.Size([1, 5, 5])
example_tensor
tensor([[[0., 0., 1., 0., 0.],
         [0., 1., 0., 0., 0.],
         [1., 0., 0., 0., 0.],
         [0., 0., 0., 0., 0.],
         [0., 0., 0., 0., 0.]]])
type(example_tensor)
torch.Tensor
example_pil = ToPILImage()(example_tensor)  # similar to np.transpose(example_tensor, (1,2,0))
print(type(example_pil))
<class 'PIL.Image.Image'>
show_image(example_pil, 'gray')

To convert between Tensor and Numpy:

example_np = example_tensor.detach().cpu().numpy()
print(type(example_np))
<class 'numpy.ndarray'>

To convert between numpy and PIL:

example_np = np.array(example_pil)
print(type(example_np))
<class 'numpy.ndarray'>
example_pil_back = Image.fromarray(example_np)
print(type(example_pil_back))
<class 'PIL.Image.Image'>

more options for different PIL modes: (L, LA, P, I, F, RGB, YCbCr, RGBA, CMYK, 1).

One has to be careful: - PIL gray images (L mode) need to be squeezed from 3D to 2D via np.squeeze() - RGB mode images should be floats between [0,1] - L mode (grayscale) images should be np.uint8 from [0,255] - 1 mode (bool) images are True or False.

# PIL_image = Image.fromarray(np.uint8(example_np)).convert('RGB')
# PIL_image = Image.fromarray(example_np.astype('uint8'), 'RGB')
show_image(example_pil_back, 'gray')

Transformations

There are many transformations that can be done on tensors and/or PIL images:

flipper = RandomHorizontalFlip(p=1.0)
show_image(flipper(example_pil), 'gray')

print(example_tensor)
normalizer = Normalize(mean=.5, std=.5)
normalized_tensor = normalizer(example_tensor)
print(normalized_tensor)
tensor([[[0., 0., 1., 0., 0.],
         [0., 1., 0., 0., 0.],
         [1., 0., 0., 0., 0.],
         [0., 0., 0., 0., 0.],
         [0., 0., 0., 0., 0.]]])
tensor([[[-1., -1.,  1., -1., -1.],
         [-1.,  1., -1., -1., -1.],
         [ 1., -1., -1., -1., -1.],
         [-1., -1., -1., -1., -1.],
         [-1., -1., -1., -1., -1.]]])

Composing transforms

composer = Compose([RandomHorizontalFlip(p=1.0),
                    Normalize(mean=(.5,), std=(.5,))])
composer(example_tensor)
tensor([[[-1., -1.,  1., -1., -1.],
         [-1., -1., -1.,  1., -1.],
         [-1., -1., -1., -1.,  1.],
         [-1., -1., -1., -1., -1.],
         [-1., -1., -1., -1., -1.]]])
torch.as_tensor(example_np/255).float()
tensor([[0., 0., 1., 0., 0.],
        [0., 1., 0., 0., 0.],
        [1., 0., 0., 0., 0.],
        [0., 0., 0., 0., 0.],
        [0., 0., 0., 0., 0.]])

If we want to convert the whole numpy array to tensor we can use:

example_tensor = torch.as_tensor(example_np / 255).float()
example_tensor
tensor([[0., 0., 1., 0., 0.],
        [0., 1., 0., 0., 0.],
        [1., 0., 0., 0., 0.],
        [0., 0., 0., 0., 0.],
        [0., 0., 0., 0., 0.]])

PyTorch default floating point dtype is torch.float32, one can change this if needed via torch.set_default_dtype(torch.float64).

Samplers

One can also define a Sampler (and it’s subclasses: SequentialSampler, RandomSampler, SubsetRandomSampler, WeightedRandomSampler, BatchSampler, and DistributedSampler), use WeightedRandomSampler for example in case data is unbalanced.

composer = Compose([RandomHorizontalFlip(p=0.5),
                    Normalize(mean=.5, std=.5)])

dataset = TransformedTensorDataset(x_tensor, y_tensor, transform=composer)

train_sampler = SubsetRandomSampler(train_idx)
val_sampler = SubsetRandomSampler(val_idx)
# Builds a loader of each set
train_loader = DataLoader(dataset=dataset, batch_size=16, sampler=train_sampler)
val_loader = DataLoader(dataset=dataset, batch_size=16, sampler=val_sampler)

Note that we need val_sampler just because we are passing the full dataset.

Also note that when using Samplers, one can’t use shuffle=True in DataLoaders.

len(iter(train_loader)), len(iter(val_loader))
(15, 4)

Some Model methods

To see the layers:

sbs_deeper.model
Sequential(
  (flatten): Flatten(start_dim=1, end_dim=-1)
  (linear1): Linear(in_features=25, out_features=10, bias=True)
  (relu): ReLU()
  (linear2): Linear(in_features=10, out_features=1, bias=True)
  (sigmoid): Sigmoid()
)
list(sbs_deeper.model.named_modules())
[('',
  Sequential(
    (flatten): Flatten(start_dim=1, end_dim=-1)
    (linear1): Linear(in_features=25, out_features=10, bias=True)
    (relu): ReLU()
    (linear2): Linear(in_features=10, out_features=1, bias=True)
    (sigmoid): Sigmoid()
  )),
 ('flatten', Flatten(start_dim=1, end_dim=-1)),
 ('linear1', Linear(in_features=25, out_features=10, bias=True)),
 ('relu', ReLU()),
 ('linear2', Linear(in_features=10, out_features=1, bias=True)),
 ('sigmoid', Sigmoid())]

To see particular weights in a layer:

sbs_deeper.model.linear2.weight.detach()
tensor([[ 0.1024,  0.2038,  0.2085, -0.2298,  0.1973, -0.2550,  0.2233, -0.0626,
         -0.0030,  0.0018]])
sbs_deeper.count_parameters()
271

The 271 comes from: (25 * 10 + 10 biases) + (10 * 1 + 1 bias) = 260 + 11 = 271

to reset parameters I implemented this way but this doesn’t reset all parameters (for example in nn.PReLU) so use caution!

sbs_deeper.reset_parameters()