03. Convolutional Neural Networks and Computer Vision with TensorFlow¶
So far we've covered the basics of TensorFlow and built a handful of models to work across different problems.
Now we're going to get specific and see how a special kind of neural network, convolutional neural networks (CNNs) can be used for computer vision (detecting patterns in visual data).
🔑 Note: In deep learning, many different kinds of model architectures can be used for different problems. For example, you could use a convolutional neural network for making predictions on image data and/or text data. However, in practice some architectures typically work better than others.
For example, you might want to:
- Classify whether a picture of food contains pizza 🍕 or steak 🥩 (we're going to do this)
- Detect whether or not an object appears in an image (e.g. did a specific car pass through a security camera?)
In this notebook, we're going to follow the TensorFlow modelling workflow we've been following so far whilst learning about how to build and use CNNs.
What we're going to cover¶
Specifically, we're going to go through the follow with TensorFlow:
- Getting a dataset to work with
- Architecture of a convolutional neural network
- A quick end-to-end example (what we're working towards)
- Steps in modelling for binary image classification with CNNs
- Becoming one with the data
- Preparing data for modelling
- Creating a CNN model (starting with a baseline)
- Fitting a model (getting it to find patterns in our data)
- Evaluating a model
- Improving a model
- Making a prediction with a trained model
- Steps in modelling for multi-class image classification with CNNs
- Same as above (but this time with a different dataset)
How you can use this notebook¶
You can read through the descriptions and the code (it should all run, except for the cells which error on purpose), but there's a better option.
Write all of the code yourself.
Yes. I'm serious. Create a new notebook, and rewrite each line by yourself. Investigate it, see if you can break it, why does it break?
You don't have to write the text descriptions but writing the code yourself is a great way to get hands-on experience.
Don't worry if you make mistakes, we all do. The way to get better and make less mistakes is to write more code.
Get the data¶
Because convolutional neural networks work so well with images, to learn more about them, we're going to start with a dataset of images.
The images we're going to work with are from the Food-101 dataset, a collection of 101 different categories of 101,000 (1000 images per category) real-world images of food dishes.
To begin, we're only going to use two of the categories, pizza 🍕 and steak 🥩 and build a binary classifier.
🔑 Note: To prepare the data we're using, preprocessing steps such as, moving the images into different subset folders, have been done. To see these preprocessing steps check out the preprocessing notebook.
We'll download the pizza_steak subset .zip file and unzip it.
import zipfile
# Download zip file of pizza_steak images
!wget https://storage.googleapis.com/ztm_tf_course/food_vision/pizza_steak.zip
# Unzip the downloaded file
zip_ref = zipfile.ZipFile("pizza_steak.zip", "r")
zip_ref.extractall()
zip_ref.close()
--2021-12-06 05:16:49-- https://storage.googleapis.com/ztm_tf_course/food_vision/pizza_steak.zip Resolving storage.googleapis.com (storage.googleapis.com)... 172.217.214.128, 172.253.114.128, 108.177.111.128, ... Connecting to storage.googleapis.com (storage.googleapis.com)|172.217.214.128|:443... connected. HTTP request sent, awaiting response... 200 OK Length: 109540975 (104M) [application/zip] Saving to: ‘pizza_steak.zip’ pizza_steak.zip 100%[===================>] 104.47M 196MB/s in 0.5s 2021-12-06 05:16:50 (196 MB/s) - ‘pizza_steak.zip’ saved [109540975/109540975]
🔑 Note: If you're using Google Colab and your runtime disconnects, you may have to redownload the files. You can do this by rerunning the cell above.
Inspect the data (become one with it)¶
A very crucial step at the beginning of any machine learning project is becoming one with the data. This usually means plenty of visualizing and folder scanning to understand the data you're working with.
Wtih this being said, let's inspect the data we just downloaded.
The file structure has been formatted to be in a typical format you might use for working with images.
More specifically:
- A
traindirectory which contains all of the images in the training dataset with subdirectories each named after a certain class containing images of that class. - A
testdirectory with the same structure as thetraindirectory.
Example of file structure
pizza_steak <- top level folder
└───train <- training images
│ └───pizza
│ │ │ 1008104.jpg
│ │ │ 1638227.jpg
│ │ │ ...
│ └───steak
│ │ 1000205.jpg
│ │ 1647351.jpg
│ │ ...
│
└───test <- testing images
│ └───pizza
│ │ │ 1001116.jpg
│ │ │ 1507019.jpg
│ │ │ ...
│ └───steak
│ │ 100274.jpg
│ │ 1653815.jpg
│ │ ...Let's inspect each of the directories we've downloaded.
To so do, we can use the command ls which stands for list.
!ls pizza_steak
test train
We can see we've got a train and test folder.
Let's see what's inside one of them.
!ls pizza_steak/train/
pizza steak
And how about insde the steak directory?
!ls pizza_steak/train/steak/
1000205.jpg 1647351.jpg 2238681.jpg 2824680.jpg 3375959.jpg 417368.jpg 100135.jpg 1650002.jpg 2238802.jpg 2825100.jpg 3381560.jpg 4176.jpg 101312.jpg 165639.jpg 2254705.jpg 2826987.jpg 3382936.jpg 42125.jpg 1021458.jpg 1658186.jpg 225990.jpg 2832499.jpg 3386119.jpg 421476.jpg 1032846.jpg 1658443.jpg 2260231.jpg 2832960.jpg 3388717.jpg 421561.jpg 10380.jpg 165964.jpg 2268692.jpg 285045.jpg 3389138.jpg 438871.jpg 1049459.jpg 167069.jpg 2271133.jpg 285147.jpg 3393547.jpg 43924.jpg 1053665.jpg 1675632.jpg 227576.jpg 2855315.jpg 3393688.jpg 440188.jpg 1068516.jpg 1678108.jpg 2283057.jpg 2856066.jpg 3396589.jpg 442757.jpg 1068975.jpg 168006.jpg 2286639.jpg 2859933.jpg 339891.jpg 443210.jpg 1081258.jpg 1682496.jpg 2287136.jpg 286219.jpg 3417789.jpg 444064.jpg 1090122.jpg 1684438.jpg 2291292.jpg 2862562.jpg 3425047.jpg 444709.jpg 1093966.jpg 168775.jpg 229323.jpg 2865730.jpg 3434983.jpg 447557.jpg 1098844.jpg 1697339.jpg 2300534.jpg 2878151.jpg 3435358.jpg 461187.jpg 1100074.jpg 1710569.jpg 2300845.jpg 2880035.jpg 3438319.jpg 461689.jpg 1105280.jpg 1714605.jpg 231296.jpg 2881783.jpg 3444407.jpg 465494.jpg 1117936.jpg 1724387.jpg 2315295.jpg 2884233.jpg 345734.jpg 468384.jpg 1126126.jpg 1724717.jpg 2323132.jpg 2890573.jpg 3460673.jpg 477486.jpg 114601.jpg 172936.jpg 2324994.jpg 2893832.jpg 3465327.jpg 482022.jpg 1147047.jpg 1736543.jpg 2327701.jpg 2893892.jpg 3466159.jpg 482465.jpg 1147883.jpg 1736968.jpg 2331076.jpg 2907177.jpg 3469024.jpg 483788.jpg 1155665.jpg 1746626.jpg 233964.jpg 290850.jpg 3470083.jpg 493029.jpg 1163977.jpg 1752330.jpg 2344227.jpg 2909031.jpg 3476564.jpg 503589.jpg 1190233.jpg 1761285.jpg 234626.jpg 2910418.jpg 3478318.jpg 510757.jpg 1208405.jpg 176508.jpg 234704.jpg 2912290.jpg 3488748.jpg 513129.jpg 1209120.jpg 1772039.jpg 2357281.jpg 2916448.jpg 3492328.jpg 513842.jpg 1212161.jpg 1777107.jpg 2361812.jpg 2916967.jpg 3518960.jpg 523535.jpg 1213988.jpg 1787505.jpg 2365287.jpg 2927833.jpg 3522209.jpg 525041.jpg 1219039.jpg 179293.jpg 2374582.jpg 2928643.jpg 3524429.jpg 534560.jpg 1225762.jpg 1816235.jpg 239025.jpg 2929179.jpg 3528458.jpg 534633.jpg 1230968.jpg 1822407.jpg 2390628.jpg 2936477.jpg 3531805.jpg 536535.jpg 1236155.jpg 1823263.jpg 2392910.jpg 2938012.jpg 3536023.jpg 541410.jpg 1241193.jpg 1826066.jpg 2394465.jpg 2938151.jpg 3538682.jpg 543691.jpg 1248337.jpg 1828502.jpg 2395127.jpg 2939678.jpg 3540750.jpg 560503.jpg 1257104.jpg 1828969.jpg 2396291.jpg 2940544.jpg 354329.jpg 561972.jpg 126345.jpg 1829045.jpg 2400975.jpg 2940621.jpg 3547166.jpg 56240.jpg 1264050.jpg 1829088.jpg 2403776.jpg 2949079.jpg 3553911.jpg 56409.jpg 1264154.jpg 1836332.jpg 2403907.jpg 295491.jpg 3556871.jpg 564530.jpg 1264858.jpg 1839025.jpg 240435.jpg 296268.jpg 355715.jpg 568972.jpg 127029.jpg 1839481.jpg 2404695.jpg 2964732.jpg 356234.jpg 576725.jpg 1289900.jpg 183995.jpg 2404884.jpg 2965021.jpg 3571963.jpg 588739.jpg 1290362.jpg 184110.jpg 2407770.jpg 2966859.jpg 3576078.jpg 590142.jpg 1295457.jpg 184226.jpg 2412263.jpg 2977966.jpg 3577618.jpg 60633.jpg 1312841.jpg 1846706.jpg 2425062.jpg 2979061.jpg 3577732.jpg 60655.jpg 1313316.jpg 1849364.jpg 2425389.jpg 2983260.jpg 3578934.jpg 606820.jpg 1324791.jpg 1849463.jpg 2435316.jpg 2984311.jpg 358042.jpg 612551.jpg 1327567.jpg 1849542.jpg 2437268.jpg 2988960.jpg 358045.jpg 614975.jpg 1327667.jpg 1853564.jpg 2437843.jpg 2989882.jpg 3591821.jpg 616809.jpg 1333055.jpg 1869467.jpg 2440131.jpg 2995169.jpg 359330.jpg 628628.jpg 1334054.jpg 1870942.jpg 2443168.jpg 2996324.jpg 3601483.jpg 632427.jpg 1335556.jpg 187303.jpg 2446660.jpg 3000131.jpg 3606642.jpg 636594.jpg 1337814.jpg 187521.jpg 2455944.jpg 3002350.jpg 3609394.jpg 637374.jpg 1340977.jpg 1888450.jpg 2458401.jpg 3007772.jpg 361067.jpg 640539.jpg 1343209.jpg 1889336.jpg 2487306.jpg 3008192.jpg 3613455.jpg 644777.jpg 134369.jpg 1907039.jpg 248841.jpg 3009617.jpg 3621464.jpg 644867.jpg 1344105.jpg 1925230.jpg 2489716.jpg 3011642.jpg 3621562.jpg 658189.jpg 134598.jpg 1927984.jpg 2490489.jpg 3020591.jpg 3621565.jpg 660900.jpg 1346387.jpg 1930577.jpg 2495884.jpg 3030578.jpg 3623556.jpg 663014.jpg 1348047.jpg 1937872.jpg 2495903.jpg 3047807.jpg 3640915.jpg 664545.jpg 1351372.jpg 1941807.jpg 2499364.jpg 3059843.jpg 3643951.jpg 667075.jpg 1362989.jpg 1942333.jpg 2500292.jpg 3074367.jpg 3653129.jpg 669180.jpg 1367035.jpg 1945132.jpg 2509017.jpg 3082120.jpg 3656752.jpg 669960.jpg 1371177.jpg 1961025.jpg 250978.jpg 3094354.jpg 3663518.jpg 6709.jpg 1375640.jpg 1966300.jpg 2514432.jpg 3095301.jpg 3663800.jpg 674001.jpg 1382427.jpg 1966967.jpg 2526838.jpg 3099645.jpg 3664376.jpg 676189.jpg 1392718.jpg 1969596.jpg 252858.jpg 3100476.jpg 3670607.jpg 681609.jpg 1395906.jpg 1971757.jpg 2532239.jpg 3110387.jpg 3671021.jpg 6926.jpg 1400760.jpg 1976160.jpg 2534567.jpg 3113772.jpg 3671877.jpg 703556.jpg 1403005.jpg 1984271.jpg 2535431.jpg 3116018.jpg 368073.jpg 703909.jpg 1404770.jpg 1987213.jpg 2535456.jpg 3128952.jpg 368162.jpg 704316.jpg 140832.jpg 1987639.jpg 2538000.jpg 3130412.jpg 368170.jpg 714298.jpg 141056.jpg 1995118.jpg 2543081.jpg 3136.jpg 3693649.jpg 720060.jpg 141135.jpg 1995252.jpg 2544643.jpg 313851.jpg 3700079.jpg 726083.jpg 1413972.jpg 199754.jpg 2547797.jpg 3140083.jpg 3704103.jpg 728020.jpg 1421393.jpg 2002400.jpg 2548974.jpg 3140147.jpg 3707493.jpg 732986.jpg 1428947.jpg 2011264.jpg 2549316.jpg 3142045.jpg 3716881.jpg 734445.jpg 1433912.jpg 2012996.jpg 2561199.jpg 3142618.jpg 3724677.jpg 735441.jpg 143490.jpg 2013535.jpg 2563233.jpg 3142674.jpg 3727036.jpg 740090.jpg 1445352.jpg 2017387.jpg 256592.jpg 3143192.jpg 3727491.jpg 745189.jpg 1446401.jpg 2018173.jpg 2568848.jpg 314359.jpg 3736065.jpg 752203.jpg 1453991.jpg 2020613.jpg 2573392.jpg 3157832.jpg 37384.jpg 75537.jpg 1456841.jpg 2032669.jpg 2592401.jpg 3159818.jpg 3743286.jpg 756655.jpg 146833.jpg 203450.jpg 2599817.jpg 3162376.jpg 3745515.jpg 762210.jpg 1476404.jpg 2034628.jpg 2603058.jpg 3168620.jpg 3750472.jpg 763690.jpg 1485083.jpg 2036920.jpg 2606444.jpg 3171085.jpg 3752362.jpg 767442.jpg 1487113.jpg 2038418.jpg 2614189.jpg 317206.jpg 3766099.jpg 786409.jpg 148916.jpg 2042975.jpg 2614649.jpg 3173444.jpg 3770370.jpg 80215.jpg 149087.jpg 2045647.jpg 2615718.jpg 3180182.jpg 377190.jpg 802348.jpg 1493169.jpg 2050584.jpg 2619625.jpg 31881.jpg 3777020.jpg 804684.jpg 149682.jpg 2052542.jpg 2622140.jpg 3191589.jpg 3777482.jpg 812163.jpg 1508094.jpg 2056627.jpg 262321.jpg 3204977.jpg 3781152.jpg 813486.jpg 1512226.jpg 2062248.jpg 2625330.jpg 320658.jpg 3787809.jpg 819027.jpg 1512347.jpg 2081995.jpg 2628106.jpg 3209173.jpg 3788729.jpg 822550.jpg 1524526.jpg 2087958.jpg 2629750.jpg 3223400.jpg 3790962.jpg 823766.jpg 1530833.jpg 2088030.jpg 2643906.jpg 3223601.jpg 3792514.jpg 827764.jpg 1539499.jpg 2088195.jpg 2644457.jpg 3241894.jpg 379737.jpg 830007.jpg 1541672.jpg 2090493.jpg 2648423.jpg 3245533.jpg 3807440.jpg 838344.jpg 1548239.jpg 2090504.jpg 2651300.jpg 3245622.jpg 381162.jpg 853327.jpg 1550997.jpg 2125877.jpg 2653594.jpg 3247009.jpg 3812039.jpg 854150.jpg 1552530.jpg 2129685.jpg 2661577.jpg 3253588.jpg 3829392.jpg 864997.jpg 15580.jpg 2133717.jpg 2668916.jpg 3260624.jpg 3830872.jpg 885571.jpg 1559052.jpg 2136662.jpg 268444.jpg 326587.jpg 38442.jpg 907107.jpg 1563266.jpg 213765.jpg 2691461.jpg 32693.jpg 3855584.jpg 908261.jpg 1567554.jpg 2138335.jpg 2706403.jpg 3271253.jpg 3857508.jpg 910672.jpg 1575322.jpg 2140776.jpg 270687.jpg 3274423.jpg 386335.jpg 911803.jpg 1588879.jpg 214320.jpg 2707522.jpg 3280453.jpg 3867460.jpg 91432.jpg 1594719.jpg 2146963.jpg 2711806.jpg 3298495.jpg 3868959.jpg 914570.jpg 1595869.jpg 215222.jpg 2716993.jpg 330182.jpg 3869679.jpg 922752.jpg 1598345.jpg 2154126.jpg 2724554.jpg 3306627.jpg 388776.jpg 923772.jpg 1598885.jpg 2154779.jpg 2738227.jpg 3315727.jpg 3890465.jpg 926414.jpg 1600179.jpg 2159975.jpg 2748917.jpg 331860.jpg 3894222.jpg 931356.jpg 1600794.jpg 2163079.jpg 2760475.jpg 332232.jpg 3895825.jpg 937133.jpg 160552.jpg 217250.jpg 2761427.jpg 3322909.jpg 389739.jpg 945791.jpg 1606596.jpg 2172600.jpg 2765887.jpg 332557.jpg 3916407.jpg 947877.jpg 1615395.jpg 2173084.jpg 2768451.jpg 3326734.jpg 393349.jpg 952407.jpg 1618011.jpg 217996.jpg 2771149.jpg 3330642.jpg 393494.jpg 952437.jpg 1619357.jpg 2193684.jpg 2779040.jpg 3333128.jpg 398288.jpg 955466.jpg 1621763.jpg 220341.jpg 2788312.jpg 3333735.jpg 40094.jpg 9555.jpg 1623325.jpg 22080.jpg 2788759.jpg 3334973.jpg 401094.jpg 961341.jpg 1624450.jpg 2216146.jpg 2796102.jpg 3335013.jpg 401144.jpg 97656.jpg 1624747.jpg 2222018.jpg 280284.jpg 3335267.jpg 401651.jpg 979110.jpg 1628861.jpg 2223787.jpg 2807888.jpg 3346787.jpg 405173.jpg 980247.jpg 1632774.jpg 2230959.jpg 2815172.jpg 3364420.jpg 405794.jpg 982988.jpg 1636831.jpg 2232310.jpg 2818805.jpg 336637.jpg 40762.jpg 987732.jpg 1645470.jpg 2233395.jpg 2823872.jpg 3372616.jpg 413325.jpg 996684.jpg
Woah, a whole bunch of images. But how many?
🛠 Practice: Try listing the same information for the
pizzadirectory in thetestfolder.
import os
# Walk through pizza_steak directory and list number of files
for dirpath, dirnames, filenames in os.walk("pizza_steak"):
print(f"There are {len(dirnames)} directories and {len(filenames)} images in '{dirpath}'.")
There are 2 directories and 0 images in 'pizza_steak'. There are 2 directories and 0 images in 'pizza_steak/test'. There are 0 directories and 250 images in 'pizza_steak/test/steak'. There are 0 directories and 250 images in 'pizza_steak/test/pizza'. There are 2 directories and 0 images in 'pizza_steak/train'. There are 0 directories and 750 images in 'pizza_steak/train/steak'. There are 0 directories and 750 images in 'pizza_steak/train/pizza'.
# Another way to find out how many images are in a file
num_steak_images_train = len(os.listdir("pizza_steak/train/steak"))
num_steak_images_train
750
# Get the class names (programmatically, this is much more helpful with a longer list of classes)
import pathlib
import numpy as np
data_dir = pathlib.Path("pizza_steak/train/") # turn our training path into a Python path
class_names = np.array(sorted([item.name for item in data_dir.glob('*')])) # created a list of class_names from the subdirectories
print(class_names)
['pizza' 'steak']
Okay, so we've got a collection of 750 training images and 250 testing images of pizza and steak.
Let's look at some.
🤔 Note: Whenever you're working with data, it's always good to visualize it as much as possible. Treat your first couple of steps of a project as becoming one with the data. Visualize, visualize, visualize.
# View an image
import matplotlib.pyplot as plt
import matplotlib.image as mpimg
import random
def view_random_image(target_dir, target_class):
# Setup target directory (we'll view images from here)
target_folder = target_dir+target_class
# Get a random image path
random_image = random.sample(os.listdir(target_folder), 1)
# Read in the image and plot it using matplotlib
img = mpimg.imread(target_folder + "/" + random_image[0])
plt.imshow(img)
plt.title(target_class)
plt.axis("off");
print(f"Image shape: {img.shape}") # show the shape of the image
return img
# View a random image from the training dataset
img = view_random_image(target_dir="pizza_steak/train/",
target_class="steak")
Image shape: (384, 512, 3)
After going through a dozen or so images from the different classes, you can start to get an idea of what we're working with.
The entire Food101 dataset comprises of similar images from 101 different classes.
You might've noticed we've been printing the image shape alongside the plotted image.
This is because the way our computer sees the image is in the form of a big array (tensor).
# View the img (actually just a big array/tensor)
img
array([[[ 37, 7, 5],
[ 37, 7, 5],
[ 37, 7, 5],
...,
[ 61, 52, 57],
[109, 100, 105],
[102, 92, 100]],
[[ 36, 6, 4],
[ 36, 6, 4],
[ 36, 6, 4],
...,
[105, 96, 101],
[124, 115, 120],
[ 90, 80, 88]],
[[ 36, 6, 4],
[ 36, 6, 4],
[ 36, 6, 4],
...,
[155, 146, 151],
[130, 120, 128],
[ 72, 62, 70]],
...,
[[ 54, 15, 10],
[ 54, 15, 10],
[ 54, 15, 10],
...,
[222, 224, 221],
[219, 221, 218],
[217, 219, 216]],
[[ 55, 16, 11],
[ 55, 16, 11],
[ 55, 16, 11],
...,
[218, 220, 217],
[215, 217, 214],
[212, 214, 211]],
[[ 55, 16, 11],
[ 55, 16, 11],
[ 56, 17, 12],
...,
[214, 216, 213],
[210, 212, 209],
[207, 209, 206]]], dtype=uint8)
# View the image shape
img.shape # returns (width, height, colour channels)
(384, 512, 3)
Looking at the image shape more closely, you'll see it's in the form (Width, Height, Colour Channels).
In our case, the width and height vary but because we're dealing with colour images, the colour channels value is always 3. This is for different values of red, green and blue (RGB) pixels.
You'll notice all of the values in the img array are between 0 and 255. This is because that's the possible range for red, green and blue values.
For example, a pixel with a value red=0, green=0, blue=255 will look very blue.
So when we build a model to differentiate between our images of pizza and steak, it will be finding patterns in these different pixel values which determine what each class looks like.
🔑 Note: As we've discussed before, many machine learning models, including neural networks prefer the values they work with to be between 0 and 1. Knowing this, one of the most common preprocessing steps for working with images is to scale (also referred to as normalize) their pixel values by dividing the image arrays by 255.
# Get all the pixel values between 0 & 1
img/255.
array([[[0.14509804, 0.02745098, 0.01960784],
[0.14509804, 0.02745098, 0.01960784],
[0.14509804, 0.02745098, 0.01960784],
...,
[0.23921569, 0.20392157, 0.22352941],
[0.42745098, 0.39215686, 0.41176471],
[0.4 , 0.36078431, 0.39215686]],
[[0.14117647, 0.02352941, 0.01568627],
[0.14117647, 0.02352941, 0.01568627],
[0.14117647, 0.02352941, 0.01568627],
...,
[0.41176471, 0.37647059, 0.39607843],
[0.48627451, 0.45098039, 0.47058824],
[0.35294118, 0.31372549, 0.34509804]],
[[0.14117647, 0.02352941, 0.01568627],
[0.14117647, 0.02352941, 0.01568627],
[0.14117647, 0.02352941, 0.01568627],
...,
[0.60784314, 0.57254902, 0.59215686],
[0.50980392, 0.47058824, 0.50196078],
[0.28235294, 0.24313725, 0.2745098 ]],
...,
[[0.21176471, 0.05882353, 0.03921569],
[0.21176471, 0.05882353, 0.03921569],
[0.21176471, 0.05882353, 0.03921569],
...,
[0.87058824, 0.87843137, 0.86666667],
[0.85882353, 0.86666667, 0.85490196],
[0.85098039, 0.85882353, 0.84705882]],
[[0.21568627, 0.0627451 , 0.04313725],
[0.21568627, 0.0627451 , 0.04313725],
[0.21568627, 0.0627451 , 0.04313725],
...,
[0.85490196, 0.8627451 , 0.85098039],
[0.84313725, 0.85098039, 0.83921569],
[0.83137255, 0.83921569, 0.82745098]],
[[0.21568627, 0.0627451 , 0.04313725],
[0.21568627, 0.0627451 , 0.04313725],
[0.21960784, 0.06666667, 0.04705882],
...,
[0.83921569, 0.84705882, 0.83529412],
[0.82352941, 0.83137255, 0.81960784],
[0.81176471, 0.81960784, 0.80784314]]])
A (typical) architecture of a convolutional neural network¶
Convolutional neural networks are no different to other kinds of deep learning neural networks in the fact they can be created in many different ways. What you see below are some components you'd expect to find in a traditional CNN.
Components of a convolutional neural network:
| Hyperparameter/Layer type | What does it do? | Typical values |
|---|---|---|
| Input image(s) | Target images you'd like to discover patterns in | Whatever you can take a photo (or video) of |
| Input layer | Takes in target images and preprocesses them for further layers | input_shape = [batch_size, image_height, image_width, color_channels] |
| Convolution layer | Extracts/learns the most important features from target images | Multiple, can create with tf.keras.layers.ConvXD (X can be multiple values) |
| Hidden activation | Adds non-linearity to learned features (non-straight lines) | Usually ReLU (tf.keras.activations.relu) |
| Pooling layer | Reduces the dimensionality of learned image features | Average (tf.keras.layers.AvgPool2D) or Max (tf.keras.layers.MaxPool2D) |
| Fully connected layer | Further refines learned features from convolution layers | tf.keras.layers.Dense |
| Output layer | Takes learned features and outputs them in shape of target labels | output_shape = [number_of_classes] (e.g. 3 for pizza, steak or sushi) |
| Output activation | Adds non-linearities to output layer | tf.keras.activations.sigmoid (binary classification) or tf.keras.activations.softmax |
How they stack together:
A simple example of how you might stack together the above layers into a convolutional neural network. Note the convolutional and pooling layers can often be arranged and rearranged into many different formations.
An end-to-end example¶
We've checked out our data and found there's 750 training images, as well as 250 test images per class and they're all of various different shapes.
It's time to jump straight in the deep end.
Reading the original dataset authors paper, we see they used a Random Forest machine learning model and averaged 50.76% accuracy at predicting what different foods different images had in them.
From now on, that 50.76% will be our baseline.
🔑 Note: A baseline is a score or evaluation metric you want to try and beat. Usually you'll start with a simple model, create a baseline and try to beat it by increasing the complexity of the model. A really fun way to learn machine learning is to find some kind of modelling paper with a published result and try to beat it.
The code in the following cell replicates and end-to-end way to model our pizza_steak dataset with a convolutional neural network (CNN) using the components listed above.
There will be a bunch of things you might not recognize but step through the code yourself and see if you can figure out what it's doing.
We'll go through each of the steps later on in the notebook.
For reference, the model we're using replicates TinyVGG, the computer vision architecture which fuels the CNN explainer webpage.
📖 Resource: The architecture we're using below is a scaled-down version of VGG-16, a convolutional neural network which came 2nd in the 2014 ImageNet classification competition.
import tensorflow as tf
from tensorflow.keras.preprocessing.image import ImageDataGenerator
# Set the seed
tf.random.set_seed(42)
# Preprocess data (get all of the pixel values between 1 and 0, also called scaling/normalization)
train_datagen = ImageDataGenerator(rescale=1./255)
valid_datagen = ImageDataGenerator(rescale=1./255)
# Setup the train and test directories
train_dir = "pizza_steak/train/"
test_dir = "pizza_steak/test/"
# Import data from directories and turn it into batches
train_data = train_datagen.flow_from_directory(train_dir,
batch_size=32, # number of images to process at a time
target_size=(224, 224), # convert all images to be 224 x 224
class_mode="binary", # type of problem we're working on
seed=42)
valid_data = valid_datagen.flow_from_directory(test_dir,
batch_size=32,
target_size=(224, 224),
class_mode="binary",
seed=42)
# Create a CNN model (same as Tiny VGG - https://poloclub.github.io/cnn-explainer/)
model_1 = tf.keras.models.Sequential([
tf.keras.layers.Conv2D(filters=10,
kernel_size=3, # can also be (3, 3)
activation="relu",
input_shape=(224, 224, 3)), # first layer specifies input shape (height, width, colour channels)
tf.keras.layers.Conv2D(10, 3, activation="relu"),
tf.keras.layers.MaxPool2D(pool_size=2, # pool_size can also be (2, 2)
padding="valid"), # padding can also be 'same'
tf.keras.layers.Conv2D(10, 3, activation="relu"),
tf.keras.layers.Conv2D(10, 3, activation="relu"), # activation='relu' == tf.keras.layers.Activations(tf.nn.relu)
tf.keras.layers.MaxPool2D(2),
tf.keras.layers.Flatten(),
tf.keras.layers.Dense(1, activation="sigmoid") # binary activation output
])
# Compile the model
model_1.compile(loss="binary_crossentropy",
optimizer=tf.keras.optimizers.Adam(),
metrics=["accuracy"])
# Fit the model
history_1 = model_1.fit(train_data,
epochs=5,
steps_per_epoch=len(train_data),
validation_data=valid_data,
validation_steps=len(valid_data))
Found 1500 images belonging to 2 classes. Found 500 images belonging to 2 classes. Epoch 1/5 47/47 [==============================] - 25s 260ms/step - loss: 0.5519 - accuracy: 0.7047 - val_loss: 0.3863 - val_accuracy: 0.8460 Epoch 2/5 47/47 [==============================] - 11s 234ms/step - loss: 0.4160 - accuracy: 0.8073 - val_loss: 0.3462 - val_accuracy: 0.8500 Epoch 3/5 47/47 [==============================] - 11s 236ms/step - loss: 0.3920 - accuracy: 0.8193 - val_loss: 0.3200 - val_accuracy: 0.8740 Epoch 4/5 47/47 [==============================] - 11s 235ms/step - loss: 0.3531 - accuracy: 0.8467 - val_loss: 0.2980 - val_accuracy: 0.8840 Epoch 5/5 47/47 [==============================] - 11s 241ms/step - loss: 0.3209 - accuracy: 0.8620 - val_loss: 0.3026 - val_accuracy: 0.8680
🤔 Note: If the cell above takes more than ~12 seconds per epoch to run, you might not be using a GPU accelerator. If you're using a Colab notebook, you can access a GPU accelerator by going to Runtime -> Change Runtime Type -> Hardware Accelerator and select "GPU". After doing so, you might have to rerun all of the above cells as changing the runtime type causes Colab to have to reset.
Nice! After 5 epochs, our model beat the baseline score of 50.76% accuracy (our model got ~85% accuaracy on the training set and ~85% accuracy on the test set).
However, our model only went through a binary classificaiton problem rather than all of the 101 classes in the Food101 dataset, so we can't directly compare these metrics. That being said, the results so far show that our model is learning something.
🛠 Practice: Step through each of the main blocks of code in the cell above, what do you think each is doing? It's okay if you're not sure, we'll go through this soon. In the meantime, spend 10-minutes playing around the incredible CNN explainer website. What do you notice about the layer names at the top of the webpage?
Since we've already fit a model, let's check out its architecture.
# Check out the layers in our model
model_1.summary()
Model: "sequential"
_________________________________________________________________
Layer (type) Output Shape Param #
=================================================================
conv2d (Conv2D) (None, 222, 222, 10) 280
conv2d_1 (Conv2D) (None, 220, 220, 10) 910
max_pooling2d (MaxPooling2D (None, 110, 110, 10) 0
)
conv2d_2 (Conv2D) (None, 108, 108, 10) 910
conv2d_3 (Conv2D) (None, 106, 106, 10) 910
max_pooling2d_1 (MaxPooling (None, 53, 53, 10) 0
2D)
flatten (Flatten) (None, 28090) 0
dense (Dense) (None, 1) 28091
=================================================================
Total params: 31,101
Trainable params: 31,101
Non-trainable params: 0
_________________________________________________________________
What do you notice about the names of model_1's layers and the layer names at the top of the CNN explainer website?
I'll let you in on a little secret: we've replicated the exact architecture they use for their model demo.
Look at you go! You're already starting to replicate models you find in the wild.
Now there are a few new things here we haven't discussed, namely:
- The
ImageDataGeneratorclass and therescaleparameter - The
flow_from_directory()method- The
batch_sizeparameter - The
target_sizeparameter
- The
Conv2Dlayers (and the parameters which come with them)MaxPool2Dlayers (and their parameters).- The
steps_per_epochandvalidation_stepsparameters in thefit()function
Before we dive into each of these, let's see what happens if we try to fit a model we've worked with previously to our data.
Using the same model as before¶
To examplify how neural networks can be adapted to many different problems, let's see how a binary classification model we've previously built might work with our data.
🔑 Note: If you haven't gone through the previous classification notebook, no troubles, we'll be bringing in the a simple 4 layer architecture used to separate dots replicated from the TensorFlow Playground environment.
We can use all of the same parameters in our previous model except for changing two things:
- The data - we're now working with images instead of dots.
- The input shape - we have to tell our neural network the shape of the images we're working with.
- A common practice is to reshape images all to one size. In our case, we'll resize the images to
(224, 224, 3), meaning a height and width of 224 pixels and a depth of 3 for the red, green, blue colour channels.
- A common practice is to reshape images all to one size. In our case, we'll resize the images to
# Set random seed
tf.random.set_seed(42)
# Create a model to replicate the TensorFlow Playground model
model_2 = tf.keras.Sequential([
tf.keras.layers.Flatten(input_shape=(224, 224, 3)), # dense layers expect a 1-dimensional vector as input
tf.keras.layers.Dense(4, activation='relu'),
tf.keras.layers.Dense(4, activation='relu'),
tf.keras.layers.Dense(1, activation='sigmoid')
])
# Compile the model
model_2.compile(loss='binary_crossentropy',
optimizer=tf.keras.optimizers.Adam(),
metrics=["accuracy"])
# Fit the model
history_2 = model_2.fit(train_data, # use same training data created above
epochs=5,
steps_per_epoch=len(train_data),
validation_data=valid_data, # use same validation data created above
validation_steps=len(valid_data))
Epoch 1/5 47/47 [==============================] - 11s 215ms/step - loss: 1.2197 - accuracy: 0.4947 - val_loss: 0.6932 - val_accuracy: 0.5000 Epoch 2/5 47/47 [==============================] - 10s 210ms/step - loss: 0.6932 - accuracy: 0.5000 - val_loss: 0.6932 - val_accuracy: 0.5000 Epoch 3/5 47/47 [==============================] - 10s 208ms/step - loss: 0.6932 - accuracy: 0.5000 - val_loss: 0.6932 - val_accuracy: 0.5000 Epoch 4/5 47/47 [==============================] - 10s 210ms/step - loss: 0.6932 - accuracy: 0.5000 - val_loss: 0.6932 - val_accuracy: 0.5000 Epoch 5/5 47/47 [==============================] - 10s 210ms/step - loss: 0.6932 - accuracy: 0.5000 - val_loss: 0.6931 - val_accuracy: 0.5000
Hmmm... our model ran but it doesn't seem like it learned anything. It only reaches 50% accuracy on the training and test sets which in a binary classification problem is as good as guessing.
Let's see the architecture.
# Check out our second model's architecture
model_2.summary()
Model: "sequential_1"
_________________________________________________________________
Layer (type) Output Shape Param #
=================================================================
flatten_1 (Flatten) (None, 150528) 0
dense_1 (Dense) (None, 4) 602116
dense_2 (Dense) (None, 4) 20
dense_3 (Dense) (None, 1) 5
=================================================================
Total params: 602,141
Trainable params: 602,141
Non-trainable params: 0
_________________________________________________________________
Wow. One of the most noticeable things here is the much larger number of parameters in model_2 versus model_1.
model_2 has 602,141 trainable parameters where as model_1 has only 31,101. And despite this difference, model_1 still far and large out performs model_2.
🔑 Note: You can think of trainable parameters as patterns a model can learn from data. Intuitiely, you might think more is better. And in some cases it is. But in this case, the difference here is in the two different styles of model we're using. Where a series of dense layers have a number of different learnable parameters connected to each other and hence a higher number of possible learnable patterns, a convolutional neural network seeks to sort out and learn the most important patterns in an image. So even though there are less learnable parameters in our convolutional neural network, these are often more helpful in decphering between different features in an image.
Since our previous model didn't work, do you have any ideas of how we might make it work?
How about we increase the number of layers?
And maybe even increase the number of neurons in each layer?
More specifically, we'll increase the number of neurons (also called hidden units) in each dense layer from 4 to 100 and add an extra layer.
🔑 Note: Adding extra layers or increasing the number of neurons in each layer is often referred to as increasing the complexity of your model.
# Set random seed
tf.random.set_seed(42)
# Create a model similar to model_1 but add an extra layer and increase the number of hidden units in each layer
model_3 = tf.keras.Sequential([
tf.keras.layers.Flatten(input_shape=(224, 224, 3)), # dense layers expect a 1-dimensional vector as input
tf.keras.layers.Dense(100, activation='relu'), # increase number of neurons from 4 to 100 (for each layer)
tf.keras.layers.Dense(100, activation='relu'),
tf.keras.layers.Dense(100, activation='relu'), # add an extra layer
tf.keras.layers.Dense(1, activation='sigmoid')
])
# Compile the model
model_3.compile(loss='binary_crossentropy',
optimizer=tf.keras.optimizers.Adam(),
metrics=["accuracy"])
# Fit the model
history_3 = model_3.fit(train_data,
epochs=5,
steps_per_epoch=len(train_data),
validation_data=valid_data,
validation_steps=len(valid_data))
Epoch 1/5 47/47 [==============================] - 11s 220ms/step - loss: 2.3542 - accuracy: 0.6367 - val_loss: 0.5904 - val_accuracy: 0.7700 Epoch 2/5 47/47 [==============================] - 10s 211ms/step - loss: 0.9569 - accuracy: 0.6980 - val_loss: 0.6021 - val_accuracy: 0.7560 Epoch 3/5 47/47 [==============================] - 10s 211ms/step - loss: 0.6688 - accuracy: 0.7360 - val_loss: 1.0608 - val_accuracy: 0.5780 Epoch 4/5 47/47 [==============================] - 10s 211ms/step - loss: 0.6921 - accuracy: 0.7120 - val_loss: 0.5879 - val_accuracy: 0.7380 Epoch 5/5 47/47 [==============================] - 10s 210ms/step - loss: 0.6880 - accuracy: 0.7327 - val_loss: 0.5331 - val_accuracy: 0.7800
Woah! Looks like our model is learning again. It got ~70% accuracy on the training set and ~70% accuracy on the validation set.
How does the architecute look?
# Check out model_3 architecture
model_3.summary()
Model: "sequential_2"
_________________________________________________________________
Layer (type) Output Shape Param #
=================================================================
flatten_2 (Flatten) (None, 150528) 0
dense_4 (Dense) (None, 100) 15052900
dense_5 (Dense) (None, 100) 10100
dense_6 (Dense) (None, 100) 10100
dense_7 (Dense) (None, 1) 101
=================================================================
Total params: 15,073,201
Trainable params: 15,073,201
Non-trainable params: 0
_________________________________________________________________
My gosh, the number of trainable parameters has increased even more than model_2. And even with close to 500x (~15,000,000 vs. ~31,000) more trainable parameters, model_3 still doesn't out perform model_1.
This goes to show the power of convolutional neural networks and their ability to learn patterns despite using less parameters.
Binary classification: Let's break it down¶
We just went through a whirlwind of steps:
- Become one with the data (visualize, visualize, visualize...)
- Preprocess the data (prepare it for a model)
- Create a model (start with a baseline)
- Fit the model
- Evaluate the model
- Adjust different parameters and improve model (try to beat your baseline)
- Repeat until satisfied
Let's step through each.
1. Import and become one with the data¶
Whatever kind of data you're dealing with, it's a good idea to visualize at least 10-100 samples to start to building your own mental model of the data.
In our case, we might notice that the steak images tend to have darker colours where as pizza images tend to have a distinct circular shape in the middle. These might be patterns that our neural network picks up on.
You an also notice if some of your data is messed up (for example, has the wrong label) and start to consider ways you might go about fixing it.
📖 Resource: To see how this data was processed into the file format we're using, see the preprocessing notebook.
If the visualization cell below doesn't work, make sure you've got the data by uncommenting the cell below.
# import zipfile
# # Download zip file of pizza_steak images
# !wget https://storage.googleapis.com/ztm_tf_course/food_vision/pizza_steak.zip
# # Unzip the downloaded file
# zip_ref = zipfile.ZipFile("pizza_steak.zip", "r")
# zip_ref.extractall()
# zip_ref.close()
# Visualize data (requires function 'view_random_image' above)
plt.figure()
plt.subplot(1, 2, 1)
steak_img = view_random_image("pizza_steak/train/", "steak")
plt.subplot(1, 2, 2)
pizza_img = view_random_image("pizza_steak/train/", "pizza")
Image shape: (384, 512, 3) Image shape: (512, 384, 3)
2. Preprocess the data (prepare it for a model)¶
One of the most important steps for a machine learning project is creating a training and test set.
In our case, our data is already split into training and test sets. Another option here might be to create a validation set as well, but we'll leave that for now.
For an image classification project, it's standard to have your data seperated into train and test directories with subfolders in each for each class.
To start we define the training and test directory paths.
# Define training and test directory paths
train_dir = "pizza_steak/train/"
test_dir = "pizza_steak/test/"
Our next step is to turn our data into batches.
A batch is a small subset of the dataset a model looks at during training. For example, rather than looking at 10,000 images at one time and trying to figure out the patterns, a model might only look at 32 images at a time.
It does this for a couple of reasons:
- 10,000 images (or more) might not fit into the memory of your processor (GPU).
- Trying to learn the patterns in 10,000 images in one hit could result in the model not being able to learn very well.
Why 32?
A batch size of 32 is good for your health.
No seriously, there are many different batch sizes you could use but 32 has proven to be very effective in many different use cases and is often the default for many data preprocessing functions.
To turn our data into batches, we'll first create an instance of ImageDataGenerator for each of our datasets.
# Create train and test data generators and rescale the data
from tensorflow.keras.preprocessing.image import ImageDataGenerator
train_datagen = ImageDataGenerator(rescale=1/255.)
test_datagen = ImageDataGenerator(rescale=1/255.)
The ImageDataGenerator class helps us prepare our images into batches as well as perform transformations on them as they get loaded into the model.
You might've noticed the rescale parameter. This is one example of the transformations we're doing.
Remember from before how we imported an image and it's pixel values were between 0 and 255?
The rescale parameter, along with 1/255. is like saying "divide all of the pixel values by 255". This results in all of the image being imported and their pixel values being normalized (converted to be between 0 and 1).
🔑 Note: For more transformation options such as data augmentation (we'll see this later), refer to the
ImageDataGeneratordocumentation.
Now we've got a couple of ImageDataGenerator instances, we can load our images from their respective directories using the flow_from_directory method.
# Turn it into batches
train_data = train_datagen.flow_from_directory(directory=train_dir,
target_size=(224, 224),
class_mode='binary',
batch_size=32)
test_data = test_datagen.flow_from_directory(directory=test_dir,
target_size=(224, 224),
class_mode='binary',
batch_size=32)
Found 1500 images belonging to 2 classes. Found 500 images belonging to 2 classes.
Wonderful! Looks like our training dataset has 1500 images belonging to 2 classes (pizza and steak) and our test dataset has 500 images also belonging to 2 classes.
Some things to here:
- Due to how our directories are structured, the classes get inferred by the subdirectory names in
train_dirandtest_dir. - The
target_sizeparameter defines the input size of our images in(height, width)format. - The
class_modevalue of'binary'defines our classification problem type. If we had more than two classes, we would use'categorical'. - The
batch_sizedefines how many images will be in each batch, we've used 32 which is the same as the default.
We can take a look at our batched images and labels by inspecting the train_data object.
# Get a sample of the training data batch
images, labels = train_data.next() # get the 'next' batch of images/labels
len(images), len(labels)
(32, 32)
Wonderful, it seems our images and labels are in batches of 32.
Let's see what the images look like.
# Get the first two images
images[:2], images[0].shape
(array([[[[0.47058827, 0.40784317, 0.34509805],
[0.4784314 , 0.427451 , 0.3647059 ],
[0.48627454, 0.43529415, 0.37254903],
...,
[0.8313726 , 0.70980394, 0.48627454],
[0.8431373 , 0.73333335, 0.5372549 ],
[0.87843144, 0.7725491 , 0.5882353 ]],
[[0.50980395, 0.427451 , 0.36078432],
[0.5058824 , 0.42352945, 0.35686275],
[0.5137255 , 0.4431373 , 0.3647059 ],
...,
[0.82745105, 0.7058824 , 0.48235297],
[0.82745105, 0.70980394, 0.5058824 ],
[0.8431373 , 0.73333335, 0.5372549 ]],
[[0.5254902 , 0.427451 , 0.34901962],
[0.5372549 , 0.43921572, 0.36078432],
[0.5372549 , 0.45098042, 0.36078432],
...,
[0.82745105, 0.7019608 , 0.4784314 ],
[0.82745105, 0.7058824 , 0.49411768],
[0.8352942 , 0.7176471 , 0.5137255 ]],
...,
[[0.77647066, 0.5647059 , 0.2901961 ],
[0.7803922 , 0.53333336, 0.22352943],
[0.79215693, 0.5176471 , 0.18039216],
...,
[0.30588236, 0.2784314 , 0.24705884],
[0.24705884, 0.23137257, 0.19607845],
[0.2784314 , 0.27450982, 0.25490198]],
[[0.7843138 , 0.57254905, 0.29803923],
[0.79215693, 0.54509807, 0.24313727],
[0.8000001 , 0.5254902 , 0.18823531],
...,
[0.2627451 , 0.23529413, 0.20392159],
[0.24313727, 0.227451 , 0.19215688],
[0.26666668, 0.2627451 , 0.24313727]],
[[0.7960785 , 0.59607846, 0.3372549 ],
[0.7960785 , 0.5647059 , 0.26666668],
[0.81568635, 0.54901963, 0.22352943],
...,
[0.23529413, 0.19607845, 0.16078432],
[0.3019608 , 0.26666668, 0.24705884],
[0.26666668, 0.2509804 , 0.24705884]]],
[[[0.38823533, 0.4666667 , 0.36078432],
[0.3921569 , 0.46274513, 0.36078432],
[0.38431376, 0.454902 , 0.36078432],
...,
[0.5294118 , 0.627451 , 0.54509807],
[0.5294118 , 0.627451 , 0.54509807],
[0.5411765 , 0.6392157 , 0.5568628 ]],
[[0.38431376, 0.454902 , 0.3529412 ],
[0.3921569 , 0.46274513, 0.36078432],
[0.39607847, 0.4666667 , 0.37254903],
...,
[0.54509807, 0.6431373 , 0.5686275 ],
[0.5529412 , 0.6509804 , 0.5764706 ],
[0.5647059 , 0.6627451 , 0.5882353 ]],
[[0.3921569 , 0.46274513, 0.36078432],
[0.38431376, 0.454902 , 0.3529412 ],
[0.4039216 , 0.47450984, 0.3803922 ],
...,
[0.5764706 , 0.67058825, 0.6156863 ],
[0.5647059 , 0.6666667 , 0.6156863 ],
[0.5647059 , 0.6666667 , 0.6156863 ]],
...,
[[0.47058827, 0.5647059 , 0.4784314 ],
[0.4784314 , 0.5764706 , 0.4901961 ],
[0.48235297, 0.5803922 , 0.49803925],
...,
[0.39607847, 0.42352945, 0.3019608 ],
[0.37647063, 0.40000004, 0.2901961 ],
[0.3803922 , 0.4039216 , 0.3019608 ]],
[[0.45098042, 0.5529412 , 0.454902 ],
[0.46274513, 0.5647059 , 0.4666667 ],
[0.47058827, 0.57254905, 0.47450984],
...,
[0.40784317, 0.43529415, 0.3137255 ],
[0.39607847, 0.41960788, 0.31764707],
[0.38823533, 0.40784317, 0.31764707]],
[[0.47450984, 0.5764706 , 0.47058827],
[0.47058827, 0.57254905, 0.4666667 ],
[0.46274513, 0.5647059 , 0.4666667 ],
...,
[0.4039216 , 0.427451 , 0.31764707],
[0.3921569 , 0.4156863 , 0.3137255 ],
[0.4039216 , 0.42352945, 0.3372549 ]]]], dtype=float32),
(224, 224, 3))
Due to our rescale parameter, the images are now in (224, 224, 3) shape tensors with values between 0 and 1.
How about the labels?
# View the first batch of labels
labels
array([1., 1., 0., 1., 0., 0., 0., 1., 0., 1., 0., 0., 1., 0., 0., 0., 1.,
1., 0., 1., 0., 1., 1., 1., 0., 0., 0., 0., 0., 1., 0., 1.],
dtype=float32)
Due to the class_mode parameter being 'binary' our labels are either 0 (pizza) or 1 (steak).
Now that our data is ready, our model is going to try and figure out the patterns between the image tensors and the labels.
3. Create a model (start with a baseline)¶
You might be wondering what your default model architecture should be.
And the truth is, there's many possible answers to this question.
A simple heuristic for computer vision models is to use the model architecture which is performing best on ImageNet (a large collection of diverse images to benchmark different computer vision models).
However, to begin with, it's good to build a smaller model to acquire a baseline result which you try to improve upon.
🔑 Note: In deep learning a smaller model often refers to a model with less layers than the state of the art (SOTA). For example, a smaller model might have 3-4 layers where as a state of the art model, such as, ResNet50 might have 50+ layers.
In our case, let's take a smaller version of the model that can be found on the CNN explainer website (model_1 from above) and build a 3 layer convolutional neural network.
# Make the creating of our model a little easier
from tensorflow.keras.optimizers import Adam
from tensorflow.keras.layers import Dense, Flatten, Conv2D, MaxPool2D, Activation
from tensorflow.keras import Sequential
# Create the model (this can be our baseline, a 3 layer Convolutional Neural Network)
model_4 = Sequential([
Conv2D(filters=10,
kernel_size=3,
strides=1,
padding='valid',
activation='relu',
input_shape=(224, 224, 3)), # input layer (specify input shape)
Conv2D(10, 3, activation='relu'),
Conv2D(10, 3, activation='relu'),
Flatten(),
Dense(1, activation='sigmoid') # output layer (specify output shape)
])
Great! We've got a simple convolutional neural network architecture ready to go.
And it follows the typical CNN structure of:
Input -> Conv + ReLU layers (non-linearities) -> Pooling layer -> Fully connected (dense layer) as OutputLet's discuss some of the components of the Conv2D layer:
- The "
2D" means our inputs are two dimensional (height and width), even though they have 3 colour channels, the convolutions are run on each channel invididually. filters- these are the number of "feature extractors" that will be moving over our images.kernel_size- the size of our filters, for example, akernel_sizeof(3, 3)(or just 3) will mean each filter will have the size 3x3, meaning it will look at a space of 3x3 pixels each time. The smaller the kernel, the more fine-grained features it will extract.stride- the number of pixels afilterwill move across as it covers the image. Astrideof 1 means the filter moves across each pixel 1 by 1. Astrideof 2 means it moves 2 pixels at a time.padding- this can be either'same'or'valid','same'adds zeros the to outside of the image so the resulting output of the convolutional layer is the same as the input, where as'valid'(default) cuts off excess pixels where thefilterdoesn't fit (e.g. 224 pixels wide divided by a kernel size of 3 (224/3 = 74.6) means a single pixel will get cut off the end.
What's a "feature"?
A feature can be considered any significant part of an image. For example, in our case, a feature might be the circular shape of pizza. Or the rough edges on the outside of a steak.
It's important to note that these features are not defined by us, instead, the model learns them as it applies different filters across the image.
📖 Resources: For a great demonstration of these in action, be sure to spend some time going through the following:
- CNN Explainer Webpage - a great visual overview of many of the concepts we're replicating here with code.
- A guide to convolutional arithmetic for deep learning - a phenomenal introduction to the math going on behind the scenes of a convolutional neural network.
- For a great explanation of padding, see this Stack Overflow answer.
Now our model is ready, let's compile it.
# Compile the model
model_4.compile(loss='binary_crossentropy',
optimizer=Adam(),
metrics=['accuracy'])
Since we're working on a binary classification problem (pizza vs. steak), the loss func