Class 21: Woof, Bark, Meow?
Classifying complex image - Transfer learning
We have already seen several examples of classifying image corpora. When detecting whether our class photographs were taken inside or outside, the algorithm primarily used color to detect the brightness and spectrum to see if it looked like sunlight (more blue) or artificial light (more red). With the flowers dataset we used color as well as texture to figure out what type of flower was shown in the image. With the MNIST, Fashion MNIST, and EMNIST datasets neural networks were used to detect and classify more complex features such as shape and location in the image.
In the world of image classification, all of these tasks are fairly easy. The first few only needed average values of pixels at a very low resolution. The latter ones were zoomed in, centered, and standardized in size and orientation. Most classification tasks out in the wild are much more complex.
How would we go about solving these types of problems? The simple answer is that you need millions of images, a very deep and large convolutional neural network, and a large computing cluster with access to many expensive GPUs.
The good news is that there is a solution for all of us that do not have access to such large datasets. Recall that a convolutional neural network usually consists of several convolutional layers followed by a flattening operation and dense layers. It turns out that if a deep neural network is trained with a large corpus of images, the lower convolutional layers pick out features that can be used for other image processing tasks. We can actually think of the CNN layers as doing feature extraction, much like we did with the textual data when counting tokens and character shingles.
The reason this is so helpful is that many groups (those with access to large computing resources) have published their trained models. We can fit just lower levels of these models to new training data, and then use the output of this in our own models. These could be more neural networks, but there is nothing stopping us from using a different model on the learned features. This process is called transfer learning. It is one of the most important features of neural networks. Otherwise it would only be possible for a limited number of research groups to make real use of these techniques.
Today, we will apply transfer learning to two tasks: face detection in our class dataset and detecting whether an image contains a dog or a cat.
The keras package provides several pre-trained models for us to use. At the current moment there are:
application_inception_v3: Szegedy, Christian, et al. “Rethinking the inception architecture for computer vision.” Proceedings of the IEEE Conference on Computer Vision and Pattern Recognition. 2016.
application_vgg16: Simonyan, Karen, and Andrew Zisserman. “Very deep convolutional networks for large-scale image recognition.” arXiv preprint arXiv:1409.1556 (2014).
application_vgg19: Simonyan, Karen, and Andrew Zisserman. “Very deep convolutional networks for large-scale image recognition.” arXiv preprint arXiv:1409.1556 (2014).
application_mobilenet: Howard, Andrew G., et al. “Mobilenets: Efficient convolutional neural networks for mobile vision applications.” arXiv preprint arXiv:1704.04861 (2017).
application_resnet50: He, Kaiming, et al. “Deep residual learning for image recognition.” Proceedings of the IEEE conference on computer vision and pattern recognition. 2016.
application_xception: Chollet, François. “Xception: Deep Learning with Depthwise Separable Convolutions.” arXiv preprint arXiv:1610.02357 (2016).
These were all trained on the same corpus from the ImageNet Challenge, which includes images from 1000 categories all scaled to 224 by 224 pixels. The images look something like this:
For more information about the corpus, see the paper here:
- Olga Russakovsky, Jia Deng, Hao Su, Jonathan Krause, Sanjeev Satheesh, Sean Ma, Zhiheng Huang, Andrej Karpathy, Aditya Khosla, Michael Bernstein, Alexander C. Berg and Li Fei-Fei. ImageNet Large Scale Visual Recognition Challenge. IJCV, 2015.
We are going to use the VGG16 model. It comes from the Visual Geometry Group at Oxford. The 16 refers to their being 16 trainable layers in the neural network. Let’s load the model, which includes trained weights and look at the architecture.
Notice that there are over 131 million weights that need to trained in this model. Most of them, however, are in the last two dense layers. The convolutions can be grouped into five blocks, defined by how large a subset (in pixels) the convolution is looking at. Each successive convolution is grabbed larger and more complex features than the previous one. Very roughly, the five blocks can be thought of as capturing the following aspects of the image:
- 2x2: double convolution, gradients
- 4x4: double convolution, edges
- 8x8: triple convolution, shapes and textures
- 16x16: triple convolution, objects and object textures
- 32x32: triple convolution, objects in context and entities
We will fit the convolutional parts of the VGG16 model to our images and then use these learned features to detect faces and dogs.
Faces in the Class Dataset
Now, let’s load the dataset from class.
Before jumping into the transfer learning, we will briefly explore what it really means to look at 2x2, 4x4, 8x8, 16x16, and 32x32 blocks of the images. This function visualizes all five of these layers centered around a particular point:
Let’s run it over two of the class images:
Hopefully you will agree that, roughly, these zoom levels correspond to my descriptions in the prior section.
Now, we will load the VGG16 model in again, but this time will not
include the top layers. I then take three images from the
dataset and preprocess these (the function scales and rotates the
images in the same way that the VGG uses before putting their
images into the neural network). We then predict the fitted values
from the VGG16 model and look at the dimension of the output.
The result is a tensor with 3 samples; for each sample we have a 7 by 7 grid with 512 filters. Try to visulize this by thinking of each image being chopped into a 7 by 7 grid. Each box has now been summarized to a sequence of 512 numbers.
At this point we could probably flatten the resulting tensors into
a matrix with
7 * 7 * 512 (25088) columns and one row per image.
If we were patient, it is likely we could build a good model with
our data for whether an image contains a person or not.
Alternatively, we can simplify things and create a much smaller dataset by apply the max function to each of the 512 filters. We lose the positional information from this aggresive pooling, but not have a very easy to use matrix with just 512 variables. To do this quickly, I can use the following function:
And we now have 512 rows and three columns.
Detecting faces with transfer learning
Now, we will actually apply transfer learning to our image dataset. I have already applied the VGG16 model to the data and uploaded a csv file with 512 columns from the max pooling over the 7 by 7 filters.
At this point we can fit a normal, dense neural network on our image data. First, we will construct the datasets:
And then build a neural network with two hidden layers, compile it, and train on our training data:
The minimal tuning and tweaking, the results are fairly convincing that transfer learning is good way of classifying images:
The only false positive is this one photo:
I know for a fact that Lily thinks she is a person, I’m not even 100% sure this should count as a negative example. It also leads us right into the second classification task for the day.
Dogs versus cats
Next, lets look at a dataset of photos showing cats and dogs. The goal is to classify which of the two a particular image is showing. I will load the metadata, the learned VGG16 features, and the raw data. The latter is only needed to create plots of the negative examples.
As an example, I will just use the same model I had above:
Not a bad start, with a classification rate of around 97.7%, with very little effort. Transfer learning is amazing!
Primarily just because they are cute, there are the dogs classified as cats:
And the cats classified as dogs:
Do you see any patterns that show what might be difficult about this classification task?
A History of Image Processing with Neural Networks
A model out of the University of Toronto, now known as AlexNet, became the first CNN to produce state-of-the-art classification rates on the ILSVRC-2012 dataset:
- Krizhevsky, Alex, Ilya Sutskever, and Geoffrey E. Hinton. “Imagenet classification with deep convolutional neural networks.” In Advances in neural information processing systems, pp. 1097-1105. 2012.
AlexNet was the first to put together several key advances, many of which we have already used or discussed in this class:
- relu units
- data augmentation
- multiple GPUs While not all invented by the AlexNet group, they were the first to put them all together and figure out how to train a deep neural network.
Visualizing CNNs (2013)
Following the success of AlexNet, the year 2013 saw a much larger number of neural network entrants into the ILSVRC competition. The winning entry came about due to the visualization techniques described in the following paper:
- Zeiler, Matthew D., and Rob Fergus. “Visualizing and understanding convolutional networks.” Computer vision–ECCV 2014. Springer International Publishing, 2014. 818-833.
Their incredibly diverse set of techniques allowed the team to tweak the AlexNet architecture to get even better results.
A demo of applying these techniques to the MNIST dataset with ConvNetJS:
The 2013 competition also brought about the very influential OverFeat model from a team based at NYU:
- Sermanet, Pierre, David Eigen, Xiang Zhang, Michaël Mathieu, Rob Fergus, and Yann LeCun. “Overfeat: Integrated recognition, localization and detection using convolutional networks.” arXiv preprint arXiv:1312.6229 (2013).
This won the image localization task, by trying to solve localization and identification in a unified process. I’ll give a very simplified version of what they did (the paper is a great read, and I suggest working through it if you are interested in computer vision).
VGG-16, VGG-19 (2014)
One of the top entries from 2014, by an Oxford-based team, took advantage of significantly deeper models.
- Simonyan, Karen, and Andrew Zisserman. “Very deep convolutional networks for large-scale image recognition.” arXiv preprint arXiv:1409.1556 (2014).
The winning entry from 2014, by Google, also took advantage of much deeper architectures:
- Szegedy, Christian, Wei Liu, Yangqing Jia, Pierre Sermanet, Scott Reed, Dragomir Anguelov, Dumitru Erhan, Vincent Vanhoucke, and Andrew Rabinovich. “Going deeper with convolutions.” In Proceedings of the IEEE Conference on Computer Vision and Pattern Recognition, pp. 1-9. 2015. They called their model GoogLeNet in honor of the original LeNet architecture.
ResNet-50, -101, -152 (2015)
Finally, in the 2015 competition, Microsoft produced an model which is extremely deeper than any previously used. These models are known as ResNet, with their depth given as an suffix.
- He, Kaiming, Xiangyu Zhang, Shaoqing Ren, and Jian Sun. “Deep Residual Learning for Image Recognition.” arXiv preprint arXiv:1512.03385 (2015).
Stochastic Depth Models (2016)
Another tweak on the ResNet architecture, which subsamples layers in the network:
- Gao Huang, Yu Sun, Zhuang Liu, Daniel Sedra, Kilian Weinberger. “Deep Networks with Stochastic Depth”, arXiv preprint arXiv:1603.09382 (2016). Notice how this seems like an almost obvious thing to try given the ResNet architecture, but less-so in a generic neural network.
ResNet-200, -1001 (2016)
Microsoft’s update to the past year’s model:
- He, Kaiming, et al. “Identity Mappings in Deep Residual Networks.” arXiv preprint arXiv:1603.05027 (2016).
A new line of research involves looking at ways to produce near state-of-the-art results with a minimal model size (or minimal computational cost):
- Iandola, Forrest N., et al. “SqueezeNet: AlexNet-level accuracy with 50x fewer parameters and <1MB model size.” arXiv preprint arXiv:1602.07360 (2016).