CNN History

LeNet (1998)

• Image followed by multiple convolutional / pooling layers

  • Build up hierarchical filter structures

  • Subsampling / pooling increases robustness

• Fully connected layers towards the end

  • Brings all information together, combines it once more

• Output layer of 10 units, one for each digit class

ImageNet Dataset (2009)

Standard benchmark for vision:

• 1.2 M images
• 1000 classes
• > 500 images per class

ImageNet Large Scale Visual Recognition Challenge (ILSVRC)

• ILSVRC Classification Task
  • 1000 object classes
  • 1.2 million training images (732 – 1300 per class)
  • 50 thousand validation images (50 per class)
  • 100 thousand test images (100 per class)

AlexNet (2012)

• Multiple convolutional layers

• Couple fully connected (dense) layers

• Final classification using a “soft-max” layer

• Train end-to-end via back propagation

• Details

  • first use of ReLU
  • used Norm layers (not common anymore)
  • heavy data augmentation
  • dropout 0.5
  • batch size 128
  • SGD Momentum 0.9
  • Learning rate 1e-2, reduced by a factor of 10 manually when val accuracy plateaus
  • L2 weight decay 5e-4

VGG Net (2014)

Small filters, Deeper networks

• 8 layers (AlexNet) -> 16 - 19 layers (VGG16Net)
• Only 3x3 CONV stride 1, pad 1
• and 2x2 MAX POOL stride 2

ResNet (2015)

Residual blocks

How can we train such deep networks?

Solution: Use network layers to fit a residual mapping instead of directly trying to fit a

desired underlying mapping

• Use layers to fit residual $F(x) = H(x) – x$ instead of $H(x)$ directly
• Initially, $F(x)$ is set to 0, so the layer just computes the identity
• I.e. adding more layers does not harm 👏

ResNet Architecture

• Stack residual blocks
• Every residual block has two 3x3 conv layers
• Periodically, double # of filters and downsample spatially using stride 2 (/2 in each dimension)
• Additional conv layer at the beginning
• No FC layers at the end (only FC 1000 to output classes)

Training ResNet in practice

• Batch Normalization after every CONV layer (not covered)
• Xavier 2/ initialization from He et al.
• SGD + Momentum (0.9)
• Learning rate: 0.1, divided by 10 when validation error plateaus
• Mini-batch size 256
• Weight decay of 1e-5
• No dropout used

Transfer Learning

ImageNet has 1.2 million images! Typically, we do not have that many! Can we also use these methods with less images?

Yes! With transfer learning!

• Features (conv layers) are generic and can be reused!

How?

• Train on huge data-set (e.g. Imagenet)
• Freeze layers and adapt only last (FC) layers

Pratical Advice

• Very little data, very similar dataset:

  • Use Linear classifier on top layer

• Very little data, very different dataset:

  • You’re in trouble… Try linear classifier from different stages and pray 🤪

• A lot of data, very similar dataset:

  • Finetune a few layers

• A lot of data, very different dataset:

  • Finetune a larger number of layers

Example: Image Captioning

Example: Face Recognition

• Siamese Networks (FaceNet)

• Distance

  $$ d\left(x\_{1}, x\_{2}\right)=\left\|f\left(x\_{1}\right)-f\left(x\_{2}\right)\right\|\_{2}^{2} $$
  • If $d(x\_1, x\_2)$ small: same person
  • Otherwise different person

• Training: Triplet loss

  $$ L(A, P, N)=\max \left(\|f(A)-f(P)\|^{2}-\|f(A)-f(N)\|^{2}+\alpha, 0\right) $$