Mask RCNN takes off from where Faster RCNN left, with some augmentations aimed at bettering instance segmentation (which was out of scope for FRCNN). Instance segmentation was achieved remarkably well in *DeepMask* , *SharpMask* and later *Feature Pyramid Networks* (FPN).

Faster RCNN was not designed for pixel-to-pixel alignment between network inputs and outputs. This is most evident in how RoIPool , the de facto core operation for attending to instances, performs coarse spatial quantization for feature extraction. Mask RCNN fixes that by introducing RoIAlign in place of RoIPool.

#### Methodology

Mask RCNN retains most of the architecture of Faster RCNN. It adds the a third branch for segmentation. The third branch takes the output from RoIAlign layer and predicts binary class masks for each class.

##### Major Changes and intutions

**Mask prediction**

Mask prediction segmentation predicts a binary mask for each RoI using fully convolution - and the stark difference being usage of *sigmoid* activation for predicting final mask instead of *softmax*, implies masks don't compete with each other. This *decouples* segmentation from classification. The class prediction branch is used for class prediction and for calculating loss, the mask of predicted loss is used calculating Lmask.

Also, they show that a single class agnostic mask prediction works almost as effective as separate mask for each class, thereby supporting their method of decoupling classification from segmentation

**RoIAlign**

RoIPool first quantizes a floating-number RoI to the discrete granularity of the feature map, this quantized RoI is then subdivided into spatial bins which are themselves quantized, and finally feature values covered by each bin are aggregated (usually by max pooling). Instead of  quantization of the RoI boundaries
or bin bilinear interpolation is used to compute the exact values of the input features at four regularly sampled locations in each RoI bin, and aggregate the result (using max or average).

**Backbone architecture**

Faster RCNN uses a VGG like structure for extracting features from image, weights of which were shared among RPN and region detection layers. Herein, authors experiment with 2 backbone architectures - ResNet based VGG like in FRCNN and ResNet based [FPN](http://www.shortscience.org/paper?bibtexKey=journals/corr/LinDGHHB16) based. FPN uses convolution feature maps from previous layers and recombining them to produce pyramid of feature maps to be used for prediction instead of single-scale feature layer (final output of conv layer before connecting to fc layers was used in Faster RCNN) 

**Training Objective**

The training objective looks like this 
![](https://i.imgur.com/snUq73Q.png)

Lmask is the addition from Faster RCNN. The method to calculate was mentioned above

#### Observation

Mask RCNN performs significantly better than COCO instance segmentation winners *without any bells and whiskers*. Detailed results are available in the paper

Deeper networks should never have a higher **training** error than smaller ones. In the worst case, the layers should "simply" learn identities. It seems as this is not so easy with conventional networks, as they get much worse with more layers. So the idea is to add identity functions which skip some layers. The network only has to learn the **residuals**. 

Advantages:

* Learning the identity becomes learning 0 which is simpler
* Loss in information flow in the forward pass is not a problem anymore
    * No vanishing / exploding gradient
* Identities don't have parameters to be learned

## Evaluation

The learning rate starts at 0.1 and is divided by 10 when the error plateaus. Weight decay of 0.0001 ($10^{-4}$), momentum of 0.9. They use mini-batches of size 128.

* ImageNet ILSVRC 2015: 3.57% (ensemble)
* CIFAR-10: 6.43%
* MS COCO: 59.0% mAp@0.5 (ensemble)
* PASCAL VOC 2007: 85.6% mAp@0.5
* PASCAL VOC 2012: 83.8% mAp@0.5

## See also

* [DenseNets](http://www.shortscience.org/paper?bibtexKey=journals/corr/1608.06993)

This paper presents a feed-forward neural network architecture for processing graphs as inputs, inspired from previous work on Graph Neural Networks.

In brief, the architecture of the GG-NN corresponds to $T$ steps of GRU-like (gated recurrent units) updates, where T is a hyper-parameter. At each step, a vector representation is computed for all nodes in the graph, where a node's representation at step t is computed from the representation of nodes at step $t-1$. Specifically, the representation of a node will be updated based on the representation of its neighbors in the graph. Incoming and outgoing edges in the graph are treated differently by the neural network, by using different parameter matrices for each. Moreover, if edges have labels, separate parameters can be learned for the different types of edges (meaning that edge labels determine the configuration of parameter sharing in the model). Finally, GG-NNs can incorporate node-level attributes, by using them in the initialization (time step 0) of the nodes' representations.

GG-NNs can be used to perform a variety of tasks on graphs. The per-node representations can be used to make per-node predictions by feeding them to a neural network (shared across nodes). A graph-level predictor can also be obtained using a soft attention architecture, where per-node outputs are used as scores into a softmax in order to pool the representations across the graph, and feed this graph-level representation to a neural network. The attention mechanism can be conditioned on a "question" (e.g. on a task to predict the shortest path in a graph, the question would be the identity of the beginning and end nodes of the path to find), which is fed to the node scorer of the soft attention mechanism. Moreover, the authors describe how to chain GG-NNs to go beyond predicting individual labels and predict sequences.

Experiments on several datasets are presented. These include tasks where a single output is required (on a few bAbI tasks) as well as tasks where a sequential output is required, such as outputting the shortest path or the Eulerian circuit of a graph. Moreover, experiments on a much more complex and interesting program verification task are presented.

This paper explores the use of so-called Monte Carlo objectives for training directed generative models with latent variables. Monte Carlo objectives take the form of the logarithm of a Monte Carlo estimate (i.e. an average over samples) of the marginal probability $P(x)$. One important motivation for using Monte Carlo objectives is that they can be shown (see the Importance Weighted Variational Autoencoder paper \cite{journals/corr/BurdaGS15} and my notes on it) to correspond to bounds on the true likelihood of the model, and one can tighten the bound simply by drawing more samples in the Monte Carlo objective. Currently, the most successful application of Monte Carlo objectives is based on an importance sampling estimate, which involves training a proposal distribution $Q(h|x)$ in addition to the model $P(x,h)$.

This paper considers the problem of training with gradient descent on such objectives, in the context of a model to which the reparametrization trick cannot be used (e.g. for discrete latent variables). They analyze the sources of variance in the estimation of the gradients (see Equation 5) and propose a very simple approach to reducing the variance of a sampling-based estimator of these gradients. 

First, they argue that gradients with respect to the $P(x,h)$ parameters are less susceptible to problems due to high variance gradients.

Second, and most importantly, they derive a multi-sample estimate of the gradient that is meant to reduce the variance of gradients on the proposal distribution parameters $Q(h|x)$. The end result is the gradient estimate of Equations 10-11. It is based on the observation that the first term of the gradient of Equation 5 doesn't distinguish between the contribution of each sampled latent hi. The key contribution is this: they notice that one can incorporate a variance reducing baseline for each sample hi, corresponding to the Monte Carlo estimate of the log-likelihood when removing hi from the estimate (see Equation 10). The authors show that this is a proper baseline, in that using it doesn't introduce a bias in the estimation for the gradients.

Experiments show that this approach yields better performance than training based on Reweighted Wake Sleep \cite{journals/corr/BornscheinB14} or the use of NVIL baselines \cite{conf/icml/MnihG14}, when training sigmoid belief networks as generative models or as structured output prediction (image completion) models on binarized MNIST.

## General stuff about face recognition

Face recognition has 4 main tasks:

* **Face detection**: Given an image, draw a rectangle around every face
* **Face alignment**: Transform a face to be in a canonical pose
* **Face representation**: Find a representation of a face which is suitable for follow-up tasks (small size, computationally cheap to compare, invariant to irrelevant changes)
* **Face verification**: Images of two faces are given. Decide if it is the same person or not.

The face verification task is sometimes (more simply) a face classification task (given a face, decide which of a fixed set of people it is).

Datasets being used are:

* **LFW** (Labeled Faces in the Wild): 97.35% accuracy; 13 323 web photos of 5 749 celebrities
* **YTF** (YouTube Faces): 3425 YouTube videos of 1 595 subjects
* **SFC** (Social Face Classification): 4.4 million labeled faces from 4030 people, each 800 to 1200 faces
* **USF** (Human-ID database): 3D scans of faces

## Ideas in this paper

This paper deals with face alignment and face representation.

**Face Alignment**

They made an average face with the USF dataset. Then, for each new face, they apply the following procedure:

* Find 6 points in a face (2 eyes, 1 nose tip, 2 corners of the lip, 1 middle point of the bottom lip)
* Crop according to those
* Find 67 points in the face / apply them to a normalized 3D model of a face
* Transform (=align) face to a normalized position

**Representation**

Train a neural network on 152x152 images of faces to classify 4030 celebrities. Remove the softmax output layer and use the output of the second-last layer as the transformed representation.

The network is:

* C1 (convolution): 32 filters of size $11 \times 11 \times 3$ (RGB-channels) (returns $142\times 142$ "images")
* M2 (max pooling): $3 \times 3$, stride of 2  (returns $71\times 71$ "images")
* C3 (convolution): 16 filters of size $9 \times 9 \times 16$ (returns $63\times 63$ "images")
* L4 (locally connected): $16\times9\times9\times16$ (returns $55\times 55$ "images")
* L5 (locally connected): $16\times7\times7\times16$ (returns $25\times 25$ "images")
* L6 (locally connected): $16\times5\times5\times16$ (returns $21\times 21$ "images")
* F7 (fully connected): ReLU, 4096 units
* F8 (fully connected): softmax layer with 4030 output neurons

The training was done with:

* Stochastic Gradient Descent (SGD)
* Momentum of 0.9
* Performance scheduling (LR starting at 0.01, ending at 0.0001)
* Weight initialization: $w \sim \mathcal{N}(\mu=0, \sigma=0.01)$, $b = 0.5$
* ~15 epochs ($\approx$ 3 days) of training


## Evaluation results

* **Quality**:
  * 97.35% accuracy (or mean accuracy?) with an Ensemble of DNNs for LFW
  * 91.4% accuracy with a single network on YTF
* **Speed**: DeepFace runs in 0.33 seconds per image (I'm not sure which size). This includes image decoding, face detection and alignment, **the** feed forward network (why only one? wasn't this the best performing Ensemble?) and final classification output

## See also

* Andrew Ng: [C4W4L03 Siamese Network](https://www.youtube.com/watch?v=6jfw8MuKwpI)