People Detection: Deep Learning Approaches

Deep Learning for Object Detection

• People detections is a special case of object detection (one of the most challenging object classes to detect)
• Recently, most detectors are trained for the more challenging task of multi-object detection
  • Goal: Given an image, detect all instances of, say, 1000 different object classes
  • “Person” always one of the classes
• Speed is an issue
  • Sliding Window: Look at each position, each scale
  • Cascades look at each position too
    • They just take a shorter look at most positions/scales
  • Region Proposals: Avoid useless positions/scales from the beginning

Region Proposals

• 💡Idea

  • Identify image regions that are likely to contain an object
  • Don’t care about the object class in the regions at this point

• Characterization of a general object

  • Find “blobby” regions
  • Find connected regions that are somehow distinct from their surroundings

• Requirements

  • FAST!!!
  • High recall
  • Can allow a relatively high amount of false positives

• 2 main categories

  • Grouping methods

    • Generate proposals based on hierarchically grouping meaningful image regions

    • Often better localization

    • E.g. Selective search

      

  • Window scoring methods

    • Generate a large amount of windows
    • Use a quickly computed cue to discard unlikely windows (“objectness” measure)
    • Often faster

R-CNN ¹

Idea and structure

Training

1. Train AlexNet on ImageNet (1000 classes)

   

2. Re-initialize last layers to a different dimension (depending on the #classes of the new classifier) and train new model

   

3. Train a classifier

   • Binary SVMs (e.g. is human? yes/no) for each object class $\rightarrow$ $C$ SVMs in our case

   • The outputs of pool5 of the retrained AlexNet are used as features

     

4. Improve the region proposals

   • Use a regression model to improve the estimated locatin of the object

     • Input: features of proposed region (pool5)
     • output: x, y, width, height of the estimated region

     

Downsides

1. Speed: Need to forward-pass EACH region proposal through entire CNN!!!

2. SVM & BBox regressor are trained after CNN is fixed

   • No simultaneous update/adaptation of CNN features possible

3. Complexity: multi-stage approach

Improvement:

• For 1: Can we make (part of) the CNN run only once for all proposals?

• For 2&3: Can we make the CNN perform these steps?

Fast R-CNN ²

Overview

ROI pooling

• Conv layers don’t care about input size, FC layers do
• ROI pooling: warp the variable size ROIs into in a predefined fix size shape.

End-to-end training

• Instead of SVM & Regressor just add corresponding losses and train the system for both (multitask)
• Gradients can backprop. into feature layers through ROI pooling layers (just as with normal maxpool layers)
• End-to-end brings slight improvement 👏
• Softmax (integrated) loss slightly but consistently outperforms external classifier 👏

Fast R-CNN vs R-CNN

Speed:

Downsides

• Majority of time is lost for region proposals

• Model is also not fully end-to-end: proposals come from “outside”

  (Can we include them in the CNN as well? 🤔)

Faster R-CNN³

Overview

Region Proposal Network (RPN)

• Input: Feature map from larger conv network of size $C \times W \times H$

• Output

  • List of $p$ proposals
  • “Objectness” score of size $p \times 6$
    • $p \times 4$ coordinates (top-left and bottom-right $(x,y) $ coordinates) for bounding box
    • $p \times 2$ for objectness (with vs. without object) per location

• General approach:

  • Take a mini net (RPN) and slide it over the feature map (stepsize 1)
  • At each position evaluate $k$ different window sizes for objectness
  • Results in approx. $W \times H \times k$ windows/proposals

  

• Fully convolutional network

• Anchors: tackle the scale problem of the feature map

  • Initial reference boxes consisting of aspect ratio and scale, centered at sliding window
  • 3 scales and 3 aspect ratios = 9 anchors

• Layers

  • reg layer: regression of the reference anchor
  • cls layer: object/no object score

Loss

• Need a label for each anchor to train the objectness classification

• Labelling anchors

  • Positive: highest IoU with groundtruth or IoU > 0.7 (can be more than one)
    • Also store the association between anchor and groundtruth box
  • Negative: others, if their IoU < 0.3
  • Other anchors do not contribute to training

  $\rightarrow$ Convert to classification problem

• RPN multitask loss:

  

  • $N\_{cls}$: Batch size (256)
  • $N\_{reg}$: number of window positions ($\approx$ 2400)
  • $\lambda = 10$

Training

As in paper

Jointly

• Train everything in one go

• Combination of four losses

  • objectness classification
  • anchor regression
  • object class classification
  • detection regression
  

Why two regression losses?

Anchor regression directly impacts the feature used for detection. Detection regression merely improves final localization

Comparison between all the R-CNNs

SSD Detector ⁴

Motivation

Thus far, deep multiclass detectors rely on variants of three steps:

• generate bounding boxes (proposals)
• resample pixels/features in boxes to uniform size
• apply high quality classifier

Can we avoid / speed up any of those steps to increase overall speed?

Overview

• 💡Core Idea: Use a set of fixed default boxes at each position in a feature map (similar to anchors)
• Classify object class and box regression for each default box
• pply boxes at different layers in the ConvNet
  • Use layers of different sizes
  • Avoids the need for rescaling

Structure

• Detectors at various stages with varying numbers of default boxes
• Resulting number of detections is fixed
• Reduced by non maximum suppression