<p>We directly use a pre-trained VGGNet or ResNet backbone. Although the backbone is pre-trained for classification task, the hidden layers learn features which can be used for object detection also. Initial layers will learn low level features such as lines, dots, curves etc. Next layer will learn learn high-level features that are built on top of low-level features to detect objects and larger shapes in the image.</p>
<p><a href="https://i.stack.imgur.com/K03Wx.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/K03Wx.jpg" alt="enter image description here" /></a></p>
<p>Then the last layers are modified to output the object detection coordinates rather than class.</p>
<p>There are object detection specific backbones too. Check these papers:</p>
<ul>
<li><a href="https://arxiv.org/abs/1804.06215" rel="nofollow noreferrer">DetNet: A Backbone network for Object Detection</a></li>
<li><a href="https://github.com/PKUbahuangliuhe/CBNet" rel="nofollow noreferrer">CBNet: A Novel Composite Backbone Network Architecture for Object Detection</a></li>
<li><a href="https://papers.nips.cc/paper/2019/file/228b25587479f2fc7570428e8bcbabdc-Paper.pdf" rel="nofollow noreferrer">DetNAS: Backbone Search for Object Detection</a></li>
<li><a href="https://www.microsoft.com/en-us/research/blog/high-resolution-network-a-universal-neural-architecture-for-visual-recognition/" rel="nofollow noreferrer">High-Resolution Network: A universal neural architecture for visual recognition</a></li>
</ul>
<p>Lastly, the pretrained weights will be useful only if you are using them for similar images. E.g.: weights trained on Image-net will be useless on ultrasound medical image data. In this case we would rather train from scratch.</p>
Backbone network in Object detection
<p>I am trying to understand the training process of a object deetaction deeplearng algorithm and I am having some problems understanding how the backbone network (the network that performs feature extraction) is trained.</p>
<p>I understand that it is common to use CNNs like AlexNet, VGGNet, and ResNet but I don't understand if these networks are pre-trained or not. If they are not trained what does the training consist of?</p>