VGG-Net Architecture Explained

The Visual Geometry Group (VGG) models, particularly VGG-16 and VGG-19, have significantly influenced the field of computer vision since their inception. These models, introduced by the Visual Geometry Group from the University of Oxford, stood out in the 2014 ImageNet Large Scale Visual Recognition Challenge (ILSVRC) for their deep convolutional neural networks (CNNs) with a uniform architecture. VGG-19, the deeper variant of the VGG models, has garnered considerable attention due to its simplicity and effectiveness.

This article delves into the architecture of VGG-19, its evolution, and its impact on the development of deep learning models.

Evolution of VGG Models

Before the advent of VGG models, CNN architectures like LeNet-5 and AlexNet laid the groundwork for deep learning in computer vision. LeNet-5, introduced in the 1990s, was one of the first successful applications of CNNs in recognizing handwritten digits. AlexNet, which won the ILSVRC in 2012, marked a significant breakthrough by leveraging deeper architectures and GPU acceleration.

The VGG models were introduced by Karen Simonyan and Andrew Zisserman in their 2014 paper titled "Very Deep Convolutional Networks for Large-Scale Image Recognition." The primary objective was to investigate the effect of increasing the depth of CNNs on large-scale image recognition tasks. VGG-16 and VGG-19, with 16 and 19 weight layers respectively, were among the most notable models presented in the paper. Their design was characterized by using small 3x3 convolution filters consistently across all layers, which simplified the network structure and improved performance.

You can refer to - VGG-16 | CNN model to study the architecture of VGG-16 Architecture.

VGG-19 Architecture

VGG-19 is a deep convolutional neural network with 19 weight layers, comprising 16 convolutional layers and 3 fully connected layers. The architecture follows a straightforward and repetitive pattern, making it easier to understand and implement.

The key components of the VGG-19 architecture are:

1. Convolutional Layers: 3x3 filters with a stride of 1 and padding of 1 to preserve spatial resolution.
2. Activation Function: ReLU (Rectified Linear Unit) applied after each convolutional layer to introduce non-linearity.
3. Pooling Layers: Max pooling with a 2x2 filter and a stride of 2 to reduce the spatial dimensions.
4. Fully Connected Layers: Three fully connected layers at the end of the network for classification.
5. Softmax Layer: Final layer for outputting class probabilities.

Detailed Layer-by-Layer Architecture of VGG-Net 19

The VGG-19 model consists of five blocks of convolutional layers, followed by three fully connected layers. Here is a detailed breakdown of each block:

Block 1

• Conv1_1: 64 filters, 3x3 kernel, ReLU activation
• Conv1_2: 64 filters, 3x3 kernel, ReLU activation
• Max Pooling: 2x2 filter, stride 2

Block 2

• Conv2_1: 128 filters, 3x3 kernel, ReLU activation
• Conv2_2: 128 filters, 3x3 kernel, ReLU activation
• Max Pooling: 2x2 filter, stride 2

Block 3

• Conv3_1: 256 filters, 3x3 kernel, ReLU activation
• Conv3_2: 256 filters, 3x3 kernel, ReLU activation
• Conv3_3: 256 filters, 3x3 kernel, ReLU activation
• Conv3_4: 256 filters, 3x3 kernel, ReLU activation
• Max Pooling: 2x2 filter, stride 2

Block 4

• Conv4_1: 512 filters, 3x3 kernel, ReLU activation
• Conv4_2: 512 filters, 3x3 kernel, ReLU activation
• Conv4_3: 512 filters, 3x3 kernel, ReLU activation
• Conv4_4: 512 filters, 3x3 kernel, ReLU activation
• Max Pooling: 2x2 filter, stride 2

Block 5

• Conv5_1: 512 filters, 3x3 kernel, ReLU activation
• Conv5_2: 512 filters, 3x3 kernel, ReLU activation
• Conv5_3: 512 filters, 3x3 kernel, ReLU activation
• Conv5_4: 512 filters, 3x3 kernel, ReLU activation
• Max Pooling: 2x2 filter, stride 2

Fully Connected Layers

• FC1: 4096 neurons, ReLU activation
• FC2: 4096 neurons, ReLU activation
• FC3: 1000 neurons, softmax activation (for 1000-class classification)

Architectural Design Principles

The VGG-19 architecture follows several key design principles:

1. Uniform Convolution Filters: Consistently using 3x3 convolution filters simplifies the architecture and helps maintain uniformity.
2. Deep Architecture: Increasing the depth of the network enables learning more complex features.
3. ReLU Activation: Introducing non-linearity helps in learning complex patterns.
4. Max Pooling: Reduces the spatial dimensions while preserving important features.
5. Fully Connected Layers: Combines the learned features for classification.

Impact and Legacy of VGG-19

Influence on Subsequent Models

The simplicity and effectiveness of VGG-19 influenced the design of subsequent deep learning models. Architectures like ResNet and Inception drew inspiration from the depth and uniformity principles established by VGG models. VGG-19's deep yet straightforward architecture demonstrated that increasing depth could significantly improve performance in image recognition tasks.

Use in Transfer Learning

VGG-19 has been extensively used in transfer learning due to its robust feature extraction capabilities. Pre-trained VGG-19 models on large datasets like ImageNet are often fine-tuned for various computer vision tasks, including object detection, image segmentation, and style transfer.

Research and Industry Applications

VGG-19 has found applications in numerous research and industry projects. Its architecture has been used as a baseline in academic research, enabling comparisons with newer models. In industry, VGG-19's pre-trained weights serve as powerful feature extractors in applications ranging from medical imaging to autonomous vehicles.

Additional Information about VGGNet-19

1. Model Simplicity and Effectiveness: The VGG-19 architecture's simplicity, characterized by its uniform use of 3x3 convolution filters and repetitive block structure, makes it a highly effective and easy-to-implement model for various computer vision tasks.
2. Computational Requirements: One of the key trade-offs of the VGG-19 model is its computational demand. Due to its depth and the use of small filters, it requires significant memory and computational power, making it more suited for environments with robust hardware capabilities.
3. Robust Feature Extraction: The depth of the VGG-19 model allows it to capture intricate features in images, making it an excellent feature extractor. This capability is particularly useful in transfer learning, where pre-trained VGG-19 models are fine-tuned for specific tasks, leveraging the rich feature representations learned from large datasets.
4. Data Augmentation: To enhance the performance and generalization capability of VGG-19, data augmentation techniques such as random cropping, horizontal flipping, and color jittering are often employed during training. These techniques help the model to better handle variations and improve its robustness.
5. Influence on Network Design: The principles established by the VGG-19 architecture, such as the use of small convolution filters and deep networks, have influenced the design of subsequent state-of-the-art models. Researchers have built upon these concepts to develop more advanced architectures that continue to push the boundaries of what is possible in computer vision.

Conclusion

In conclusion, VGG-19 stands as a landmark model in the history of deep learning, combining simplicity with depth to achieve remarkable performance. Its architecture serves as a foundation for many modern neural networks, highlighting the enduring impact of its design principles on the field of computer vision.