# Review — ResNeSt: Split-Attention Networks

## Outperforms EfficientNet

ResNeSt: Split-Attention NetworksResNeSt, by Facebook, UC Davis, Snap, Amazon, ByteDance, and SenseTime2020 arXiv v2, Over 500 Citations(Sik-Ho Tsang @ Medium)

Image Classification, Residual Network, ResNet, ResNeXt, Object Detection, Semantic Segmentation, Instance Segmentation

- ResNeSt is proposed, in which a modularized architecture is designed to apply the
**channel-wise attention on different network branches.**to leverage their success in**capturing cross-feature interactions**and**learning diverse representations**.

# Outline

**ResNeSt****Experimental Results**

**1. ResNeSt**

## 1.1. ResNeSt Block

- In prior
**ResNeXt****the number of featuremap groups**is given by**a cardinality hyperparameter**.*K*

In this

ResNeSt,a new radix hyperparameterthat indicates the number of splits within a cardinal group, soRthe total number of feature groupsis.G=KR

- (Setting radix
*R*=2, the Split-Attention block applies SKNet-like attention to each cardinal group.) - Thus, there is
**a series of transformations {**to each individual group, then the intermediate representation of each group is*F*1,*F*2, …,*FG*}, for*Ui*=*Fi*(*X*)*i*∈{1, 2, …,*G*}. **Global contextual information**with embedded**channel-wise statistics**can be gathered with global average pooling across spatial dimensions. The*c*-th component is calculated as:

- This attention idea is similar to the one in SENet.
- And
**a weighted fusion of the cardinal group representation**has a size of*Vk**H*×*W*×*C*/*K*is aggregated using**channel-wise soft attention**, where**each featuremap channel is produced using a weighted combination over splits**. Then the*c*-th channel is calculated as:

- where
denotes a*aki*(*c*)**(soft) assignment weight**given by:

- and mapping
*Gci*determines the weight of each split for the*c*-th channel based on the global context representation*sk*. **The final output**of the proposed Split-Attention block is produced using a*Y***shortcut connection:***Y*=*V*+*X*.- Practically,
**the group transformation**is a*Fi***1×1 convolution followed by a 3×3 convolution**. - And
**the attention weight function**is parameterized using*G***two fully connected layers with ReLU**activation. - Though the cardinality-major implementation is straightforward and intuitive, but is
**difficult to modularize and accelerate using standard CNN operators**. A radix-major implementation of ResNeSt block is proposed as follows.

## 1.2. Radix-Major Implementation of ResNeSt Block

- The featuremap groups with
**same radix index but different cardinality are next to each other physically**. A**summation across different splits**is conducted, so that the featuremap groups with the same cardinality-index but different radixindex are**fused together**. - A
**global pooling layer**aggregates over the spatial dimension. - Then
**two consecutive fully connected (FC or dense) layers**with number of groups equal to cardinality are added after pooling layer to predict the attention weights for each splits.

With this implementation, the

first 1×1 convolutional layers can be unified into one layerand the 3×3 convolutional layerscan be implementedusinga single grouped convolution with the number of groups ofTherefore, the Split-Attention block isRK.modularized using standard CNN operators.

# 2. Experimental Results

## 2.1. Image Classification

- mixup is used. AutoAugment is used.
- For example
**2s2x40d**denotes**radix=2**,**cardinality=2**and**width=40**.

Increasing the radix from 0 to 4 continuously improves the top-1 accuracy, while also increasing latency and memory usage.

**Split-Attention with the 2s1x64d setting is used**in the following experiments.

ResNeSt model displays the

best tradeoff.

## 2.2. Object Detection

Compared to the baselines using standard ResNet, ResNeSt is able to

boost mean average precision by around 3%on both Faster R-CNNs and Cascade R-CNNs.

## 2.3. Instance Segmentation

**For****Mask R-CNN****, ResNeSt50 outperforms the baseline with a gain of 2.85%/2.09% for box/mask performance**, and**ResNeSt101**exhibits**even better**improvement of**4.03%/3.14%.****For****Cascade R-CNN****, the gains**produced by switching to**ResNeSt50 or ResNeSt101**are**3.13%/2.36% or 3.51%/3.04%**, respectively.

## 2.4. Semantic Segmentation

**DeepLabv3****model using ResNeSt-50**backbone already achieves**better performance**than DeepLabv3 with a much larger ResNet-101 backbone.

## 2.5. More Detailed Results in Appendix of the Paper

- More results are shown in appendix of the paper. Please feel free to read the paper directly.

## Reference

[2020 arXiv] [ResNeSt]

ResNeSt: Split-Attention Networks

## Image Classification

**1989–2019 … 2020**: [Random Erasing (RE)] [SAOL] [AdderNet] [FixEfficientNet] [BiT] [RandAugment] [ImageNet-ReaL] [ciFAIR] [ResNeSt]**2021**: [Learned Resizer] [Vision Transformer, ViT] [ResNet Strikes Back] [DeiT] [EfficientNetV2] [MLP-Mixer] [T2T-ViT] [Swin Transformer]