# SimSiam¶

## Abstract¶

Siamese networks have become a common structure in various recent models for unsupervised visual representation learning. These models maximize the similarity between two augmentations of one image, subject to certain conditions for avoiding collapsing solutions. In this paper, we report surprising empirical results that simple Siamese networks can learn meaningful representations even using none of the following: (i) negative sample pairs, (ii) large batches, (iii) momentum encoders. Our experiments show that collapsing solutions do exist for the loss and structure, but a stop-gradient operation plays an essential role in preventing collapsing. We provide a hypothesis on the implication of stop-gradient, and further show proof-of-concept experiments verifying it. Our “SimSiam” method achieves competitive results on ImageNet and downstream tasks. We hope this simple baseline will motivate people to rethink the roles of Siamese architectures for unsupervised representation learning.

## Results and Models¶

In this page, we provide benchmarks as much as possible to evaluate our pre-trained models. If not mentioned, all models are pre-trained on ImageNet-1k dataset.

### Classification¶

The classification benchmarks includes 4 downstream task datasets, VOC, ImageNet, iNaturalist2018 and Places205. If not specified, the results are Top-1 (%).

#### VOC SVM / Low-shot SVM¶

The Best Layer indicates that the best results are obtained from which layers feature map. For example, if the Best Layer is feature3, its best result is obtained from the second stage of ResNet (1 for stem layer, 2-5 for 4 stage layers).

Besides, k=1 to 96 indicates the hyper-parameter of Low-shot SVM.

Self-Supervised Config Best Layer SVM k=1 k=2 k=4 k=8 k=16 k=32 k=64 k=96
resnet50_8xb32-coslr-100e feature5 84.64 39.65 49.86 62.48 69.50 74.48 78.31 81.06 82.56
resnet50_8xb32-coslr-200e feature5 85.20 39.85 50.44 63.73 70.93 75.74 79.42 82.02 83.44

#### ImageNet Linear Evaluation¶

The Feature1 - Feature5 don’t have the GlobalAveragePooling, the feature map is pooled to the specific dimensions and then follows a Linear layer to do the classification. Please refer to resnet50_mhead_linear-8xb32-steplr-90e_in1k for details of config.

The AvgPool result is obtained from Linear Evaluation with GlobalAveragePooling. Please refer to resnet50_linear-8xb512-coslr-90e_in1k for details of config.

Self-Supervised Config Feature1 Feature2 Feature3 Feature4 Feature5 AvgPool
resnet50_8xb32-coslr-100e 16.27 33.77 45.80 60.83 68.21 68.28
resnet50_8xb32-coslr-200e 15.57 37.21 47.28 62.21 69.85 69.84

#### Places205 Linear Evaluation¶

The Feature1 - Feature5 don’t have the GlobalAveragePooling, the feature map is pooled to the specific dimensions and then follows a Linear layer to do the classification. Please refer to resnet50_mhead_8xb32-steplr-28e_places205.py for details of config.

Self-Supervised Config Feature1 Feature2 Feature3 Feature4 Feature5
resnet50_8xb32-coslr-100e 21.32 35.66 43.05 50.79 53.27
resnet50_8xb32-coslr-200e 21.17 35.85 43.49 50.99 54.10

#### ImageNet Nearest-Neighbor Classification¶

The results are obtained from the features after GlobalAveragePooling. Here, k=10 to 200 indicates different number of nearest neighbors.

Self-Supervised Config k=10 k=20 k=100 k=200
resnet50_8xb32-coslr-100e 57.4 57.6 55.8 54.2
resnet50_8xb32-coslr-200e 60.2 60.4 58.8 57.4

### Detection¶

The detection benchmarks includes 2 downstream task datasets, Pascal VOC 2007 + 2012 and COCO2017. This benchmark follows the evluation protocols set up by MoCo.

#### Pascal VOC 2007 + 2012¶

Please refer to faster_rcnn_r50_c4_mstrain_24k_voc0712.py for details of config.

Self-Supervised Config AP50
resnet50_8xb32-coslr-100e 79.80
resnet50_8xb32-coslr-200e 79.85

#### COCO2017¶

resnet50_8xb32-coslr-100e 38.6 57.6 42.3 34.6 54.8 36.9
resnet50_8xb32-coslr-200e 38.8 58.0 42.3 34.9 55.3 37.6

### Segmentation¶

The segmentation benchmarks includes 2 downstream task datasets, Cityscapes and Pascal VOC 2012 + Aug. It follows the evluation protocols set up by MMSegmentation.

#### Pascal VOC 2012 + Aug¶

Please refer to fcn_r50-d8_512x512_20k_voc12aug.py for details of config.

Self-Supervised Config mIOU
resnet50_8xb32-coslr-100e 48.35
resnet50_8xb32-coslr-200e 46.27

## Citation¶

@inproceedings{chen2021exploring,
title={Exploring simple siamese representation learning},
author={Chen, Xinlei and He, Kaiming},
booktitle={CVPR},
year={2021}
}