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Title: Computation Reallocation for Object Detection

Paper content:
The allocation of computation resources in the backbone is a crucial issue in object detection.
However, classification allocation pattern is usually adopted directly to object detector, which is proved to be sub-optimal.
In order to reallocate the engaged computation resources in a more efficient way, we present CR-NAS (Computation Reallocation Neural Architecture Search) that can learn computation reallocation strategies across different feature resolution and spatial position diectly on the target detection dataset.
A two-level reallocation space is proposed for both stage and spatial reallocation.
A novel hierarchical search procedure is adopted to cope with the complex search space.
We apply CR-NAS to multiple backbones and achieve consistent improvements.
Our CR-ResNet50 and CR-MobileNetV2 outperforms the baseline by 1.9% and 1.7% COCO AP respectively without any additional computation budget.
The models discovered by CR-NAS can be equiped to other powerful detection neck/head and be easily transferred to other dataset, e.g. PASCAL VOC, and other vision tasks, e.g. instance segmentation.
Our CR-NAS can be used as a plugin to improve the performance of various networks, which is demanding.
Object detection is one of the fundamental tasks in computer vision.
The backbone feature extractor is usually taken directly from classification literature (Girshick, 2015; Ren et al., 2015; Lin et al., 2017a; Lu et al., 2019) .
However, comparing with classification, object detection aims to know not only what but also where the object is.
Directly taking the backbone of classification network for object detectors is sub-optimal, which has been observed in .
To address this issue, there are many approaches either manually or automatically modify the backbone network.
proposes a neural architecture search (NAS) framework for detection backbone to avoid expert efforts and design trails.
However, previous works rely on the prior knowledge for classification task, either inheriting the backbone for classification, or designing search space similar to NAS on classification.
This raises a natural question: How to design an effective backbone dedicated to detection tasks?
To answer this question, we first draw a link between the Effective Receptive Field (ERF) and the computation allocation of backbone.
The ERF is only small Gaussian-like factor of theoretical receptive field (TRF), but it dominates the output (Luo et al., 2016) .
The ERF of image classification task can be easily fulfilled, e.g. the input size is 224×224 for the ImageNet data, while the ERF of object detection task need more capacities to handle scale variance across the instances, e.g. the input size is 800×1333 and the sizes of objects vary from 32 to 800 for the COCO dataset.
Lin et al. (2017a) allocates objects of different scales into different feature resolutions to capture the appropriate ERF in each stage.
Here we conduct an experiment to study the differences between the ERF of several FPN features.
As shown in Figure 1 , we notice the allocation of computation across different resolutions has a great impact on the ERF.
Furthermore, appropriate computation allocation across spacial position (Dai et al., 2017) boost the performance of detector by affecting the ERF.
Figure 1: Following the instructions in Luo et al. (2016) , we draw the ERF of FPN in different resolution features.
The size of base plate is 512×512, with respective anchor boxes ({64, 128, 256} for {p 3 , p 4 , p 5 }) drawn in.
The classification CNNs ResNet50 tends to have redundant ERF for high resolution features p 3 and limited ERF for low resolution features p 5 .
After stage reallocation, our SCR-ResNet50 has more balanced ERF across all resolutions which leads to a high performance.
Based on the above observation, in this paper, we aim to automatically design the computation allocation of backbone for object detectors.
Different from existing detection NAS works (Ghiasi et al., 2019; Ning Wang & Shen, 2019 ) which achieve accuracy improvement by introducing higher computation complexity, we reallocate the engaged computation cost in a more efficient way.
We propose computation reallocation NAS (CR-NAS) to search the allocation strategy directly on the detection task.
A two-level reallocation space is conducted to reallocate the computation across different resolution and spatial position.
In stage level, we search for the best strategy to distribute the computation among different resolution.
In operation level, we reallocate the computation by introducing a powerful search space designed specially for object detection.
The details about search space can be found in Sec. 3.2.
We propose a hierarchical search algorithm to cope with the complex search space.
Typically in stage reallocation, we exploit a reusable search space to reduce stage-level searching cost and adapt different computational requirements.
Extensive experiments show the effectiveness of our approach.
Our CR-NAS offers improvements for both fast mobile model and accurate model, such as ResNet (He et al., 2016) , MobileNetV2 (Sandler et al., 2018) , ResNeXt (Xie et al., 2017) .
On the COCO dataset, our CR-ResNet50 and CR-MobileNetV2 can achieve 38.3% and 33.9% AP, outperforming the baseline by 1.9% and 1.7% respectively without any additional computation budget.
Furthermore, we transfer our CR-ResNet and CR-MobileNetV2 into the another ERF-sensitive task, instance segmentation, by using the Mask RCNN framework.
Our CR-ResNet50 and CR-MobileNetV2 yields 1.3% and 1.2% COCO segmentation AP improvement over baseline.
To summarize, the contributions of our paper are three-fold:
• We propose computation reallocation NAS(CR-NAS) to reallocate engaged computation resources.
To our knowledge, we are the first to dig inside the computation allocation across different resolution.
• We develop a two-level reallocation space and hierarchical search paradigm to cope with the complex search space.
Typically in stage reallocation, we exploit a reusable model to reduce stage-level searching cost and adapt different computational requirements.
• Our CR-NAS offers significant improvements for various types of networks.
The discovered models show great transferablity over other detection neck/head, e.g. NAS-FPN (Cai & Vasconcelos, 2018) , other dataset, e.g. PASCAL VOC (Everingham et al., 2015) and other vision tasks, e.g. instance segmentation .
In this paper, we present CR-NAS (Computation Reallocation Neural Architecture Search) that can learn computation reallocation strategies across different resolution and spatial position.
We design a two-level reallocation space and a novel hierarchical search procedure to cope with the complex search space.
Extensive experiments show the effectiveness of our approach.
The discovered model has great transfer-ability to other detection neck/head, other dataset and other vision tasks.
Our CR-NAS can be used as a plugin to other detection backbones to further booster the performance under certain computation resources.

assistant: text: We propose CR-NAS to reallocate engaged  computation resources in different resolution and spatial position.