RESEARCH

End-to-end object detection with Transformers

May 27, 2020

Transformers are a deep learning architecture that has gained popularity in recent years. They rely on a simple yet powerful mechanism called attention, which enables AI models to selectively focus on certain parts of their input and thus reason more effectively. Transformers have been widely applied on problems with sequential data, in particular in natural language processing (NLP) tasks such as language modeling and machine translation, and have also been extended to tasks as diverse as speech recognition, symbolic mathematics, and reinforcement learning. But, perhaps surprisingly, computer vision has not yet been swept up by the Transformer revolution.

To help bridge this gap, we are releasing Detection Transformers (DETR), an important new approach to object detection and panoptic segmentation. DETR completely changes the architecture compared with previous object detection systems. It is the first object detection framework to successfully integrate Transformers as a central building block in the detection pipeline.

DETR matches the performance of state-of-the-art methods, such as the well-established and highly optimized Faster R-CNN baseline on the challenging COCO object detection data set, while also greatly simplifying and streamlining the architecture.

DETR directly predicts (in parallel) the final set of detections by combining a common CNN with a Transformer architecture. During training, bipartite matching uniquely assigns predictions with ground truth boxes. Predictions with no match should yield a “no object” class prediction.

DETR offers a simpler, more flexible pipeline architecture that requires fewer heuristics. Inference can be boiled down to 50 lines of simple Python code using elementary architectural blocks. Moreover, because Transformers have proven to be a powerful tool for dramatically improving performance of models in other domains, we believe additional performance gains and improved training efficiency will be possible with additional tuning.

We are providing the source code as well as pretrained models in PyTorch here.

Reframing the task of object detection

DETR casts the object detection task as an image-to-set problem. Given an image, the model must predict an unordered set (or list) of all the objects present, each represented by its class, along with a tight bounding box surrounding each one.

This formulation is particularly suitable for Transformers. We chain a convolutional neural network (CNN), which extracts the local information from the image, with a Transformer encoder-decoder architecture, which reasons about the image as a whole and then generates the predictions.

Traditional computer vision models typically use a complex, partly handcrafted pipeline that relies on custom layers in order to localize objects in an image and then extract features. DETR replaces this with a simpler neural network that offers a true end-to-end deep learning solution to the problem.

Traditional two-stage detection systems, such as Faster R-CNN, predict object bounding boxes by filtering over a large number of coarse candidate regions, which are generally a function of the CNN features. Each selected region is used in a refinement step, which involves cropping the CNN features in the location defined by the region, classifying each region independently, and refining its location. Finally, a non-maximum suppression step is applied to remove duplicate boxes. DETR simplifies the detection pipeline by leveraging a standard Transformer architecture to perform the (potentially non-differentiable) operations that are traditionally specific to object detection.

The DETR framework consists of a set-based global loss, which forces unique predictions via bipartite matching, and a Transformer encoder-decoder architecture. Given a fixed small set of learned object queries, DETR reasons about the relations of the objects and the global image context to directly output the final set of predictions in parallel. Previous attempts to use architectures such as recurrent neural networks for object detection were much slower and less effective, because they made predictions sequentially rather than in parallel.

Transformers’ self-attention mechanisms allow DETR to perform global reasoning on the image as well as on the specific objects that are predicted. For example, the model may look at other regions of the image to help make a decision about the object in a bounding box. It can also make predictions based on relationships or correlations between objects in an image. If DETR predicts that an image contains a person standing on the beach, for example, it knows that a partially occluded object is more likely to be a surfboard. In contrast, other detection models predict each object in isolation.

We also demonstrate that this pipeline can be extended to related tasks such as panoptic segmentation, which aims at segmenting distinct foreground objects while simultaneously labeling all the pixels from the background. DETR treats foreground items, such as animals or people, and background items, such as sky or grass, in a truly unified manner.

Advancing toward a unified approach for NLP and computer vision tasks

Using Transformers in vision may help bridge the gap between the NLP and computer vision communities, both in terms of research innovations and in the way information is reasoned about. There is significant interest in tasks that involve simultaneously handling visual and textual inputs, such as with Facebook’s recently launched Hateful Memes Challenge. These bimodal tasks have been historically known to be difficult with conventional architectures.

DETR’s novel architecture can also help improve the interpretability of computer vision models. Because it relies on attention mechanisms, it is easy to see what parts of an image the network is looking at when it makes a prediction.

We hope that DETR will help enable progress in object detection and lead to new methods for bimodal tasks. We’re excited to explore these applications and to see how others leverage this approach.

Written By

Nicolas Carion

Research Assistant

Sergey Zagoruyko

Researcher

Francisco Massa

Research Engineer