# What is an object detection problem with only one class called?

Object detection is defined as the problem in which a model needs to figure out the bounding boxes and the class for each object. A lot of ML solutions for object detection base around having "two passes" - one for creating the bounding box of the region and another for classifying it.

I was wondering if there is a name for a subset of this problem where $$n_{classes} = 1$$. I feel like there is an interesting opportunity here as the whole classification part of the model can (basically) be ignored. Obviously, I can just train a typical object detection model with one class, but was just interested to see if there are any more specialized methods.

If you are talking about "two-stage" obejct detectors like Faster R-CNN, note that the second phase is not only for classification, but to obtain more accurate results (https://stackoverflow.com/a/61965140/4762996).

in addition, I guess training a detector with many classes acts like a regularizer, which results in much better accuracies.

The only benefit of explicitly training with one class could be the reduzed model size (and a corresponding speedup).

Note also that there are one stage object detectors (CornerNet: https://arxiv.org/abs/1808.01244, YOLOv3: https://pjreddie.com/media/files/papers/YOLOv3.pdf, DETR: https://arxiv.org/abs/2005.12872, and many more). I just wanted to stress that just because you only have one class, it does not make sense to take a two-stage detection architecture and skip the second stage.

Finally, have a look at: https://stats.stackexchange.com/q/351079

An interesting direction would be to incorporate one-class classification approaches, e.g. as mentioned here: https://stackoverflow.com/a/61965358/4762996.

• Makes sense, I feel like there is room for improvement by doing it "in one pass". Perhaps this is an area to explore! Jun 6 '20 at 11:49
• Note then that there are also one-stage object detectors. Jun 6 '20 at 11:50