What are the advantages of setting proj_xy?

[vansin]

mmrotate/mmrotate/models/task_modules/coders/delta_xywht_rbbox_coder.py

Lines 165 to 170 in 4922856

	if proj_xy:
	dx = (torch.cos(pt) * (gx - px) + torch.sin(pt) * (gy - py)) / pw
	dy = (-torch.sin(pt) * (gx - px) + torch.cos(pt) * (gy - py)) / ph
	else:
	dx = (gx - px) / pw
	dy = (gy - py) / ph

oc define is False, and le90 and le135 define is True

[yangxue0827]

这个应该是有一段历史的：
oc定义法应该是我最早在做舰船检[1]的时候引入的，当时采用的框的编解码是最简单那种，也就是对应proj_xy=False；
le135和le90其实都是长边定义法只不过取值范围不同，le135最早我实在RRPN[2]文章中看到的，至于它们都用了proj_xy=True，是出自RoI Transformer[3]文章的设置：


主要是考虑相对偏移量。

le135和le90之前其实还有一个差异，就是edge_swap，le135设置的是False，le90则是True。总结一下就是
oc：edge_swap=False, proj_xy=False
le90: edge_swap=True, proj_xy=True
le135: edge_swap=False, proj_xy=True
mmrotate都是按照原始出处设置的，其实所有oc的方法采用le90或le135的设置都会有性能的提升。

[1] Automatic ship detection in remote sensing images from google earth of complex scenes based on multiscale rotation dense feature pyramid networks, Remote Sensing, 2018
[2] Arbitrary-Oriented Scene Text Detection via Rotation Proposals, TMM, 2018
[3] Learning RoI Transformer for Detecting Oriented Objects in Aerial Images, CVPR, 2019