I found this is not very useful especially when we're annotating
concave-shaped objects.

Windows by default uses GB2312.

For old files, you can use iconv:

  iconv -f gb18030 -t utf-8 1.json > 1_utf8.json

https://github.com/wkentaro/labelme/issues/687

* Added a tool to adjust image brightness and contrast. Useful for dark images.

* Refactor brightness and contrast adjuster

* Fix syntax error in Python2
Co-authored-by: NKentaro Wada <www.kentaro.wada@gmail.com>