Why is facial expression analysis in the wild challenging?

Tobias Gehrig, Hazim Kemal Ekenel. Why is facial expression analysis in the wild challenging?. In Abhinav Dhall, Roland Goecke, Jyoti Joshi, Michael Wagner 0004, Tom Gedeon, editors, Proceedings of the 2013 on Emotion recognition in the wild challenge and workshop, EmotiW 2013, Sydney, Australia, December 9, 2013. pages 9-16, ACM, 2013. [doi]

@inproceedings{GehrigE13,
  title = {Why is facial expression analysis in the wild challenging?},
  author = {Tobias Gehrig and Hazim Kemal Ekenel},
  year = {2013},
  doi = {10.1145/2531923.2531924},
  url = {http://doi.acm.org/10.1145/2531923.2531924},
  researchr = {https://researchr.org/publication/GehrigE13},
  cites = {0},
  citedby = {0},
  pages = {9-16},
  booktitle = {Proceedings of the 2013 on Emotion recognition in the wild challenge and workshop, EmotiW 2013, Sydney, Australia, December 9, 2013},
  editor = {Abhinav Dhall and Roland Goecke and Jyoti Joshi and Michael Wagner 0004 and Tom Gedeon},
  publisher = {ACM},
  isbn = {978-1-4503-2564-6},
}