You can fight the case, your mouth was inside the box, vertical up height.
Your leg cannot smoke so how can say my leg outside the box is smoking...
The purpose of leg is supporting the upper torso only.
Complaint base on thread of counter sue the authority failing to provide adequate railings, min 3 m high in height, around the premise to ensure safety of smokers and cause of foot to lean forward outside the yellow line.
Write or create a blog to write about this and write a formal letter to them, first. This will give them them to think about suing you and to embarrass the government of Singapore.
Ridicule them with alot of make believe assumptions to degrade the fine officer with petulant child behaviour unfit to carry out his duty diligently and integrity of the authority is undermine in the case....
Write to them they hv to response to your letter first and carry out their own investigation before respond to you in letter.
Write as if the fine officer has mental health problem to carry out his offical duty to make good assessment of you and poise you are standing....