I'm watching a movie and in one of the scenes a person wants to attract public attention and taps against a glass.
The subtitles say "Jemand klopft gegen ein Glas.". But in DWDS it's "an das Glas klopfen".
What is the difference between "gegen ein Glas klopfen" and "an ein Glas klopfen"? Which one is correct in this situation?
»ans Glas klopfen« and »gegen ein Glas klopfen« can describe two different things.
The second one, which you encountered in the movie, is correct here, because in Germany people typically tap their spoon against a glass at larger gatherings to attract attention, e.g. if they want to give a short speech or if they want the wedding couple to kiss (in this case the other guests will join into the glass-tapping).
»ans Glas klopfen« can mean, more generally, to knock on (anything made from) glass, e.g. the glass of a door or window to attract the attention of somebody inside the house.
Gegen in general means a direction (like towards), while an strongly indicates a contact/touch.
An example for this general meaning is:
Die Fenster gehen gegen Osten
In your special case there would be no knocking noise in case you stopped earlier, so it is pretty clear, that gegen here also includes contact (like against).
This is confirmed by DWDS, meaning I/1/d, where it is stated:
bezeichnet die Berührung, den Endpunkt einer zielgerichteten Bewegung, Handlung
So as already stated an is more common and shorter too.
