What a day. Spent the day hunting a bug in my code, only to find out that it wasn’t in my code. (Update: Turns out my code was wrong in two ways, more below the original entry.) (Update 2: There are options to set this in the Arduino IDE Preferences)
There’s an error in the sprintf and snprintf implementation on Arduino that occurs when more than 8 varargs are passed in after the format specifier.
The above command will rescale the video to a quarter of Full HD resolution in portrait orientation, set a duration of 30 seconds (the -t option) and set the output video stream 0.0 framerate to 30 frames per second.