I'm using frequently unmanned cameras, and frequently those cams deliver a crooked pic (= why do some breed of people have the urgent need to touch a 'lonesome' cam on a tripod??)
To my surprise, there's no 'horizon adjuster' avail (wrong info: there's a 30$ plugin avail ... )
So, to teach me Motion in general and OSCs in detail, I built myself this lil' toy:
Install it into 'Effects', drag it on your clip which needs adjustment, and you're offered just a few knobs'n sliders:
In the Inspector, you can determine the 'thickness' of the two rulers; and a switch to make'em invisible. The OSC lets you move these 100% straight lines, to give you some orientation. Here in my example, the white line of 'the box' should be horizontal.
The other OSC, with the round red arrow, allows you to rotate the frame (to some degree, not meant for repairing vertical videos!); move it up and down; while doing so, some zoom is added to avoid 'black corners' ...
On final delivery, you have to switchOFF the on-screen elements .
When you open that plug-in in Motion, it probably looks a bit .... convoluted ; sorry, I'm no 'pro' in programming, tried to make it international by (mostly) using English descriptions. And if you wonder about the parameters: adjusted by trial'n error over time to make it fit my needs. Feel free to change …
hope, you find it useful...
and tell us/me, if you adjust/change/apply any features!