Crop video that has been taken the vertical way on iPhone I frequently capture video holding my phone as usual, which means vertically. 
But when I want to watch it on a computer screen, I would like to have it full screen - even if it means loosing the top and bottom of the scene.

I'm sure there a lot of softwarz that do this on a mac but I wanted to be able to do it directlty in my picture library of my phone.

Is there an app that makes it ? Or is this an hiddent trick to do it with the Photo App?
no not really, you could find a third party app, doing this on a comp is easy however! why not use it in landscape to begin with? it drives me crazy when people film in portrait, why? because almost every screen you watch your media on is "landscape"