You know the pain of watching a vertical video in a world based on horizontal players. Google took a swing at fixing that with the Google Camera app, which warns users not to do that. Horizon actually fixes the issue by letting you take a proper horizontal video while holding the phone in any orientation. It's magic!
Horizon uses your phone's accelerometer to detect the tilt and adjusts the video frame in real time. The result is that you can rock the phone back and forth, and the frame stays perfectly level. You can even hold the phone in portrait orientation and take a horizontal video. Horizon supports a variety of video resolutions, depending on the device. For example, the G3 allows up to 1440p.
The free version of Horizon is only a demo. You can take 15 second videos and there's a Horizon watermark on them. To get unlimited recording and no watermark, it'll cost you $1.00 via an in-app purchase.