Vertical video is helpful on smartphones and vertical video platforms such as TikTok. It is encoded and transmitted for free with the datarhei Restreamer.
YouTube: Quickstart on Raspberry Pi 4 to produce vertical video with filter.
In this video, a 16:9 video is rotated 90° using the datarhei Restreamer streaming server. In the video settings of the active channel, the encoding is changed to 9:16. There are different rotation presets available. In the example, a Raspberry Pi 4 is used, which performs the encoding via the GPU OpenMax.
An encoding of the video is always necessary to get rotated video output.
You have already started your Restreamer and added a video source.
Edit the video source settings.
Various setting options are available besides rotation, also horizontal and vertical flipping.