Guys,
The reason is to use display rotation like that is to have the monitor oriented vertically, meaning rotate it 90 or 270 degrees, to run the application in a kiosk.
The first natural action you wan to make is to rotate the whole interface in the operating system. As you can check in other thread, the experiments demonstrated that this rotation of the operating system alone reduces the AppGameKit game framerate in a half for the raspberry pi 4 with buster, bullseye or bookworm. I didn't have any reduction on the Raspberry pi 3b+ with Jessie.
In the raspberry pi 4, making the application full screen reduced the framerate by another 50%. So, OS display rotated plus fullscreen made the FPS only 25% of the maximum FPS in landscape and not full screen.
In other words, the reason is not to lose FPS in such a dramatic percentage.
Yeshu777 is looking for alternatives to rotate everything in code, so it is just a matter of physically rotating the monitor but not the OS interface.
VirtualNomad, do you have a simple example of how to achieve this with RenderToImage() to render 2d sprites and animation? (Not the Snow flakes from the example projects) In theory it was the ideal thing to do (render to the image and then rotate camera) but I could not make the 2D world animation to an image, for sure am doing something wrong. In my test, 2D always render in the normal orientation, on top of 3D. Even if successful, we need to keep an acceptable Frame rate.
Let's go to Mars