My iMac has 2GB of GPU memory because it’s for home-use.
It’s insufficient size for larger image output.
It can output a little bit larger result when its display resolution set down to 640px x 480px.
It still needed to split images into 4 parts for 720p high quality video.
It caused a side effect which each frame has a border at the joint of the parts."
You seem to be correct on why the frame is split into four quadrants.
My iMac has 2GB of GPU memory because it’s for home-use. It’s insufficient size for larger image output. It can output a little bit larger result when its display resolution set down to 640px x 480px.
It still needed to split images into 4 parts for 720p high quality video.
It caused a side effect which each frame has a border at the joint of the parts."
You seem to be correct on why the frame is split into four quadrants.