Comment by GranPC
18 hours ago
This is hugely dependent on whether the camera supports clean HDMI output - that is, without overlays. My Canon camera for example insists on showing a focus square over HDMI no matter what, and it is impossible to disable.
You can remove it by installing magic lantern. It lets me use my old 650D as a second camera.
Unfortunately there is no port of ML to my specific model. I did some porting work myself by running the camera firmware in QEMU, but to be able to run it on hardware I apparently needed some signing key that only the Magic Lantern lead dev has. By the time I was doing all of this he was busy with real world stuff so ultimately I just borrowed a friend's Nikon camera.
ML doesn't work on a lot of cameras - yet. It's quite far behind the last generation of SLRs and stays away from the flagship models.
The particular camera he's talking about, the G5 X Mark II, does support clean HDMI out. I used to use it as my webcam.