Pi 4 - force HDMI preview to both monitors

[root-hal9000]

Is there a way to force the HDMI Preview to go out to both monitors on Raspberry Pi 4?
Both monitors are active at boot (I have hdmi_allow_hotplug:0=1 and hdmi_allow_hotplug:1=1 set), but the preview only shows on HDMI 0.

Thanks for all your work!

[root-hal9000]

I see on this commit that enabled HDMI preview that there is a line for hdmi_preview 0 in etc/raspimjpeg/raspimjpeg.1 --- Is it a matter of adding HDMI_preview 1 to have it go to second monitor?

EDIT: I realize the stupidity of what I just said, since that's just a boolean (I was looking at the camera option above that line and thought it was a selector in that sense.)

I am looking at raspimjpeg right now, I see this commit where they added a selector for which monitor, but it doesn't seem like there is an option for both, I might follow up there

[roberttidey]

I think hdmi_preview just disables/enables hdmi_preview (0 0ff 1 on).

Normally the camera stream goes to a mjpeg stream generator for the web based preview. If hdmi_preview is 1 then a software splitter is used to also send the camera stream to the hdmi port.

I am not sure at the moment about the second monitor as this was done prior to dual monitor support. I'll have a look.

[root-hal9000]

thanks @roberttidey - yep, I edited my comment after I saw that, my mistake, a little too eager. I am looking into the docs/code over there as well to see if I can get something out of it, but I appreciate your much more experienced eyes on it. I know this might be a little niche (I am yet again using rpi_cam_web_interface to run an interactive art installation), but I figured that since the option to pick output is there on the raspicam side, there might be some possibility. Thanks again!

[roberttidey]

Not sure whether this will work but worth a try. It needs both monitors to be the same resolution.

If you go to applications/preferences/screen configuration
Now position the screens on top of each other then apply the change. Reboot.

That is supposed to duplicate the screens.