"virtual" screencasted window allowing dynamically changing which window gets shown in screencast

[elkowar]

The problem

In many cases when screensharing, you end up wanting to show the contents of one window primarily, but sometimes wanting to show some other window for context. Currently, the only real solution here is to share your entire screen, which has several drawbacks:

• If you have very high dpi, or a weird aspect-ratio (i.e. 21:9), sharing the entire screen is often a very bad experience for the viewers
• You are also sharing stuff that you might not want to, or might move a window to the shared screen accidentally, leaking some information (yes, this is mostly solved by block-out-from, however doing that for a browser window might be very fragile and not necessarily convenient)
• You're including other stuff around the window, which is distracting and takes up space

The proposed solution

Niri could create a sort of "virtual" window that is only seen by xdg-desktop-portal screencast clients. This window then mirrors the contents of whatever the user currently wants to show, allowing the user to quickly switch what is shown on stream through niri, rather than having to go through the screenshare-menu of the users video-calling software.

I'd propose three different modes here, which should be toggleable via regular actions (-> niri IPC, keybinds, etc):

1. follow-focus: niri always casts the currently focused window. When the user focuses something else, the other window gets shown. Optimally, it should be possible to constrain this to a single workspace and/or display, such that you can still have a secondary monitor or workspace that the screencast does not follow you to.
2. full-screen: the equivalent of just sharing your entire screen. This would be still be useful because it'd allow the user to quickly switch between sharing the entire screen and singular, specific windows
3. fixed / manual: share one specific window, specified via its ID. The user could then set up keybinds or other automations to switch which window is casted exactly how they want to -- i.e. creating a keybind that sets the window focused at that moment as the casted window.

This would work especially well in combination with #1080, as you'd then be able to nicely visualize which window you are sharing -- which is infinitely more important when that window might be changing very frequently and dynamically.

[YaLTeR]

Sounds like a pretty cool idea, though yeah, need to figure out how to toggle between these and configure them nicely.

follow-focus: niri always casts the currently focused window. When the user focuses something else, the other window gets shown. Optimally, it should be possible to constrain this to a single workspace and/or display, such that you can still have a secondary monitor or workspace that the screencast does not follow you to.

Where would you constrain this? Unfortunately, the xdp-gnome dialog only lets us select a "window" from a predefined list.

Do you mean, like, having a global config option for which output to restrict this to or something?

full-screen: the equivalent of just sharing your entire screen. This would be still be useful because it'd allow the user to quickly switch between sharing the entire screen and singular, specific windows

How's this different from just normal screencasting? Even in OBS, there's nowadays only a single PipeWire source that asks the portal for either screen or window:

[elkowar]

I think toggling between those via actions should be reasonably simple to design

Where would you constrain this? Unfortunately, the xdp-gnome dialog only lets us select a "window" from a predefined list.
Do you mean, like, having a global config option for which output to restrict this to or something?
basically, yea, that's what I mean -- although I'd argue that the "only follow focus to windows on one monitor" is a super specific, niche usecase that doesn't necessarily need to be considered in the first impl, if at all. I'd probably be using the manual / fixed mode 99% of the time either way.

How's this different from just normal screencasting? Even in OBS, there's nowadays only a single PipeWire source that asks the portal for either screen or window:

It wouldn't be different from normal screencasting. The main reason I'd want it to be an available mode here is such that we could switch to that mode dynamically without having to restart the stream in Discord/Jitsi/....

My proposal would mean that we could keybind to niri actions like this:

• screencast-set-follow-focus
• screencast-set-follow-focus <output ID> (for the "restrict follow-focus to one monitor" usecase
• screencast-set-casted <window-ID>
• screencast-set-fullscreen

That'd allow us to quickly switch what gets casted to the viewers by just running a couple keybinds, rather than having to restart streams constantly.

[YaLTeR]

So what I'm thinking is: in xdp-gnome UI in the window selector, show a special "niri dynamic window" (or some better wording). Then have an action like screencast-focused-window [--id=ID] (better wording) to explicitly change it. I feel like follow-focus will be way too mistrigger-prone. Also this avoids coming up with some way to restrict outputs or windows or w/e.

I guess we could also add screencast-workspace and screencast-output here?

[elkowar]

that'd be perfect, yea. I do see your point wrt. follow-focus. Having screencast-focused-window + screencast-workspace and screencast-output would be more than enough! <3