Need clarification about InputCapture barriers

[Sodivad]

Currently the documentation says

For example, consider two zones of size 1920x1080 with offsets 0,0 and 1920,0, respectively. (i.e. a left-right dual-monitor setup). The following pointer barriers are permitted:

top edge of left screen: x1=0, y1=0, x2=1919, y1=0

bottom edge of left screen: x1=0, y1=1080, x2=1919, y1=1080

top edge of right screen: x1=1920, y1=0, x2=3839, y1=0

bottom edge of right screen: x1=1920, y1=1080, x2=3839, y1=1080

left edge of left screen: x1=0, y1=0, x2=0, y1=1079

right edge of right screen: x1=3840, y1=0, x2=3840, y1=1079

This has no info about the right edge of the left screen and left edge of the right screen

My first implementation assumed that

• right edge of the left screen x1=1919, y1=0, x2=1919, y2=1079
• left edge of the right screen x1=1920, y1=0, x2=1920, y2=1079

However when testing I found that input-leap requests

[2024-03-26T09:26:42] DEBUG: Zone at 1920x1080@0,0
[2024-03-26T09:26:42] DEBUG: Barrier (top) 0 at 0,0-1919,0
[2024-03-26T09:26:42] DEBUG: Barrier (right) 1 at 1920,0-1920,1079
[2024-03-26T09:26:42] DEBUG: Barrier (left) 2 at 0,0-0,1079
[2024-03-26T09:26:42] DEBUG: Barrier (bottom) 3 at 0,1080-1919,1080
[2024-03-26T09:26:42] DEBUG: Zone at 1920x1080@1920,0
[2024-03-26T09:26:42] DEBUG: Barrier (top) 4 at 1920,0-3839,0
[2024-03-26T09:26:42] DEBUG: Barrier (right) 5 at 3840,0-3840,1079
[2024-03-26T09:26:42] DEBUG: Barrier (left) 6 at 1920,0-1920,1079
[2024-03-26T09:26:42] DEBUG: Barrier (bottom) 7 at 1920,1080-3839,1080

(disregarding that 0 is an invalid id) it uses 1920,0-1920,1079 for both which means it's not possible to disambiguate both.

Looking at the examples, I think there is a similar problem for the bottom edge if there was a vertical layout, as it says y1=y2=1080 when I would expect 1079 which is used for vertical barriers.

@whot @jadahl

[jadahl]

What a coordinate means was clarified here: #1136.

This has no info about the right edge of the left screen and left edge of the right screen

Hrm, I think that a barrier in between isn't allowed. Can't remember why that was limited. Looking at the GNOME code such a barrier would be denied with a "Adjacent to multiple monitor edges" error message.

[Sodivad]

Yes, I do the same but it changes the code a bit.

[whot]

ftr, 6a29509 changed to no longer specify 0-1920 but 0-1919 instead (and that commit has the reasoning too).

InputLeap's barrier code is extremely simple (I wrote it originally 😄), it just requests any possible barrier and ignores the failed ones. That was easier to write than writing a more complex "can I actually have a barrier here" code but it's also too simple for a layout where the monitors don't align perfectly.
The vertical barrier it requests at 1920 is actually not allowed, you must not have a barrier that is not at the far edge of any direction. the 1920 one would go through the middle of the desktop.

[Sodivad]

ftr, 6a29509 changed to no longer specify 0-1920 but 0-1919 instead (and that commit has the reasoning too).

Shouldn't the same be done for the vertical position
bottom edge of left screen: x1=0, y1=1080, x2=1919, y1=1080, afaiu y=1080 is not on the screen and should be 1079

The vertical barrier it requests at 1920 is actually not allowed, you must not have a barrier that is not at the far edge of any direction. the 1920 one would go through the middle of the desktop.

I am asking in general about the right edges of screens. Currently the example says also
right edge of right screen: x1=3840, y1=0, x2=3840, y1=1079, I think 3840 is not on the screen anymore and should be
x1=3839, y1=0, x2=3839, y1=1079,

Or in the case of a single screen what should be the right edge: x=1920 or x=1919.

I think it's confusing that currently according to the examples different things seem to apply for horizontal/vertical barriers wrt to monitor dimensions

[Sodivad]

I think this would also match the pretty ASCII art in the PR that @jadahl linked and the wording, otherwise the barrier would not be on any screen.

[whot]

paging part of this back in: the barriers are on the left/top of the pixel but expand the whole pixel on the other dimension. Zoomed in, a barrier going to 0/1919 would look like this:

============
    | 1919 |
----+------+

and a barrier at 1920/1079 like this

|    | 1919 ||
+----+------|| 
|    | 1919 || 
+----+------|| 

So a horizontal barrier is always inclusive [x1, x2] (hence it has to stop at 1919, not 1920) and a vertical barrrier is inclusive [y1, y2] (hence it has to stop at 1079, not 1080 - as you said above).

If you put a vertical barrier on 3939 you're putting it on the pixel border between 3938 and 3939 and that should fail, it's not the edge of the screen. In this case you do need to go "over" by one (same for a bottom barrier).

Because we don't allow for overlaps going over is not ambiguous since a 1920 barrier cannot be shared with the right monitor, otherwise it'd fail.

[Sodivad]

Ah ok that is where my confusion stemmed from, the barriers only expanding the whole pixel in their direction but not in the other one. Thanks!

[whot]

I think this can be closed based on #1316 (reply in thread) (don't have the permissions myself)