The presence camera

The camera is not a second way into Unreal. It connects to the same control channel the tablet uses and sends the same session command as the tablet’s Start and Stop buttons. That gives it everything the tablet already has:

It sends changes only — somebody arrived, somebody left — never a repeated "still here". So an operator who presses Stop is not overruled a moment later by a visitor who has not walked away yet. When its connection comes back, after Unreal restarts for instance, it states the current answer once more, so a visitor who arrived during the restart is not left facing a kiosk that never woke up.

Like any client, it counts against MaxClients (four by default).

On the Unreal machine, run it with no arguments. It finds the control channel on that machine, ws://127.0.0.1:9010/link, and a connection from the Unreal machine needs no token.

Supervise it like the rest of the kiosk: point Promethist Watchdog, or whatever restarts your programs, straight at vision.exe. No launcher script is needed. It keeps retrying until Unreal answers, so the start-up order does not matter. When it cannot run at all — no camera, say — it exits with an error rather than sitting idle, so the supervisor sees a failure.

Started hidden — Watchdog’s hidden visibility — it opens no window at all, not even for a moment. An uncalibrated camera then still runs, detecting nothing and telling Unreal nothing, rather than refusing to start as it does with --no-window. To aim or calibrate, start it visibly.

It starts by naming the camera it found, Camera: Intel RealSense D415 (serial …, firmware …, USB 3.2), and then prints one line per change, PRESENT inner=… outer=… or NOT_PRESENT inner=… outer=…, the moment it happens. Under Watchdog that lands in the app’s app.out.log (see Logs), and it is the local record of what the camera saw and when — even while Unreal is unreachable.

If the camera is on another machine, tell it where the channel is and give it the token, as any client on the network must:

The token is the AuthToken from the Unreal machine, the same value tablet.bat carries. See Locking the control channel.

The window shows what the camera sees, the two ellipses of floor it watches, and what it currently decides. You aim the running detector: the kiosk does not stop for it, and what you see is what is deciding.

Every change saves itself.

The inner ellipse starts the conversation, the outer one ends it, a second after nobody is left in either. So walking through the ring between them starts nothing, and a visitor who steps back half a pace is not cut off. Make the inner one the spot where a visitor stands to talk, and the outer one the area they may move around in. The inner one always stays inside the outer.

It calibrates by itself two seconds after you last move or reshape an ellipse, just as C does — so keep the floor clear while you aim. Press C to calibrate at any other time, after the lighting has changed, say.

Until it is calibrated it detects nothing and tells Unreal nothing — nor while it calibrates, when the floor is deliberately empty — so it never ends a conversation somebody started from the tablet. Moving or reshaping an ellipse drops the calibration: through other ellipses the empty floor would look like somebody standing there. A margin change keeps it.

The settings belong to this installation. They are saved to visionConfig.json beside vision.exe, because the ceiling height and the camera’s angle are particular to this place. Do not copy that file to another kiosk. To update, replace vision.exe and leave visionConfig.json where it is.

The calibration remembers the camera it was measured with. A different camera — another model, or another unit of the same one — sees the floor differently, and reading it through the old calibration would look like somebody standing there. So with a different camera plugged in, vision.exe sets the calibration aside: the window says CAMERA CHANGED - press C, the log names both cameras, and it detects nothing and tells Unreal nothing until it is calibrated again — press C, or re-aim it. With --no-window it refuses to start, as an uncalibrated camera does.

Nothing is deleted. Put the original camera back and its calibration applies again.

All optional. On the Unreal machine it usually takes none.

Two more options each take the other half of the system out of the picture. That is the only way to tell a camera fault from a wiring one, so on a new machine run them in this order:

ERROR: no RealSense camera found means none is plugged in, or it is a model this version cannot use. When Windows can see one of the retired models, the error names it instead: ERROR: an Intel RealSense L515 is plugged in, but this version cannot use it.

ERROR: the depth camera is not available means it found a camera but could not stream from it — almost always the USB cable, or another program holding the camera. The Camera: line it prints when it starts says which camera it found and whether it is connected over USB 3 or USB 2.