What a sentry "sees" is not everything in front of it on screen. Sight is defined on the board's graph, as an edge chain. This page explains how that chain is built and where it breaks.
How the chain is built
The watching entity starts on its own node and reaches to the neighbouring node in the direction it faces. From there it continues along whichever edge carries on in the same direction, and then the next. The level data defines, for each pair of edges, whether one is the continuation of the other; the chain follows that relationship.
Every node the chain enters is under watch. There is no distance limit — it runs until something breaks it.
Where the chain breaks
Three things stop it:
- A sight blocker. A node tagged
blocker: a stall, a wall, a rock. Sight does not pass over it. - A corner. A node with no continuation in that direction. If the path turns, the gaze does not.
- A closed door. A door edge blocks sight as well as movement until the matching key is collected. The moment you open it, chains start running through — which is why opening a door sometimes shortens your route and makes the board more dangerous at the same time.
Hiding
Standing on a node tagged hide makes you invisible to the sight check.
Even if a chain runs over that node, it does not find you. The contact rule
still applies, but enemies never step onto hiding nodes anyway — that guarantee
is enforced in the engine, so not even badly written level data can break it.
Why there is no free geometry
Because the model you can build in your head has to match the one the solver builds. A sight system based on angles looks good on screen but makes it impossible to look at the board and answer "am I safe here" with certainty.
With chains the answer is countable: you follow the nodes, you see the blocker, you know.
The threat highlight
When you turn the threat highlight on, what gets painted is exactly the result of the calculation described on this page. It is not an approximation; the chain you see is the chain that gets checked.
Next question: does standing inside a chain always mean the same thing? No — see Capture.