How do you know if a cannon is good or bad in a given position?
These answers restate the rules implemented by our open-source engine, which follows the WXF laws and is regression-tested against the official 110-diagram repetition casebook. Where an answer refers to a specific position, that position lives on the linked reference page, where the build verifies it against the engine before publishing.
Direct answer
Count screens and real targets, not squares. A cannon that can reach far along an empty line is not doing anything if nothing sits one piece away from a genuine target; a cannon boxed in close to home but sitting exactly one screen from the enemy general is doing a great deal. Raw mobility is close to meaningless for this piece.
The quick test
Ask two questions of every cannon on the board: which of its lines currently has exactly one piece on it, and what sits beyond that piece. A cannon that answers "none" to the first question is contributing nothing right now, however open its other lines look.
Watching it change move to move
Because any piece landing on a line can switch a cannon's threat on or off, in the same way moving requires a clear line while capturing needs exactly one piece on it, a cannon's quality is not fixed the way a chariot's is. Reassess it after every move nearby, not just once at the start of your calculation.
Related questions
Is a cannon with no screens simply worthless?
Temporarily weak, not worthless; the position can change with the very next move on either side.
Should I judge a cannon by how many squares it controls?
No, that measures its potential to move, not its ability to threaten anything, which is a different question entirely.
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