Concepts

Rules & events

A rule binds a zone, an object class, and a trigger. When it fires, you get an event: a discrete, timestamped, countable fact about the scene.

Anatomy of a rule#

rule.json

{
  "id": "rule_gate_count",
  "zone": "zone_a",
  "classes": ["car", "van", "truck"],
  "trigger": "enter",
  "min_dwell_s": 0
}

Four triggers exist today:

Trigger	Fires when
`enter`	An object transitions from outside the zone to inside it
`exit`	An object that was inside leaves
`dwell`	An object stays inside longer than `min_dwell_s`
`count`	Continuously; maintains a running total of distinct entries

The naive test, box center inside polygon, undercounts. A car can clip the corner of a zone with a third of its body and never put its center inside. Praxis instead samples a 5×5 grid across the object's box and requires at least 25% of the samples to land inside the zone's polygon for that frame. Entries register when a car is genuinely in the zone, including partial passes along an edge.

One car, one event#

The counting failure modes come in mirrored pairs, and both get handled explicitly:

Double counting. A track ID switch mid-crossing looks like two entries. Candidate duplicate events get merged, but only when their two tracks never coexist in the same frames. Coexistence proves two physical objects.
Undercounting. Two cars entering side by side at the same moment are the classic casualty of aggressive merging. The coexistence test keeps them separate: both tracks were alive simultaneously, so both entries stand.

This exact logic produces the Praxis number in the challenge. On the 15-second sample feed it registers 15 distinct entries, including two side-by-side pairs that most people miss on the first try.

The event payload#

event.json

{
  "id": "evt_9f2c",
  "type": "zone.entry",
  "rule": "rule_gate_count",
  "zone": "zone_a",
  "feed": "src_boulevard",
  "t": 7.307,
  "track": 41,
  "class": "car",
  "position": [0.626, 0.741],
  "evidence": {
    "crop": "/evidence/evt_9f2c/crop.jpg",
    "track_history": "/evidence/evt_9f2c/track.json",
    "zone_geometry": "/evidence/evt_9f2c/zone.json"
  }
}

Positions are normalized to frame size. The evidence block is not optional decoration; see Evidence for why every event carries it.

Rules & events

Anatomy of a rule#

What counts as inside#

One car, one event#

The event payload#